Downloaded from http://cshperspectives.cshlp.org/ on September 24, 2021 - Published by Cold Spring Harbor Laboratory Press Retrograde Traffic from the Golgi to the Endoplasmic Reticulum Anne Spang University of Basel, Biozentrum, Growth & Development, Klingelbergstrasse 70, 4056 Basel, Switzerland Correspondence: [email protected] Proteins to be secreted are transported from the endoplasmic reticulum (ER) to the Golgi apparatus. The transport of these proteins requires the localization and activityof proteins that create ER exit sites, coat proteins to collect cargo and to reshape the membrane into a transport container, and address labels—SNARE proteins—to target the vesicles specifically to the Golgi apparatus. In addition some proteins may need export chaperones or export receptors to enable their exit into transport vesicles. ER export factors, SNAREs, and mis- folded Golgi-resident proteins must all be retrieved from the Golgi to the ER again. This retrieval is also part of the organellar homeostasis pathway essential to maintaining the identity of the ER and of the Golgi apparatus. In this review, I will discuss the different processes in retrograde transport from the Golgi to the ER and highlight the mechanistic insights we have obtained in the last couple of years. roteins that are exposed at the plasma mem- It is assumed that the vesicle coat is at least Pbrane or populate a membrane-bounded or- partially destabilized through the hydrolysis of ganelle are synthesized into the endoplasmic GTP by the small GTPase Sar1 (Oka and Nakano reticulum (ER). In the ER, the folding of these 1994; Springeret al. 1999). However, some of the proteins takes place and posttranslational mod- destabilized coat components have to stayon the ifications such as N-glycosylation and disulfide vesicle until it has reached the Golgi apparatus bridge formation occur. Upon adopting a suit- because coat components participate in the rec- able, often correct, conformation, proteins des- ognition and the tethering process (Barlowe tined to locations beyond the ER are concen- 1997; Cai et al. 2007; Lord et al. 2011; Zong trated at so-called ER exit sites (ERES) and et al. 2012). Subsequently, SNARE proteins on incorporated into nascent COPII-coated vesi- the vesicles (v-SNAREs) zipper up with cognate cles. These COPII vesicles eventually bud off SNAREs on the Golgi (target SNAREs, t- the ER membrane and are transported to the SNAREs) to drive membrane fusion (Hay et al. Golgi (in yeast, Drosophila, and C. elegans)or 1998; Cao and Barlowe 2000; Parlati et al. 2002). the ER-Golgi intermediate compartment (in The content of the ER-derived COPII vesicles mammalian cells) (Schweizer et al. 1990; Kon- is thereby released into the lumen of the cis- dylis and Rabouille 2003; Spang 2009; Witte cisterna of the Golgi apparatus. Most proteins et al. 2011). will continue their journey through the Golgi Editors: Susan Ferro-Novick, Tom A. Rapoport, and Randy Schekman Additional Perspectives on The Endoplasmic Reticulum available at www.cshperspectives.org Copyright # 2013 Cold Spring Harbor Laboratory Press; all rights reserved; doi: 10.1101/cshperspect.a013391 Cite this article as Cold Spring Harb Perspect Biol 2013;5:a013391 1 Downloaded from http://cshperspectives.cshlp.org/ on September 24, 2021 - Published by Cold Spring Harbor Laboratory Press A. Spang apparatus and encounter further modifications coat is recruited with the ability to deform the such as extension of the glycosylation tree or membrane of the donor compartment. This lipidation. However, some proteins, especially outer coat layer also plays an important role in those involved in the fusion process, i.e., the v- determining the size of the transport vesicle. SNAREs or proteins that act as export factors of The binding of the second layer/outer shell the ER, such as Vma21, which is essential for drives polymerization of the coat, which leads export of the correctly folded and assembled to the deformation of the membrane. Finally,the V0 sector of the V-ATPase, need to be recycled transport vesicle is formed and fission from the back to the ER for another round of transport donor compartment needs to occur. In the case (Ballensiefen et al. 1998; Malkus et al. 2004). of clathrin-coated vesicles dynamin, a GTPase Moreover, cis-Golgi proteins are returned to with the ability to polymerize and form a tight the ER for quality/functional control (Todorow constriction, binds to the base of the vesicle and et al. 2000; Sato et al. 2004; Valkova et al. 2011). drives scission (van der Bliek et al. 1993; Hin- Finally, some ER-resident proteins, such as the shaw and Schmid 1995; Takei et al. 1995). In all ER Hsp70 chaperone BiP/Kar2, can escape the other vesicle budding scenarios, the mechanistic ER, but are captured at the cis-Golgi by the H/ basis for severing the vesicle from the donor KDEL receptor Erd2 and returned to the ER membrane is much less clear, although a direct (Lewis et al. 1990; Semenza et al. 1990; Aoe role of the Arf1/Sar1 GTPases has been pro- et al. 1997). posed because each of them can oligomerize Unfortunately, the retrograde transport and cause tubulation of liposomes invitro (Bielli route is also hijacked by toxins. For example, et al. 2005; Lee et al. 2005a; Beck et al. 2008, endocytosed cholera toxin subunit A contains 2011). a KDEL sequence and can thereby exploit the The players at the cis-Golgi involved in COPI system to access the ER (Majoul et al. 1996, vesicle formation follow the general scheme out- 1998). From there, it is retro-translocated into lined above, beginning with the GEF at the cis- the cytoplasm where it can exert its detrimental Golgi, GBF1 (Gea1/2 in yeast) (Peyroche et al. function. 1996, 2001; Spang et al. 2001; Zhao et al. 2006). How the GEF is itself recruited is not entirely clear. One determinant appears to be the phos- VESICLE FORMATION AT THE GOLGI phoinositide PI4P (Dumaresq-Doiron et al. APPARATUS 2010). However, lipid binding cannot account for the specific recruitment of the GEF to the cis COPI Vesicles face of the Golgi. How the temporal and spatial Most transport vesicle formation appears to fol- control of GBF1 Golgi localization is achieved low an evolutionarily and structurally conserved would be an important piece of information, as scheme. First, a small GTPase of the Arf1/Sar1 the localization of the GEF determines where family is recruited to the membrane by the ac- the small GTPase Arf1 is activated. Upon acti- tion of a guanine nucleotide exchange factor vation, Arf1 is recruited to the Golgi, where it (GEF). Then a layer of the coat binds to the initiates the binding of the heptameric coat activated small GTPase and recruits cargo at complex, coatomer. This complex can be divid- the same time. If there is enough cargo avail- ed up into two layers: a tetrameric subcomplex able, the GTPase.coat.cargo complex is stabi- resembling adaptor complexes of the clathrin lized. Interestingly, this first inner layer of the coat and a cage-like trimeric subcomplex possi- coat also contains a GTPase activating protein bly containing the membrane-deforming abili- (GAP), which thus becomes an intrinsic part of ty of the complex (Yu et al. 2012). Two Arf1 the coat complex. However, the arrangement in proteins interact with the tetrameric subcom- this prebudding complex is such that the GAP plex to initiate membrane recruitment of coat- cannot efficiently stimulate the GTPase activity omer (Yu et al. 2012). In the case of COPI ves- of the small GTPase. Next, a second layer of the icles, the first and second layers of the coat are 2 Cite this article as Cold Spring Harb Perspect Biol 2013;5:a013391 Downloaded from http://cshperspectives.cshlp.org/ on September 24, 2021 - Published by Cold Spring Harbor Laboratory Press Retrograde Traffic from the Golgi to the ER thought to be recruited simultaneously. This is liberate the nascent vesicle from the donor because the coatomer complex exists as a pre- membrane, and hence act early in vesicle bio- assembled heptameric complex in the cyto- genesis. In contrast, Glo3 (ArfGAP2/3) would plasm (Waters et al. 1991; Hosobuchi et al. stabilize the coat on the nascent vesicle and fi- 1992; Robinson and Kreis 1992; Hara-Kuge nally would initiate uncoating by stimulating et al. 1994). Furthermore, the purified hepta- GTP hydrolysis on Arf1; this latter function be- meric complex, together with activated Arf1 ing probably required late in the vesicle’s life- and guanine nucleotides, is sufficient to gener- time. ate COPI-coated vesicles from synthetic lipo- Before it can fuse with a target compart- somes (Spang et al. 1998). However, different ment, a vesicle’s coat must be at least partially types of the coatomer complexes exist, one of disassembled or destabilized. This led us to ask which appears to act at the level of endosomes when uncoating would occur. Multiple scenar- (Whitney et al. 1995; Aniento et al. 1996; Gu ios have been discussed for COPI and COPII et al. 1997). Here, we will concentrate on the vesicles. It is assumed, as discussed above, that coatomer complex at the cis-Golgi. generation and consumption of COPI and The coatomer complex by itself is unable COPII vesicles follow a very similar mechanism. to stimulate GTP hydrolysis by Arf1; ArfGAPs Therefore, I will use the following findings from have to be included and are intrinsic parts of the either COPI or COPII vesicles to illustrate our coat. Although in yeast two different ArfGAPs, current understanding of uncoating. Initially, it Gcs1 and Glo3, have partially overlapping func- was believed that uncoating occurs just before tions in COPI vesicle formation and can substi- the fusion event; the coat might therefore play tute for each other (Poon et al.