Central JSM Cell & Developmental Biology Bringing Excellence in Open Access Review Article *Corresponding author Erik RP Zuiderweg, Department of Biological Chemistry, The University of Michigan Medical School, 1500 The Amazing Multi-Valency of Medical Center Drive, Ann Arbor, MI 48109, USA, Tel: 734-276-4463; Email: Submitted: 04 November 2016 the Hsp70 Chaperones Accepted: 20 November 2016 Erik RP Zuiderweg1* and Jason E. Gestwicki2 Published: 22 November 2016 1Department of Biological Chemistry, The University of Michigan Medical School, USA ISSN: 2379-061X 2Institute for Neurodegenerative Disease, University of California at San Francisco, Copyright USA © 2016 Zuiderweg et al. OPEN ACCESS Abstract Hsp70 proteins are keys to maintaining intra-cellular protein homeostasis. To carry Keywords out this task, they employ a large number of co-chaperones and adapter proteins. Here • Hsp70 proteins we review what is known about the interaction between the chaperones and partners, • Protein chaperones with a strong slant towards structural biology. Hsp70s in general and Hsc70 (HSPA8) in particular, display an amazing array of interfaces with their protein co-factors. We also reviewed the known interactions between Hsp70s and active compounds that may become leads towards Hsp70 modulation for treatment of a variety of diseases. INTRODUCTION misfolded proteins to favor protein (re)folding cycles [6]; (ii) transporting unfolded proteins through membranes to enable Hsp70 chaperones are highly conserved in all kingdoms; delivery of cargo to organelles [7]; (iii) recruiting proteins to in animals, they are an important member of the collection of the proteasome for turnover [8] and (iv) bringing proteins to protein chaperones including Hsp60, Hsp70, Hsp90 and small Hsps [1]. The archetypical Hsp70 is called DnaK in bacteria and [9]. These diverse functions are achieved by Hsc70 interactions withthe endosome/lysosomedifferent molecular partners:for chaperone-mediated J proteins, NEFs andautophagy Hsp90 acting together with a nucleotide exchange factor (NEF), termed it functions in protein trafficking and protein refolding cycles, for function (i); membrane transporting systems for role (ii), GrpE, and a J-protein, called DnaJ. In the presence of ATP, this interaction with ubiquitin ligases and NEFs for role (iii) and trio is capable of refolding denatured luciferase in vitro, and can the prokaryotic system has a single Hsp70, NEF and J protein, interactions with phosphoserine lipids and LAMP-2A for role thus be classified as a protein refolding machinery [2]. While co-chaperones and other partners are the keys to understanding the number of genes has expanded in eukaryotes. For example, function.(iv). Even fromAnother this lessonsimplified is overviewthat each of ofHsp70, the itcategories is clear that of chaperone function involves multiple components. the yeast genome codes for 14 Hsp70s, 20 J-proteins [3] and 3 Because of these broad functions, theHsp70s are increasingly NEFNEFs trio[4]. Inseems humans, to therebe maintained are 13 Hsp70s, throughout 50 J-proteins the kingdoms, and eight being seen as drug targets across a range of diseases [10]. For butNEFs eukaryotes [5]. The original require protein more refoldingisoforms functionto enable of the the systemHsp70/J/ to example, HSPA1 expression is induced in cancer cells, where it have representatives in each organelle (e.g. ER, mitochondria, inhibits apoptosis. The levels of HSPA1 are even further elevated in response to chemotherapy, which may partially limit the chaperones seems to have allowed the system to adapt to a wider effectiveness of Hsp90 inhibitor treatments. In another example, cytoplasm/nucleus). Moreover, the expansion of the co- HSPA8 enhances the lifetime of aberrant tau protein in neurons, Finally, higher eukaryotes have also added new partners for the likely contributing to neurodegenerative disease. Hence, in Hsp70s,range of includingspecialized HOP, functions, HIP, CHIP such and as other trafficking proteins, and that signaling. couple it to additional cellular functionality, such as proteosomal and functional cycles, there are translational implications in better autophagosomal degradation system and to the Hsp90 protein understandingaddition to the basicthese scientificmechanisms. interest How in delineatingmight we selectivelythe Hsp70 folding machinery. A key to understanding Hsp70 biology, disrupt some disease-associated functions of Hsp70 and not especially in eukaryotes, is to understand how it coordinates other, housekeeping activities? Several synthetic modulators of with an expanded number of co-chaperones to enable its broad Hsp70 have recently emerged that have begun to answer this functions. question. HSPA8 (Hsc70; constitutive) and HSPA1 (Hsp70-1; inducible) In this review, we will focus on the structural and biophysical are the two major human Hsp70s of the cytoplasm and nucleus. aspects of how Hsp70s bind to their different partners, as Broadly speaking, these proteins are involved in four general activities in the cell: (i) binding and release of native and spectroscopy. Ideally, one would like to focus on the Hsp70s of revealed by X-ray crystallography and multi-dimensional NMR Cite this article: Zuiderweg ERP, Gestwicki JE (2016) The Amazing Multi-Valency of the Hsp70 Chaperones. JSM Cell Dev Biol 4(1): 1019. Zuiderweg et al. (2016) Email: Central Bringing Excellence in Open Access one species, and preferentially humans. However, much of the Regardless of the exact identity of theJ-protein orNEF, each early structural biology was focused on the E.coli partner is thought to regulate the human Hsp70protein refolding that the level of homology between the different system Hsp70 ofproteins DnaK/ [18]. At the end of the cycle,HSPA1 or HSPA8 can transfer the GrpE/DnaJ. Herein we want to cast a wide net, so we will assume substratemachinery to in Hsp90, analogous either fashion directly as or thein a complexDnaK/DnaJ/GrpE with HOP cycle(Hsp can be safely extrapolated between orthologs and paralogs. For organizing protein) [18]. HOP contains two tetratricopeptide and co-factors is sufficiently high so that structural knowledge In this review, we will use the name Hsp70 if we do not want to motifs at the C-terminal tails of Hsp70 and Hsp90, respectively. example, Hsc70 is 80% homologous and 55% identical to DnaK. discriminate between homologues, and will use the systematic Unfoldedrepeats (termed Hsp70 substrateTPR1 and is TPR2A), also being which transferred bind to EEVD-COOHto TRIC, the mammalian GroEL homologue, for refolding [19]. namesTHE ATPASE as proposed CYCLE by [5] OFif we HSP70S do. The ATP hydrolysis cycle of DnaK is illustrated in Figure Why does the human Hsp70 system have so many co- (1). The Hsp70 chaperones consist of a Nucleotide Binding current ideas. The six BAG proteins contain an alpha-helical domainchaperones? of approximately We use the BAG100 aminofamily acidsof NEFs called to illustrateBAG domain, the the LID open, the exposed hydrophobic cleft in the SBD binds which conveys the NEF function. However, BAG proteins have Domain (NBD), Substrate Binding Domain (SBD) and a LID. With other functionalities as well. All of the BAG-family proteins function as adapter proteins forming complexes with different chaperoneto the exposed DnaJ helpshydrophobic to stabilize regions this interaction of misfolded by also/ unfolded binding signaling molecules and the Hsp70’s. For instance, BAG-1 has toproteins unfolded with areas a (weak) of the affinitysame substrate, of approximately and by stimulating, 10 µM. The with co- besides its BAG domain a TXEEX domain involved in DNA binding its 70-residue J-domain, ATP hydrolysis in the NBD. DnaK, now in and transcription activation and an Ubiquitin-like domain, the ADP-state, closes its LID and binds the substrate with a 100-to which can target BAG1-Hsp70 complexes to the proteasome for of the cycle or later, is currently unknown. A process of entropic BAG proteins and contains multiple protein-protein interaction pulling,1000-fold potentially tighter affinity. involving Whether several DnaJ dissociatesHsp70s, progressively at this stage degradation of specific client proteins.BAG3 is one of the largest unfolds the substrate[11], which remains bound. Subsequently, a NEF enables ADP ATP exchange, with a concomitant decrease motifs: a WW domain, multiple PXXP motifs, and two IPV motifs, allowing for PPxY protein binding, SH3 protein binding, and small heat shock protein binding respectively [20]. The non- substratein the affinity is now of released Hsp70 infor a folding-competentits substrate. Possibly form, aidedeither byto BAG domains of BAG3 have recently been shown to interact thedirect cytosol competition or to GroEL of the for NEF refolding. with the bound substrate[12] the with HSPA8 SBD. The BAG3-HSPA8 complex stabilizes several The human Hsp70 system conforms to this basic ATPase theoncogenes turnover and of polyubiquitinatedis a particularly interestingproteins by drugproteasomal target [21]. and cycle, although there are differences in complexity. As mentioned BAG1 and BAG3 represent key players of cellular by stimulating Hsp70s are highly homologous and consist of three constitutively whichautophagic no domain degradation annotation pathways, is known, respectively is a regulator [22]. preventing BAG-4, expressedabove, the humanproteins, genome Hsc70 codes(HSPA8) for 13in theHsp70 cytosol isoforms and nucleus,[5]. The containing a single BAG domain and another 350 residues for in