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Integration of Clearance Mechanisms: The Proteasome and Autophagy
Esther Wong and Ana Maria Cuervo
Department of Developmental and Molecular Biology, Institute for Aging Studies, Albert Einstein College of Medicine, Bronx, New York 10461 Correspondence: [email protected]
Cells maintain a healthy proteome through continuous evaluation of the quality of each of their proteins. Quality control requires the coordinated action of chaperones and proteolytic systems. Chaperones identifyabnormal or unstable conformations in proteins and often assist them to regain stability. However, if repair is not possible, the aberrant protein is eliminated from the cellular cytosol to prevent undesired interactions with other proteins or its organi- zation into toxic multimeric complexes. Autophagy and the ubiquitin/proteasome system mediate the complete degradation of abnormal protein products. In this article, we describe each of these proteolytic systems and their contribution to cellular quality control. We also comment on the cellular consequences resulting from the dysfunction of these systems in common human protein conformational disorders and provide an overview on current therapeutic interventions based on the modulation of the proteolytic systems.
s described in previous articles on this sub- pathogenic proteins can all make the refolding Aject, cells count on a complex network of activity of chaperones insufficient to maintain molecular chaperones that assist proteins in proteome stability and prevent proteotoxicity folding and help stabilize the transient confor- (Morimoto 2008; Douglas et al. 2009; Koga mations that proteins adapt for trafficking et al. 2010). Under these conditions and for across membrane and during their assembly those proteins in which refolding is no longer and disassembly into functional complexes possible, cells count on proteolytic systems to (Large et al. 2009; Willis et al. 2009; Koga eliminate the unstable protein(s) and to recycle et al. 2010). However, different physiological their amino acids (Willis et al. 2009). The lyso- and pathological conditions may overwhelm somal system and the ubiquitin/proteasome the homeostatic capability of the chaperone system (UPS), the two main proteolytic systems network and favor protein aggregation. For in cells, along with the molecular chaperones, example, conditions resulting in massive pro- constitute essential components of the cellular tein unfolding such as acute oxidative stress or quality control systems (Ciechanover 2005). In heat shock, chronically proaggregating condi- this article, we briefly summarize the current tions that deplete cells of critical chaperones, knowledge regarding the molecular compo- and abnormal high levels of prone-to-aggregate nents of each of these proteolytic systems and
Editors: Richard Morimoto, Jeffrey Kelly, and Dennis Selkoe Additional Perspectives on Protein Homeostasis available at www.cshperspectives.org Copyright # 2010 Cold Spring Harbor Laboratory Press; all rights reserved; doi: 10.1101/cshperspect.a006734 Cite this article as Cold Spring Harb Perspect Biol 2010;2:a006734
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E. Wong and A.M. Cuervo
expand on recent evidence supporting the crit- in response to protein or organelle damage ical participation of both systems in the cellular and to replenish the intracellular reserve of free defense against proteotoxicity. We also describe amino acids that sustains protein synthesis recently established connections between mal- even in the absence of nutrients. Failure of the functioning of these proteolytic systems and the proteolytic systems to maintain basal cellular pathogenesis of common protein conforma- turnover or to accommodate to the degradative tional disorders with main emphasis on neuro- requirements of cells under stress conditions degenerative diseases. leads to altered cellular homeostasis, compro- mises the cellular energetic balance and often promotes intracellular accumulation of dam- INTRACELLULAR CLEARANCE aged components (Koga et al. 2010). Deposits MECHANISMS of conformationally altered proteins that or- Cells maintain a state of self-renewal, through ganize into insoluble oligomeric structures are the continuous synthesis and degradation of toxic for cells and lead to cell death in common all intracellular components, including soluble human pathologies generically known as pro- proteins and organelles (Ciechanover 2005). tein conformational disorders (Markossian Added to this regulated turnover, the activity of and Kurganov 2004; Morimoto 2008; Robinson the cellular degradative systems is up-regulated 2008) (Fig. 1).
A Cytosol E Protein Lysosomes
Unfolded Chaperone protein Ribosomes
B Protein aggregate
Chaperone
Unfolded D protein Organelle
C Chaperone
Folded Proteasome Unfolded protein protein
Figure 1. Coordinate action of chaperones and the proteolytic systems in quality control. Chaperones assist in the folding of de novo synthesized proteins (A), unfolding and refolding of proteins as they traffic into cellular compartments (B), and in the refolding of proteins when damaged by cellular aggressors (C). Proteins that fail to fold can be eliminated from the cell by two proteolytic systems: autophagy (D) and the ubiquitin/proteasome system (E).
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Cellular Clearance Mechanisms
Two systems share the proteolytic cellular that become exposed as the substrate proteins load, the lysosomes and the UPS (Ciechanover unfold and undergo degradation. 2005). Although these two systems bear unique Intracellular degradation is often the most properties, there are a series of essential steps efficient mechanism to prevent toxicity associ- and components common to both of them ated with the accumulation of conformationally and required for their functions in cellular qual- altered proteins without affecting the cellular ity control. The common steps in protein degra- reserves of amino acids (Goldberg 2003; Mi- dation are: cargo selection and tagging, cargo zushima 2005). Cells can elicit alternative mech- recognition and delivery to the proteolytic ma- anisms when the load of proteins destined for chinery, degradation in the proteolytic core, degradation surpasses the activity of the proteo- and recycling of the constituent amino acids. Se- lytic systems or when there is a primary failure lection of cargo to be degraded is a prerequisite in the functions of these systems. For example, in both systems. Although for a long time it formation of large protein inclusions has been was generally accepted that cargo selection was proposed to be used by cells in certain instances only a prerequisite for the UPS and that de- to protect themselves from the toxic effect as- gradation in the lysosomal system was in-bulk sociated to oligomeric irreversible species of and occurred in a random manner, growing pathogenic proteins (Cohen and Dillin 2008). evidence support that this is not the case. In Secretion of the toxic protein products to the fact, as described more in detail in the follow- extracellular media is also used as a mechanism ing sections, molecular chaperones and other of cellular defense against proteotoxicity. Extra- cargo-recognition molecules are often the ones cellular proteases can take care of the secreted determining the fate of cellular proteins and products up to some extent, beyond which an- their degradation in one or the other proteo- titoxic aggregation mechanisms, similar to the lytic systems (Douglas et al. 2009). Degrada- ones described inside cells, result in the for- tion tags on the substrate proteins and the mation of protein inclusions or plaques in the machinery required for tagging can also be extracellular media. shared by both the proteolytic systems (Waters et al. 2009). Following tagging, the substrate AUTOPHAGY needs to be recognized by the proteolytic com- partment. Association of different cargo recog- The degradation of intracellular components of nition molecules with the shared proteolytic any kind inside lysosomes is generically defined machinery, either the lysosomal compartment as autophagy, or self-eating (Mizushima et al. or the proteolytic core in the UPS, allows for 2008). The essential component of this proteo- variants inside each of the two proteolytic sys- lytic system are the lysosomes, single membrane tems dedicated for the degradation of partic- vesicles that contain in their lumen the larger ular subsets of proteins and organelles. Both variety of cellular hydrolases including pro- systems require catalytic activities capable of teases, lipases, glycosidases, and nucleotidases breaking the peptide bonds between amino (De Duve and Wattiaux 1966). Common to acids. Multiple proteases with different speci- these hydrolases is the fact that they all reach ficity constitute the proteolytic machinery of their higher enzymatic activity at the acidic pH the lysosomal system, whereas a single protease, of the lysosomal lumen. An ATP-dependent the 20S proteasome, bearing at least three differ- proton pump at the lysosomal membrane is re- ent proteolytic activities is responsible for pro- sponsible for the acidification of this organelle. tein breakdown in the UPS. In both systems The low lysosomal pH has been proposed to degradation is attained in a confined compart- also facilitate partial unfolding of the substrate ment, the lumen of the lysosome or the catalytic proteins allowing endoproteases to gain access chamber of the proteasome, which prevents to internal peptide bonds. Degradation in the nonspecific associations of other cellular pro- lysosome is highly processive as it results from teins to the hydrophobic patches of amino acids the combined action of endo- and exoproteases
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E. Wong and A.M. Cuervo
leading to conversion of proteins into small macroautophagy, microautophagy, and chap- di- and tri-peptides and free amino acids that erone-mediated autophagy (CMA) (Fig. 2). are released into the cytosol through permeases Variants of each of this type of autophagy have at the lysosomal membrane (Maggi and Hart been described and named to indicate the cargo 1973). preferentially degraded: mitophagy (autophagy of mitochondria), pexophagy (autophagy of peroxisomes), lipophagy (autophagy of lipid Autophagic Pathways: Characteristics droplets), and aggregophagy (autophagy of ag- and Molecular Dissection gregates). A dedicated subset of cargo recog- The lysosome is the catalytic component of nition molecules is involved in each of this the autophagic system and consequently all cargo autophagic variants, but the basic mechanisms is delivered to this compartment for degrada- and essential gene products are shared with the tion. Cargo recognition and delivery occurs by general forms of autophagy (Klionsky et al. different mechanisms depending on the type 2003). of cargo and the cellular conditions, giving Only proteins can be delivered to lysosomes rise to different modalities of autophagy (Mizu- via CMA (Cuervo 2010), whereas macro- and shima et al. 2008; Yangand Klionsky 2009). The microautophagy participate in the degrada- best characterized in mammalian cells are: tion of both proteins and organelles (Yang and
BCMICROAUTOPHAGY CMA
Soluble Soluble proteins proteins
Organelles
A Protein aggregates Organelles
Translocation complex
Lysosome Soluble proteins MACROAUTOPHAGY
Figure 2. Autophagic pathways. Cytosolic proteins can reach the lysosomal lumen for degradation via autophagy through three different mechanisms. (A) In macroautophagy, a whole region of the cytosol is sequestered into a double membrane vesicle that fuses with lysosomes for cargo delivery. (B) In microautophagy, the lysosomal membrane invaginates to trap regions of the cytosol that are internalized into the lysosomal lumen as single membrane vesicles. (C) In chaperone-mediated autophagy, a targeting motif in the substrate proteins is recog- nized by a cytosolic chaperone that delivers it to the surface of the lysosome. Once there, the substrate protein binds to a lysosomal receptor that multimerizes to form a translocation complex. A luminal chaperone mediates the translocation of the substrate protein into the lumen where it is rapidly degraded.
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Cellular Clearance Mechanisms
Klionsky 2009). CMA and macroautophagy Complexes recruited to the membrane include: were initially described as stress-induced forms (1) Atgs involved in two conjugation cascades, a of autophagy, but recent studies support the co- protein-protein conjugation and a protein-lipid existence of basal activity of both autophagic conjugation system, along with the enzymes pathways in most cell types. Selectivity in cargo that catalyze these conjugation events (Mizu- recognition, originally only attributed to CMA, shima etal.1998);(2)abeclin-containing kinase has also been shown for the other autopha- complex that brings along a phosphatidyl- gic pathways (Noda et al. 2008; Lamark et al. inositol 3 kinase type III responsible for the 2009; Tolkovsky 2009). enrichment in this lipid modification on the In macroautophagy, cytosolic cargo is se- surface of the membrane that will give rise to questered inside a de novo formed double the autophagosome (Itakura et al. 2008); and membrane vesicle or autophagosome that then (3) a second kinase complex, that on activation fuses with late endosomes or lysosomes (Yang of autophagy dissociates from the negative and Klionsky 2009) (Fig. 2). Mixing of the regulator of autophagy, mTOR, and mobilizes luminal content of autophagosomes and lyso- to the region of autophagosome formation somes allows lysosomal hydrolases to gain ac- (Hosokawa et al. 2009). Although self-phos- cess to the sequestered cytosolic cargo and phorylation of this complex has been reported, initiate its degradation. Degradation of cargo other targets of the kinase activity of this second progresses in this mix compartment (autopha- complex are currently under investigation. Atg golysosome) until returning to the enzyma- complexes do not assemble irreversibly at the tic enrichment characteristic of a secondary site of autophagosome formation but rather lysosome. Although macroautophagy was de- most of them undergo continuous shuttling scribed almost in parallel to the discovery from other intracellular membranes to these of the lysosome (Deter et al. 1967), it is only regions (Suzuki and Ohsumi 2010). This shut- recently, through yeast genetic screenings, that tling is believed to contribute the lipids required the molecular components that participate in for the elongation of the limiting membrane this process have been identified and charac- that then seals around the cargo through mech- terized. About 35 genes, generically known anisms still poorly characterized. Fusion of the as autophagy-related genes or ATG, have been autophagosome with lysosomes involves micro- shown to participate in macroautophagy tubules, and proteins in the membranes of both (Klionsky et al. 2003). Their protein products, autophagosomes and lysosomes that contribute or Atg proteins, organize into functional to modulate the fusion process and the mixing complexes that regulate each of the steps of of content between both compartments. Most macroautophagy. Formation of the limiting forms of autophagy are subjected to the nega- membrane of the autophagosome initiates by tive regulatory effect of one of the major kina- the recruitment of different autophagic com- ses in the cell, mTOR, and the components plexes to specific regions of intracellular mem- associated to this kinase as part of the TORC1 branes (Mizushima et al. 1998; Mizushima complex (Meijer and Codogno 2004). The et al. 2002). The endoplasmic reticulum, mito- diverse array of cellular and extracellular cues chondria, and the plasma membrane are con- sensed by mTOR, insulin, amino acids, ATP, firmed sites of autophagosome formation hormones, glucose, and stress factors, matches (Axe et al. 2008; Hailey et al. 2010; Ravikumar with the stimuli that modulate autophagic et al. 2010). Preferential formation from one activity in cells. or another site may depend on the stimulus Autophagosomes and autophagolysosomes that activates autophagy and could determine are the morphological signature of macroau- the type of cargo sequestered inside the auto- tophagy and have been used as direct indicators phagosome. In the three sites for autophago- of the changes in the activity of this autophagic some formation, specific Atgs act as platform pathway in cells and tissues. The identification of assembly of other Atgs to the membrane. of the proteins that participate in the formation
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E. Wong and A.M. Cuervo
and cellular dynamics of these vesicles has now substrates to the lysosomal membrane recep- allowed tracking these compartments in real- tor. On substrate binding, the receptor protein time by using tagged forms of these proteins multimerizes to form a complex required for (Yang and Klionsky 2009). Furthermore, over- substrate translocation (Bandyopadhyay et al. expression and knock-down or knock-out of 2008). Specific membrane proteins regulate essential Atgs has provided a better understand- the assembly and disassembly of this receptor ing of the cellular consequences of changes in and contribute to modulate the activity of this macroautophagic activity under physiological pathway (Bandhyopadhyay et al. 2010). A cer- and pathological conditions (Komatsu 2005; tain level of basal CMA is detectable in all cells, Hara et al. 2006). but this pathway is maximally activated in re- Less information is currently available about sponse to stress (Cuervo and Dice 1996; Dice microautophagy, a form of autophagy that also 2007). involves sequestration of whole regions of the cytosol but directly by the lysosomal membrane Physiological Functions of Autophagy (Ahlberg and Glaumann 1985) (Fig. 2). This process has been better characterized in yeast In recent years, a growing number of functions where a subset of gene products have been have been attributed to autophagy, but almost shown to contribute to the formation of the all of them can be included in one of the follow- membrane projections from the surface of the ing four categories: quality control, cellular vacuole (the equivalent of the lysosome in yeast) source of energy, cell and tissue remodeling, that sequester soluble proteins and organelles and cellular defense (Mizushima et al. 2008). and internalize them inside small vesicles in The ability of different autophagic pathways the lumen of the vacuole (Tuttle and Dunn to break down intracellular components (e.g., 1995; Dubouloz et al. 2005). Although most mi- proteins, lipids, sugars, and nucleic acids) and croautophagy degradation occurs probably “in recycle their constituent elements back to the bulk,” selective removal of certain organelles cytosol makes it an ideal mechanism to supply has also been described (i.e., micropexophagy cells with this elements when nutrients are for the selective degradation of peroxisomes) scarce (Mizushima 2005). Both macroauto- (Sakai et al. 1998). The absence of mammalian phagy and CMA are maximally up-regulated homologs for the microautophagy yeast genes in response to nutrient deprivation. Autophagy, has made it difficult in gaining a better under- in particular macroautophagy, also contributes standing of the pathophysiology of this process. to the elimination of large portions of cytosol, In mammalian cells a third type of auto- organelles, plasma membrane, or even cellular phagy selective for the degradation of a subset corpses in processes such as cellular differen- of cytosolic proteins has been named as chaper- tiation, tissue remodeling, and embryogenesis one-mediated autophagy (CMA) (Dice 2007; (Levine and Klionsky 2004). Similarly, patho- Cuervo 2010) (Fig. 2). This process requires gens (e.g., bacteria, parasites, and viruses) that the recognition of the targeting motif in the reach the cellular cytosol through phagocytosis substrate protein by a cytosolic chaperone and or directly across the plasma membrane can be its subsequent targeting to the surface of the successfully eliminated by the autophagic sys- lysosome where it binds to a membrane re- tems, acting thus at the forefront of cellular ceptor protein (Cuervo and Dice 1996). Inter- defense (Deretic 2009). nalization of the substrate into the lysosomal Of relevance to this article is the important lumen is mediated by a lysosome resident role of the autophagic system in cellular quality protein and, in contrast with the other two control (Fig. 2). The fact that altered organelles autophagic processes, it requires complete un- and cytosolic components are eliminated in folding of the substrate protein before translo- lysosomes has been known since the identifica- cation (Salvador et al. 2000). The limiting step tion of this organelle. However, it has only been in this form of autophagy is the binding of the recently that the contribution of this “cleaning”
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Cellular Clearance Mechanisms
function to the maintenance of cellular ho- aggregates of certain pathogenic proteins are meostasis has been conclusively documented. positive for ubiquitin and the cargo recognition Cells knocked down for different essential auto- molecules and still fail to be recognized by the phagic genes show accumulation of abnormal autophagic system (Wong et al. 2008). It is pos- organelles and protein deposits in their cytosol, sible that additional molecules or posttransla- even when maintained in the absence of any tional modifications in the aggregated proteins other aggravating factor (Ravikumar et al. are required for autophagy recognition. 2002; Iwata et al. 2005). Similar results have Compromised CMA also leads to altera- been observed in whole animal knockout for tions in cellular homeostasis. Studies in cells autophagy genes in a specific tissue (Komatsu knocked down for the lysosomal receptor of 2005; Hara et al. 2006). Although the pheno- this pathway show accumulation of cytosolic type of these animals depends on the affected proteins, protein aggregation, and increased organ or tissue, the changes at the cellular level sensitivity to different stressors (Fig. 2). In con- are the same for all tissues. These studies sup- trast to macroautophagy-impaired cells, the or- port that normal autophagic activity is essential ganelle compromise in these cells is minimal for the maintenance of cellular homeostasis and mainly secondary to the accumulation of through the continuous turnover of organelles the protein products (Massey et al. 2006). and of damaged or altered proteins. Interestingly, the protein inclusions obser- Pathophysiology of the Quality Control ved in the animal models with impaired auto- Through Autophagy phagy are often enriched in ubiquitin, a small protein that, as described in detail in the follow- In light of the important role that the autopha- ing sections, can be used for tagging of cytosolic gic system plays under normal physiological proteins for degradation through the UPS. It is conditions, it is not surprising that alterations still controversial whether the accumulation of of autophagy have been identified in many hu- ubiquitinated proteins in aggregates in macro- man pathologies. In fact, dysfunctional auto- autophagy-incompetent cells reflects that aggre- phagy underlies the basis of a growing list of gates are normally degraded by this pathway, protein conformational disorders (Wong and or if it is possible that soluble ubiquitinated Cuervo 2010). The first connection between proteins are also substrate for macroautophagy these disorders and the autophagic system (Ferguson et al. 2009; Korolchuk et al. 2009). originated from studies showing up-regulated The degradation of aggregated proteins by macroautophagic activity in cells expressing macroautophagy has been extensively reported pathogenic forms of different proteins associ- and it is currently referred to as aggregophagy ated to disorders such as Parkinson’s or Hunt- (Ravikumar et al. 2002; Iwata et al. 2005). The ington’s disease (Ravikumar et al. 2002; Iwata presence of ubiquitin molecules on the surface et al. 2005). This increase in macroautophagy of these protein inclusions has been shown seemed of protective nature because when to facilitate the recruitment of components of precluded, cellular viability was often com- the macroautophagic machinery to these ag- promised. These observations along with the gregates leading to the in situ formation of the previously described degradation of protein ag- autophagosome. Selective degradation of the gregates by macroautophagy lead to the postu- aggregates is also mediated by cargo recognition lation that enhancement of macroautophagy proteins such as p62 or NBR1, which can inter- could be an effective intervention in these dis- act directly with ubiquitin moieties and with orders. In fact, studies in fly and mouse models LC3, one of the essential autophagy proteins of Huntington’s disease revealed that chemical that associate with the autophagosome mem- up-regulation of macroautophagy in these mo- brane (Lamark et al. 2009). However, although dels slowed down disease progression by re- all these molecules are necessary for autophagy ducing proteotoxicity and increasing cellular of aggregates they may not be sufficient, because viability (Ravikumar et al. 2004). At the same
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E. Wong and A.M. Cuervo
time, these studies also suggested that com- once delivered to lysosomes (Wong and Cuervo promised macroautophagy could underlie the 2010). Furthermore, in some instances the pathogenesis of some of these disorders, some- macroautophagic defect is primarily caused thing that has now been extensively documen- by alteration in one of the autophagy compo- ted. Reduced macroautophagic activity has been nents, whereas in other instances the autopha- reported in Parkinson’s disease, Huntington’s gic failure is secondary to alterations in the disease, Alzheimer’s disease, polyglutamine dis- other quality control mechanisms. For example, eases, amyotrophic lateral sclerosis and prion defective autophagy in some familial forms of diseases, among others (Sarkar et al. 2009) Alzheimer’s disease carrying mutations in pre- (Fig. 3). However, interestingly, a developing senilin 1, is a consequence of a primary defect theme is that the autophagic defect is not uni- in lysosomal acidification (Lee et al. 2010). In form across all these diseases. Alterations in contrast, the initial up-regulation of the auto- autophagy in these disorders spread across pro- phagic system in some familial forms of Parkin- blems in autophagosome formation, cargo rec- son’s disease is likely a compensatory response ognition, autophagosome mobilization toward for the failure in the ubiquitin/proteasome sys- lysosomes, autophagosome/lysosome fusion tem and CMA in the affected cells (Stefanis et al. or in the degradation of the autophagic cargo 2001; Cuervo et al. 2004).
ABMACROAUTOPHAGY CMA α-synuclein
Inefficient hsc70 induction Abnormal binding
Protein aggregate
Pathogenic α-synuclein
Autophagosome Lysosome Failed cargo Mutant recognition Tau
Incomplete Inefficient Protein translocation Autophagosome fusion/degradation aggregate
Figure 3. The autophagic system in quality control. Autophagy contributes to the removal of both soluble cyto- solic proteins and proteins organized into irreversible complexes or aggregates. Impairment of the autophagic system leads to the accumulation of damaged proteins in the form of protein inclusions. Failure of both macro- autophagy (A) and chaperone-mediated autophagy (B) has been described to contribute to pathogenesis in dif- ferent protein conformational disorders. Some of the steps described to be affected in each of the autophagic pathways are illustrated here.
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Cellular Clearance Mechanisms
Compromised CMA has also been linked effect of lithium on this process (Sarkar et al. to neurodegenerative disorders (Fig. 3). a-Syn- 2005). Ongoing small molecule screenings uclein, the protein that accumulates in the should soon render a battery of compounds protein inclusions observed in the affected that could be applied to enhance macroauto- neurons in Parkinson’s disease (PD), undergoes phagic activity in vivo. However, one of the degradation via CMA. In contrast, mutant limitations of most of these compounds is that forms of this protein and pathogenic variants they all act on early steps of macroautophagy of the wild type protein are targeted to lyso- enhancing autophagosome formation, but will somes and bind to the lysosomal receptor not have a beneficial effect in those pathological but fail to be translocated (Cuervo et al. 2004; conditions in which the autophagic defect is in Martinez-Vicente et al. 2008). Furthermore, steps past autophagosome formation. In fact, because their interaction with the lysosomal up-regulation of autophagy could even be membrane displays an unusual high affinity, detrimental in those pathologies with reduced degradation of other cytosolic substrates for clearance of autophagosomes. Customized in- this pathway is also compromised. Impaired terventions, aimed at repairing the specific CMA contributes thus to altered homeostasis autophagic defect in each of the different con- in the affected cells, enhanced sensitivity to formational disorders are a more promising stressors, and could lead to cellular death. future possibility. Abnormal interaction with CMA components Only genetic manipulations have been used has also been described recently for UCH-L1, so far in animal models to enhance CMA another PD-related protein (Kabuta and Wada (Zhang and Cuervo 2008). A small-scale screen- 2008). The compromise in CMA is not limited ing of a dozen of compounds revealed enhanced to PD, as recent studies have revealed similar CMA in cells treated with some of them (Finn blockage of this pathway by certain mutant et al. 2005). However, because the targets of forms of Tau, the protein responsible for cellu- those drugs were very general and a link be- lar toxicity in some tauopathies (Wang et al. tween those targets and CMA has not yet been 2009). It is thus likely that CMA could also be established, it is not possible to determine the target of other pathogenic proteins associ- whether their effect on CMA is direct or secon- ated to other conformational protein disorders. dary to other cellular effects of these drugs.
Chemical Modulation of Autophagic THE UBIQUITIN/PROTEASOME Clearance SYSTEM The promising results obtained on up-regula- The UPS is the major pathway responsible for tion of macroautophagy in the models of Hun- the highly regulated extralysosomal degradation tington’s disease (HD) (Ravikumar et al. 2004) of cytosolic proteins and of proteins residing has generated considerable interest in the possi- in the nucleus and endoplasmic reticulum in bility of using modulators of autophagy with eukaryotic cells (Coux et al. 1996; Baumeister therapeutic purposes in protein conformational et al. 1998). The UPS degrades mostly short- disorders. So far, the amount of chemicals lived proteins through a multistep process that proven to enhance autophagy is still rather lim- requires the tagging activity of a sophisticated ited. Inhibition of mTOR by rapamycin has system. The tagging molecule is a small pro- been shown effective as macroautophagic acti- tein, ubiquitin, that once covalently linked to vator, but the large number of other cellular proteins, earmarks them for destruction by processes controlled by this cellular kinase lim- the 26S proteasome, a highly conserved multi- its its clinical applicability (Ravikumar et al. catalytic ATP-dependent protease complex 2004). Regulation of macroautophagy by mech- (Fig. 4). The rapid, precise and timely pro- anisms independent of mTOR has also been cessing of a vast extent of cellular proteins by reported and seems the basis for the stimulatory the UPS allows tight control of critical cellular
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E. Wong and A.M. Cuervo
ABC K6 K11 K27 K29 K33 K48 K63 Ubiquitin Intrinsic receptors Ub Substrate Rpn 10 ATP ** AMP+PPi UBL UIM UIM DUB Rpn 13 20S core 26S * particle proteasome E1 PRU
19S regulatory particle Shuttle factors Substrate E2 Rad23 (hHR23a/b) ** UBL UBA UBA Subunit composition STI1 RING/RING E3 -like E3 Rpn3, 5-9, Dsk2 (PLIC 1/2, Ubiquilin 1/2) 11, 12, 15 Lid E2 * UBL UBA Rpn10 Substrate STI1 STI1 Rpn1, 2, 13 Base Rpt1-6 Ddil (Ddil 1/2) * E3 UBL RVP UBA α HECT E3 α-1-7 -Ring E2 p62 β-Ring Substrate * β-1-7 PBI ZnF UBA
Figure 4. Components of the ubiquitin/proteasome system. Substrates destined for proteasomal elimination are tagged with polymers of ubiquitin (Ub) through repeated sequential reactions catalyzed by ubiquitin activating (E1), conjugated (E2), and ligating (E3) enzymes (A) RING/RING-like E3 catalyzes the transfer of Ub directly from E2 to substrate whereas HECT E3 accepts activated Ub from E2 before transferring it to the substrate. Ubiq- uitinated substrates either bind directly to the ubiquitin receptors in the proteasome regulatory particle or shut- tle to the proteasome by shuttle factors (B) Ã indicates domain binding polyubiquitin chain. Binding of substrate is followed by protein unfolding by the six ATPases forming the base of 19S regulatory particle, removal of poly- ubiquitin chain by deubiquitinating enzymes (DUBs) to regenerate free Ub, and translocation of the unfolded protein into the core proteolytic chamber, where it is cleaved into short peptides (C).
functions such as DNA repair, cell cycle progres- extremely stable and highly conserved from sion, development, apoptosis, gene transcrip- yeast to mammals. In most cases, the first ubiq- tion, signal transduction, senescence, immune uitin is attached via its carboxy-terminal glycine response, metabolism, and protein quality residue to the 1-amino group of a lysine residue control. in the substrate to generate an isopeptide bond. In substrate proteins without lysine residues, ubiquitin can be conjugated to their amino ter- Ubiquitin-Conjugation: The “Kiss of Death” minus, forming a linear peptide bond (amino- for Cellular Proteins terminal ubiquitination) (Ben-Saadon et al. Most substrate proteins are targeted to the 26S 2006). The conjugation of ubiquitin to a sub- proteasome by the covalent attachment of strate is orchestrated by the actions of three multiple ubiquitin proteins to the substrates enzymes (Hershko and Ciechanover 1998). in a process known as ubiquitination. Ubiquitin In an ATP-consuming first step, ubiquitin is is a small (8.5 kDa) globular protein that is bound by a high-energy thioester bond to E1
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Cellular Clearance Mechanisms
(ubiquitin-activating enzyme), becoming thus The Ubiquitination Language activated. Subsequently, ubiquitin is transferred to the active site of E2 (ubiquitin-conjugating The way in which ubiquitin is conjugated to enzyme). In the final step, E3 (ubiquitin-ligase the substrates (ubiquitin linkages) constitutes enzyme) catalyzes the transfer of ubiquitin to another layer of control in the degradation of the substrate protein destined for degradation substrates by the UPS. Similar to phosphory- (Fig. 4). Because of the presence of internal ly- lation, ubiquitination is a reversible modifica- sine residues, ubiquitin can be repeatedly at- tion that is rapid, specific, and diverse. Besides tached to itself through repeated actions of the UPS-mediated degradation, ubiquitin also conjugating enzyme cascade to form polymeric participates in a broad array of proteasome- ubiquitin chains. independent cellular functions (Fig. 5). The The selection of substrates for UPS degrada- versatility of the ubiquitination is endowed by tion is governed by the specificity of E3 enzymes the presence of seven lysine residues in ubiq- through two possible strategies. E3 enzymes uitin at positions 6, 11, 27, 29, 33, 48, and can recognize and bind a degradative signal 63, which can each serve as acceptors of other or “degron” in the sequence of the unstable pro- ubiquitin molecules. Quantitative proteomics tein. For example, the presence of either basic in yeast have revealed the occurrence of the or hydrophobic residues at the amino-terminal seven ubiquitin linkages although in varying of a protein (N-degrons) tends to destabilize it abundance (Xu et al. 2009). K48 and K11 link- and facilitates its recognition by E3 (Ravid and ages are the most abundant ubiquitin chain Hochstrasser 2008). The specificity of other E3 types in yeast followed by K63. K6, K27, K29, ligases such as CHIP (carboxyl terminus of and K33. This gives rise to differently linked Hsc70-interacting protein) can be modulated polyubiquitin chains with distinct quaternary through its interaction with cytosolic chaper- structures and topologies (Ikeda and Dikic ones such as Hsp70 and Hsp90. CHIP uses 2008) (Fig. 5). The different topologies of the these two chaperones as a recognition subunit ubiquitin chains serve as distinct binding sur- of unstructured regions in client proteins (Mu- faces for different classes of ubiquitin-binding rata et al. 2001). In addition, CHIP can also bind proteins (Hicke et al. 2005; Raasi et al. 2005; nonnative proteins without Hsp70 or Hsp90, Varadan et al. 2005). Besides homogenous poly- suggesting that there are multiple ways by which ubiquitin chains, cells can also generate heter- substrates can be recognized by this E3 and ogeneous chains whereby ubiquitin molecules likely other E3 enzymes in general (Rosser et al. are linked to different internal lysine residues 2007). Conceivably, more strategies of substrate within a single chain or in which more than selection may exist, judging from the multi- one ubiquitin molecule is attached to a sin- plicity of ubiquitin-related enzymes that in- gle ubiquitin forming branched chain (Ben- cludes two E1 enzymes, around one hundred Saadon et al. 2006; Kirkpatrick et al. 2006; E2 enzymes, and more than a thousand E3 en- Kim et al. 2008). Further adding to the com- zymes (Staub and Rotin 2006). As indicated plexity, the polyubiquitin chains can be of vary- in previous sections, recent studies support a ing lengths and substrate proteins can also be broader role of ubiquitin-conjugation in cellular tagged with a single ubiquitin on a single lysine quality control, because ubiquitin tagging is no residue (monoubiquitination) or on multiple longer limited to targeted degradation by the lysine residues (multimonoubiquitination). UPS, but instead it also participates in selective Different modes of ubiquitination deter- autophagic degradation. mine different cellular fates of a protein (Fig. It is noteworthy that some substrate pro- 5). In the case of proteasomal degradation, teins do not require ubiquitination to be proteins bearing chains of at least four ubiq- degraded by the proteasome, although the rel- uitin molecules are the preferred substrates of evance of this process in vivo is still unclear the 26S proteasome (Jentsch and Schlenker (Murakami et al. 1992). 1995; Hochstrasser 1996; Thrower et al. 2000).
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A Monoubiquitination and C Heterogeneous polyubiquitination multi-monoubiquitination
Substrate Mixed linkages Proteasome Substrate Substrate
Substrate Branched linkages Unknown Signaling, Endocytosis, Aggregation Peroxisome and cytosolic protein degradation by macroautophagy
B Homogenous polyubiquitination
Substrate K6 DNA repair Substrate K33 Kinase modification
Substrate K11 Proteasome, ERAD, K48 Proteasome Cell cycle Substrate
Signaling, Trafficking, Endocytosis Mitophagy K63 Substrate K27 DNA repair, Aggresome formation and Substrate removal by macroautophagy
K29 Proteasome (Ub fusion degradation), Linear Proteasome Substrate Lysosome, Kinase modification Substrate Linear chain signaling
Figure 5. The ubiquitin code. The ubiquitin molecule can be attached to a single site (A) or multisites (B-C)ona substrate to yield mono- and multi-ubiquitination respectively. In addition, the ubiquitin sequence contains seven lysine residues that can support the assembly of polyubiquitin of different chain topologies. The plethora of ubiquitin linkages makes ubiquitination a highly versatile modification that serves diverse roles in the cell. Heterogeneous polyubiquitination (C) occurs when a ubiquitin chain has alternating linkage types (mixed link- ages) or when a single ubiquitin is extended at two or more lysine residues (branched linkages).
Regarding the type of linkage, K48 has been formed by the sequential action of different identified as the canonical signal that targets E2 enzymes (Kirkpatrick et al. 2006; Jin et al. proteins to the proteasome for degradation, 2008). The different types of linkage not only but K11 linkage can also serve as a potent pro- dictate the ability of the regulatory subunits of teasomal degradation signal (Baboshina and the 26S proteasome to recognize the substrate Haas 1996; Kirkpatrick et al. 2006; Jin et al. protein, but they also affect their kinetics of deg- 2008; Kim et al. 2008; Xu et al. 2009), particu- radation in this protease complex. K48 ubiqui- larly on cell cycle regulatory proteins (Jin et al. tinated substrates are the most rapidly degraded 2008) and on endoplasmic reticulum sub- by purified proteasomes proving that this link- strates (Xu et al. 2009). K29 linkage has recently age is still the most efficient targeting signal been associated with the degradation of ami- for proteasomal degradation (Xu et al. 2009). no-terminal ubiquitinated substrates (Johnson In contrast, branched polyubiquitin chains are et al. 1995; Koegl et al. 1999). Although the not processed efficiently by the proteasome K63 linkage is typically implicated in protea- (Kim et al. 2007), hence calling into question some-independent functions, recent studies the in vivo role of these branched chains. On support that this linkage can also target some the other side of the scale, some of the linkages substrates for degradation by the proteasome, may exert inhibitory functions over the protea- at least in vitro (Crosas et al. 2006; Kirkpatrick some (i.e., K6 linkage inhibits proteasomal deg- et al. 2006; Kim et al. 2007). Interestingly, K29 radation (Shang et al. 2005). and K63 linkages have been recently implicated in substrate targeting for degradation through Decoding Ubiquitination at the Proteasome autophagy (Chastagner et al. 2008; Tan et al. 2008b). Once ubiquitin-tagged, the substrate proteins The proteasome can process both homoge- are routed to the proteasome for degradation. nous and heterogeneous polyubiquitin chains Most substrates dock at the proteasome via
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Cellular Clearance Mechanisms
binding to specific ubiquitin receptors, which proteasome (Komander et al. 2009a; Komander include the stable subunits of the proteasome, et al. 2009b). Rpn10 and Rpn13, and several transiently associated shuttle factors such as Rad23, Dsk2, The 26S Proteasome Ddi1, and p62 (Finley 2009) (Fig. 4). Direct docking of ubiquitinated substrates to the The 26S proteasome is a large 2.5MDa ATP- proteasome is possible via binding to Rpn10 dependent multisubunit protease complex that and Rpn13 whereas remote substrates can be consists of two portions: the catalytic 20S core captured and escorted to the proteasome by particle (CP) and the regulatory particle (RP) the shuttle factors. The substrate binds to (Fig. 4). The activity of the 20S CP is regulated the ubiquitin-associated (UBA) domain in the by the assembly of different RPs (19S and 11S) shuttle factor, which in turn binds to the that dock on one or both sides of the catalytic proteasome through its ubiquitin-like (UBL) core to form several proteasome species (Mu- domain. The UBA domains in these ubiquitin rata et al. 2001; Finley 2009). The 26S protea- receptors may have preferential affinities for some, formed by the association of 20S CP different ubiquitin linkages and different chain to 19S RP plays a prominent role in quality lengths (Seibenhener et al. 2004; Raasi et al. control. 2005; Varadan et al. 2005; Long et al. 2008; The CP is a barrel-like structure formed Tan et al. 2008a). Besides ubiquitin, selectivity by 28 subunits organized in two outer a-rings of UBA is also defined by determinants in the and two inner b-rings, each comprising seven conjugated substrates (Verma et al. 2002). The structurally similar a- and b-subunits respec- shuttle factors may thus impose a hierarchi- tively (Goldberg 2003; Pickart and Cohen cal order of degradation for the different ubiq- 2004) (Fig. 4). The a-rings serve as a gate for uitin chains. substrate entry into the proteolytic chamber On binding, deubiquitinating enzymes formed by the b-rings, which bear caspase-like, (DUBs) mediate the disassembly of poly- trypsin-like, and chymotrypsin-like proteolytic ubiquitin chains from the captured substrates activities. Proteasomes with different catalytic before they can gain access to the proteolytic activities can be generated by exchanging the core. DUBs provide additional regulatory con- different proteolytic active subunits of the CP. trol before protein degradation, and they play Because the active sites are located on the an important role in recycling ubiquitin and inner surface of the proteolytic chamber, the maintaining a sufficient pool of free ubiq- proteasomal substrates must first gain entry uitin in cells. Rpn11, a regulatory subunit of into this space. This event is regulated by the the proteasome, is responsible for substrate association of 19S RP which controls the open- deubiquitination at the proteasome usually re- ing and closing of the a-rings in the 20S CP. moving whole chains at once (Verma et al. The 19S RP consists of 19 subunits organized 2002; Yao and Cohen 2002). Other DUBs can in a lid and a base. The base is composed of also perform deubiquitination on the substrates six AAA’ ATPase subunits (Rpt 1-6) and four before the substrates are committed to degra- non-ATPase subunits (Rpn 1, 2, 10, and 12) dation and become subject to the activity of (Finley 2009). The ATPase subunits provide Rpn11. For example, Uch37 and Ubp6 an- the energy needed for deubiquitination and tagonize substrate breakdown by trimming unfolding of the substrates, which is a prereq- the ubiquitin chains, which reduces its binding uisite for threading through the narrow chan- affinity to the proteasome and favors their nel of the 20S CP, as well as for a-ring gate release back to the cytosol (Lam et al. 1997; opening. The high energy requirements of Hanna et al. 2006). Some DUBs show prefer- the UPS explain why conditions that promote ence for specific ubiquitin linkages making mitochondrial dysfunction and thereby en- these linkages less stable and likely reducing ergy depletion may affect proteasome-mediated their chances of degradation through the degradation.
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Pathophysiology of the Quality Control the pathogenesis of these disorders. In fact, al- Through the Ubiquitin/Proteasome System tered proteasome activity has been described in many neurodegenerative disorders, and re- Malfunctioning of the UPS, which could occur cently genetic depletion of proteasome subunits at many steps of this complex degradative pro- in mouse brain has been shown to induce a full cess, results in severe cellular alterations and, phenotype of neurodegeneration (Bedford et al. if persistent, often leads to cellular death. Part 2008). of the cellular pathology is a direct consequence Like autophagy, alterations in the UPS of the critical cellular functions modulated by could occur at very different levels and through this proteolytic system, but cellular toxicity mechanisms unique for each disease. In some in UPS-compromised cells also arises from its protein conformational disorders, the proteo- role as a quality control mechanism. In this lytic core of the proteasome becomes a target respect toxicity is not limited to the cytosol, for the toxic action of the pathogenic protein. but also to the nuclei and ER where the UPS For example, incubation of 26S proteasomes is also critical for quality control. Proteotoxi- with mutant huntingtin, tau, or a-synclein city in the nucleus can result in alterations in proteins involved in common neurodegenera- the genetic material and in major changes tive disorders, has been shown to exert an inhib- in transcriptional activity. In the case of the itory effect on this protease by directly clogging ER, secretory proteins that fail to fold in this the entrance of other substrates (Keck et al. compartment can be retrotranslocated into 2003; Landles and Bates 2004; Bennett et al. the cytosol where the UPS accounts for their 2005; Betarbet et al. 2005). In other instances, degradation. Because retrotranslocation and components of the UPS are down-regulated UPS degradation are highly coupled processes, or mutated in the affected cells. For exam- inhibition of the UPS blocks retrotranslocation ple, down-regulated expression of catalytic and and leads to the accumulation of the unfolded regulatory proteasome subunits has been des- proteins in the ER lumen. Although cells count cribed in different tissues of aging organism on exquisite mechanisms of defense against (Keller et al. 2002; Chondrogianni and Gonos ER stress, because of the critical role of this 2008). Likewise, genes mutated in familial organelle in protein synthesis, persistent ER forms of PD, include members of the UPS stress because of maintained compromise of such as the E3 ligase parkin and the ubiquitin the UPS has been shown to underlie the patho- carboxyl-terminal hydrolase (UCHL1) (Alves genesis of important protein conformational et al. 2008; Yang 2009). The functional conse- disorders. quences of these mutations on the UPS activity The consequences of UPS failure on cellular are currently under investigation. In addition, homeostasis have been widely analyzed through similar to any other cellular component, the the use of potent inhibitors of the proteolytic UPS can also be the target of undesirable post- activities of the 20S CP (Adams 2004). In fact, translational modifications that compromise its several of these inhibitors are currently used function when occurring in critical subunits as antioncogenic drugs, because of the pro- (Bulteau et al. 2001; Carrard et al. 2002). Lastly, nounced toxic effect that they exert in rapidly there are pathological conditions in which dividing cancer cells. One of the immediate malfunctioning of the UPS is not direct but consequences of proteasome inhibition is the rather a consequence of other cellular changes cytosolic accumulation of protein inclusions occurring in the disease. For example, condi- enriched in ubiquitin. These inclusions, which tions in which mitochondrial function is altered often also contain chaperones and compo- would lead to an energetic compromise because nents of the 26S proteasome, resemble those of the reduced production of ATP, and this in described in the affected cells in protein con- turn could diminish the UPS activity because formational disorders, suggesting that com- of the high energetic requirements of this sys- promised proteasome activity could be behind tem (Hoglinger et al. 2003).
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Chemical Modulation of the UPS locations, guarantees that chemical targeting of these molecules may also allow some selectiv- As mentioned in the previous section, pharma- ity. Apart from enzyme targeting, polyubiquitin cological inhibition of the proteasome has been chain recognition has also been revealed recently successfully attained and drugs such as Bortezo- as a suitable chemical target. For example ubi- mib, a tripeptide that binds the catalytic site of statin specifically binds interfaces between the proteasome, are already approved for clin- K48-linked ubiquitin molecules changing the ical use as anticancer treatments. In contrast, conformation of the ubiquitin chain to prevent interventions aimed at up-regulating UPS ac- recognition by shuttle receptors and the protea- tivity, an outcome desirable in disorders with some. Although so far most of the molecules reduced activity of this system, have been less have been used to prevent UPS degradation, it successful. To date, up-regulation of the UPS is conceivable that similar strategies could be in vivo has been performed in most cases used to enhance the affinity of ubiquitin recog- through genetic up-regulation of different com- nizing molecules for ubiquitin and favor their ponents of this system. In fact, up-regulation of targeting to this proteolytic system. critical single subunits of the proteasome seems enough to increase the proteasome content and CONCLUDING REMARKS activity. Similar to autophagy, these manipu- lations have been shown effective in reducing Sound evidence supports a critical role for the intracellular protein inclusions and cellular tox- two main cellular proteolytic systems in quality icity. For example overexpression of a regulatory control and in maintenance of cellular homeo- subunit of the 19S in cellular models of HD is stasis. These systems function in a coordinated sufficient to protect against neurodegeneration manner with the cellular chaperones, but recent (Seo et al. 2007). Even in whole organisms, studies also support the existence of a cross-talk overexpression of specific factors like Rpn11 between autophagy and the UPS. As described and CHIP in worms and flies leads to improved in this article, ubiquitin, a tag once thought cellular homeostasis and prolonged lifespan exclusive to proteasome degradation, is also (Oh et al. 2006; Min et al. 2008; Tonoki et al. used for cargo selection by some forms of au- 2009). The beneficial results of these manipu- tophagy. Some of the ubiquitin recognizing lations justify the current search for chemical molecules or shuttle factors, are shared by both modulators able to up-regulate proteasome proteolytic systems (e.g., p62 or ubiquilin can activity. deliver ubiquitinated substrates to both to the Modulation of the UPS is not reduced to proteasome and macroautophagy). The chal- mere chemical targeting of the catalytic core lenge is now to decipher the code that de- but could be exerted in other steps of this com- termines targeting through one system or the plex process. In this respect, some of the events other. Cross-talk between these two systems that lead to degradation of proteins through may also exist at other levels. Cells respond to the UPS may be more amenable to manipula- blockage of the UPS by up-regulating macroau- tion than others. Forexample, enzyme-catalyzed tophagy, whereas persistent blockage of macro- steps are attractive drug targets, in particular autophagy has been shown to compromise UPS now that the structural properties of many of activity. Discovering these many layers of inter- these enzymes are well known. When consider- action between autophagy and the UPS could ing the enzymes involved in ubiquitination, lead to the identification of new targets for ther- chemical targeting of specific E3 enzymes may apeutic interventions. offer more selectivity than targeting of E1s, Malfunctioning of the proteolytic systems which will affect the total pool of ubiquitinated has been now shown in numerous protein con- proteins. Likewise, the large number of cellular formational disorders. Failure of the clearance DUBs, each of them with preference for a partic- mechanisms, once considered a consequence ular type of linkage or active in specific cellular of the disease, has been shown to underlie the
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E. Wong and A.M. Cuervo
pathogenesis of some of these disorders. These neurodegeneration and Lewy-like inclusions resembling findings have boosted the interest in developing human pale bodies. J Neurosci 28: 8189–8198. Ben-Saadon R, Zaaroor D, Ziv T, Ciechanover A. 2006. The pharmacological interventions to up-regulate polycomb protein Ring1B generates self atypical mixed the activity of the proteolytic systems. In light ubiquitin chains required for its in vitro histone H2A of the promising results observed on genetic ligase activity. Mol cell 8: 700–710. manipulation of the proteolytic systems in ani- Bennett E, Bence N, Jayakumar R, Kopito R. 2005. Global impairment of the ubiquitin-proteasome system by mal models of different conformational dis- nuclear or cytoplasmic protein aggregates precedes inclu- orders, it is predicted that compounds able to sion body formation. Mol Cell 17: 351–365. up-regulate the activity of the proteolytic sys- Betarbet R, Sherer TB, Greenamyre JT. 2005. Ubiquitin- tems could become an efficient future treatment proteasome system and Parkinson’s diseases. Exp Neurol 191: S17–27. for these devastating disorders. Bulteau AL, Lundberg KC, Humphries KM, Sadek HA, Szweda PA, Friguet B, Szweda LI. 2001. Oxidative modification and inactivation of the proteasome during ACKNOWLEDGMENTS coronary occlusion/reperfusion. J Biol Chem 276: 30057–30063. Work in our laboratory is supported by Carrard G, Bulteau AL, Petropoulos I, Friguet B. 2002. National Institutes of Health (NIH) grants Impairment of proteasome structure and function in from NIA (AG021904, AG031782), NIDKK aging. Int J Biochem Cell Biol 34: 1461–1474. (DK041918), NINDS (NS038370), a Glenn Chastagner P, Israel A, Brou C. 2008. AIP/Itch regulates Notch receptor degradation in the absence of ligand. Foundation Award, and a Hirsch/Weill-Caulier PLoS One 3: e2735. Career Scientist Award. Chondrogianni N, Gonos ES. 2008. Proteasome activation as a novel antiaging strategy. 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E. Wong and A.M. Cuervo
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Integration of Clearance Mechanisms: The Proteasome and Autophagy
Esther Wong and Ana Maria Cuervo
Cold Spring Harb Perspect Biol 2010; doi: 10.1101/cshperspect.a006734 originally published online November 10, 2010
Subject Collection Protein Homeostasis
Proteome-Scale Mapping of Perturbed The Amyloid Phenomenon and Its Significance in Proteostasis in Living Cells Biology and Medicine Isabel Lam, Erinc Hallacli and Vikram Khurana Christopher M. Dobson, Tuomas P.J. Knowles and Michele Vendruscolo Pharmacologic Approaches for Adapting A Chemical Biology Approach to the Chaperome Proteostasis in the Secretory Pathway to in Cancer−−HSP90 and Beyond Ameliorate Protein Conformational Diseases Tony Taldone, Tai Wang, Anna Rodina, et al. Jeffery W. Kelly Cell-Nonautonomous Regulation of Proteostasis Proteostasis in Viral Infection: Unfolding the in Aging and Disease Complex Virus−Chaperone Interplay Richard I. Morimoto Ranen Aviner and Judith Frydman The Autophagy Lysosomal Pathway and The Proteasome and Its Network: Engineering for Neurodegeneration Adaptability Steven Finkbeiner Daniel Finley and Miguel A. Prado Functional Modules of the Proteostasis Network Functional Amyloids Gopal G. Jayaraj, Mark S. Hipp and F. Ulrich Hartl Daniel Otzen and Roland Riek Protein Solubility Predictions Using the CamSol Chaperone Interactions at the Ribosome Method in the Study of Protein Homeostasis Elke Deuerling, Martin Gamerdinger and Stefan G. Pietro Sormanni and Michele Vendruscolo Kreft Recognition and Degradation of Mislocalized Mechanisms of Small Heat Shock Proteins Proteins in Health and Disease Maria K. Janowska, Hannah E.R. Baughman, Ramanujan S. Hegde and Eszter Zavodszky Christopher N. Woods, et al. The Nuclear and DNA-Associated Molecular Structure, Function, and Regulation of the Hsp90 Chaperone Network Machinery Zlata Gvozdenov, Janhavi Kolhe and Brian C. Maximilian M. Biebl and Johannes Buchner Freeman
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