See online version for legend and references.

SnapShot: Endosome-to-Golgi RetrievalMatthew N.J. SeamanCambridge Institute for Medical Research, Cambridge, CB2 0XY, UK

See online version for legend and references.1198 Cell 139, December 11, 2009 ©2009 Elsevier Inc. DOI 10.1016/j.cell.2009.11.040

SnapShot: Endosome-to-Golgi RetrievalMatthew N.J. SeamanCambridge Institute for Medical Research, Cambridge, CB2 0XY, UK

1198.e1 Cell 139, December 11, 2009 ©2009 Elsevier Inc. DOI 10.1016/j.cell.2009.11.040

The endosome-to-Golgi retrieval pathway regulates the localization of many important membrane proteins, is highly conserved in eukaryotes, and represents a key point of intersection of the secretory and endocytic pathways. Endosome-to-Golgi transport plays a role in diverse processes such as lysosome biogenesis, transcytosis, Wnt signaling, and amyloid precursor protein localization and processing. The best characterized “cargo” in the endosome-to-Golgi pathway in mammals is the cation-independent mannose 6-phosphate receptor (CIMPR) that mediates sorting of lysosomal hydrolases into clathrin-coated vesicles at the Golgi/trans-Golgi network (TGN). Endosome-to-Golgi retrieval essentially comprises three discrete steps involving (1) cargo selection, (2) tubule formation and transport, and (3) tethering/docking at the Golgi/TGN; the picture is complicated, however, by evidence of at least two endosome-to-Golgi retrieval pathways that may function in series or in parallel. The three steps are shown in the figure along with some of the key machinery required for each step.

Studies in yeast (Saccharomyces cerevisiae) and mammalian cells have identified much of the core machinery that mediates endosome-to-Golgi retrieval and have revealed mechanistic similarities with other membrane trafficking pathways. In yeast, five proteins, Vps5p, Vps17p, Vps26p, Vps29p, and Vps35p, assemble into the retromer complex and localize to endosomal membranes to mediate endosome-to-Golgi retrieval of the vacuolar hydrolase receptor, Vps10p. Retromer can be biochemically dissected into two subcomplexes: the cargo-selective Vps35p/Vps29p/Vps26p subcomplex and the sorting nexin (Snx) subcomplex comprising Vps5p/Vps17p in yeast. In mammals, the Vps35, Vps29, and Vps26 proteins form a stable trimer that binds cargo and dynamically associates with sorting nexins, Snx1 and Snx2 (along with Snx5 and Snx6). The Snx proteins contain phosphotidylinositol 3-phosphate (PtdIns 3P)-binding PX domains that are necessary for membrane recruitment, demonstrating a key role for PtdIns 3-kinase activity in regulating retromer activity. Recruitment of the cargo-selective retromer subcomplex (Vps35/29/26) is catalyzed by the small GTPase Rab7. The retromer-interacting protein, TBC1D5, can displace the Vps35/29/26 complex from the membrane and may function as a GTPase-stimulating protein (GAP) for Rab7.

The recognition of cargo proteins by retromer involves hydrophobic motifs such as the Trp-Leu-Met (WLM) motif that is conserved in the cytoplasmic tail of CIMPR proteins and that has been shown to be required for endosome-to-Golgi retrieval of a reporter protein carrying the CIMPR tail. Sorting of cargo into endosome tubules occurs through the concerted action of the Vps35/29/26 complex with the Snx proteins. The Snx proteins drive tubule formation via their Bin/Amphiphysin/Rvs (BAR) domains and can link the membrane to the cytoskeleton by interacting with the p150 glued protein.

In addition to the retromer complex, the clathrin adaptor AP-1 has been implicated in retrieval from endosomes to the Golgi. AP-1 interacts with an accessory protein EpsinR that is required for the proper localization of the SNARE protein Vti1b. The role of AP-1 in endosome-to-Golgi retrieval is challenging to delineate due to the requirement for AP-1 in sorting at, and traffic from, the TGN.

Arrival at the Golgi/TGN is mediated by tethering factors and the small GTPase Rab9 that is recruited to endosomes prior to acting in the docking/tethering of endosome tubules at the Golgi. Another small GTPase, Rab6, also acts at the Golgi in the endosome-to-Golgi pathway. The tethering factor GCC185, a protein rich in predicted coiled-coil structure, interacts with both Rab9 and Rab6 and therefore is a key mediator of delivery to the Golgi. Additionally, the GARP complex comprising Vps51, Vps52, Vps53, and Vps54 proteins functions in both yeast and mammalian cells in the docking of endosomal transport intermediates. The apparent duplication of docking/tethering and SNARE machinery at the Golgi mirrors the evidence that retrieval from the endosome occurs by two pathways that can function independently of each other.

Acknowledgments

M.N.J.S. would like to thank P. Cullen (University of Bristol) for providing the GFP-Snx1 movie and D. Owen (CIMR, University of Cambridge) for the animations of VPS29, VPS26B, and the epsinR-Vti1b structures. M.N.J.S. is supported by funding from the MRC.

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