Retromer is a heteropentameric complex which in humans is composed of a less defined membrane-associated sorting nexin dimer (SNX1, SNX2, SNX5, SNX6), and a vacuolar protein sorting (Vps) trimer containing Vps26, Vps29, Vps35. Although the SNX dimer is required for the recruitment of retromer to the endosomal membrane, the cargo binding function of this complex is contributed by the core trimer through the binding of Vps35 subunit to various cargo molecules including M6PR wntless and sortilin. Early study on sorting of acid hydrolases such as carboxypeptidase Y (CPY) in S. cerevisiae mutants has led to the identification of retromer in mediating the retrograde trafficking of the pro-CPY receptor (Vps10) from the endosomes to the TGN.
The retromer complex is highly conserved: homologs have been found in C. elegans, mouse and human. The retromer complex consists of 5 proteins in yeast: Vps35p, Vps26p, Vps29p, Vps17p, Vps5p. The mammalian retromer consists of Vps26, Vps29, Vps35, SNX1 and SNX2, and possibly SNX5 and SNX6. It is proposed to act in two subcomplexes: (1) Cargo recognition complex that consist of Vps35, Vps29 and Vps26 (Vps trimer), and (2) SNX-BAR dimers which consist of SNX1 or SNX2 and SNX5 or SNX6 that facilitates endosomal membrane remodulation and curvature resulting in the formation of tubules/vesicles which transports cargo molecules to the trans-golgi network (TGN).
The retromer complex has been shown to mediate retrieval of various transmembrane receptors, such as the cation-independent mannose 6-phosphate receptor, the functional mammalian counterpart of Vps10, and the Wnt receptor Wntless. Retromer is required for the recycling of Kex2p and DPAP-A which also cycle between the trans-Golgi network and a pre-vacuolar (yeast endosome equivalent) compartment in yeast. It is also required for the recycling of the cell surface receptor CED-1, which is necessary for phagocytosis of apoptotic cells.
Retromer plays a central role in the retrieval of several different cargo proteins from the endosome to the trans-Golgi network. However, it is clear that there are other complexes and proteins that act in this retrieval process. So far it is not clear whether other components that have been identified in the retrieval pathway act with retromer in the same pathway or are involved in alternative pathways. Recent studies have implicated retromer sorting defects in Alzheimer's disease and late-onset Parkinson disease 
Retromer-mediated retrograde trafficking
The association of the Vps35-Vps29-Vps26 complex to the cytosolic domains of cargo molecules endosomal membranes initiates the activation of retrograde trafficking and cargo capture. The nucleation complex was formed through the interaction of VPS complex with GTP-activated Rab7 with clathrin, clathrin-adaptors and various binding proteins.
The SNX-BAR dimer enters the nucleation complex via direct binding or lateral movement on endosomal surface. The increased level of Retromer SNX-BARs causes a conformational switch to a curvature-inducing mode which initiates membrane tubule formation. Once the cargo carriers are matured, the carrier scission is then catalyzed by dynamin-II or EHD1, together with the mechanical forces generated by actin polymerization and motor activity.
The cargo carrier is transported to the TGN by motor proteins such as dynein. Tethering of the cargo carrier to the recipient compartment will lead to the uncoating of the carrier which is driven by ATP-hydrolysis and Rab7-GTP hydrolysis. Once released from the carrier, the Vps35-Vps29-Vps26 complex and the SNX-BAR dimers get recycled back onto the endosomal membranes.
- Seaman MN (Feb 2005). "Recycle your receptors with retromer". Trends in Cell Biology 15 (2): 68–75. doi:10.1016/j.tcb.2004.12.004. PMID 15695093.
- Pfeffer SR (Feb 2001). "Membrane transport: retromer to the rescue". Current Biology 11 (3): R109–11. doi:10.1016/S0960-9822(01)00042-2. PMID 11231171.
- Seaman MN (Apr 2004). "Cargo-selective endosomal sorting for retrieval to the Golgi requires retromer". The Journal of Cell Biology 165 (1): 111–22. doi:10.1083/jcb.200312034. PMC 2172078. PMID 15078902.
- Arighi CN, Hartnell LM, Aguilar RC, Haft CR, Bonifacino JS (Apr 2004). "Role of the mammalian retromer in sorting of the cation-independent mannose 6-phosphate receptor". The Journal of Cell Biology 165 (1): 123–33. doi:10.1083/jcb.200312055. PMC 2172094. PMID 15078903.
- Belenkaya TY, Wu Y, Tang X, Zhou B, Cheng L, Sharma YV, Yan D, Selva EM, Lin X (Jan 2008). "The retromer complex influences Wnt secretion by recycling wntless from endosomes to the trans-Golgi network". Developmental Cell 14 (1): 120–31. doi:10.1016/j.devcel.2007.12.003. PMID 18160348.
- Canuel M, Korkidakis A, Konnyu K, Morales CR (Aug 2008). "Sortilin mediates the lysosomal targeting of cathepsins D and H". Biochemical and Biophysical Research Communications 373 (2): 292–7. doi:10.1016/j.bbrc.2008.06.021. PMID 18559255.
- Seaman MN, McCaffery JM, Emr SD (Aug 1998). "A membrane coat complex essential for endosome-to-Golgi retrograde transport in yeast". The Journal of Cell Biology 142 (3): 665–81. doi:10.1083/jcb.142.3.665. PMC 2148169. PMID 9700157.
- Wassmer T, Attar N, Bujny MV, Oakley J, Traer CJ, Cullen PJ (Jan 2007). "A loss-of-function screen reveals SNX5 and SNX6 as potential components of the mammalian retromer". Journal of Cell Science 120 (Pt 1): 45–54. doi:10.1242/jcs.03302. PMID 17148574.
- Eaton S (Jan 2008). "Retromer retrieves wntless". Developmental Cell 14 (1): 4–6. doi:10.1016/j.devcel.2007.12.014. PMID 18194646.
- Chen D, Xiao H, Zhang K, Wang B, Gao Z, Jian Y, Qi X, Sun J, Miao L, Yang C (Mar 2010). "Retromer is required for apoptotic cell clearance by phagocytic receptor recycling". Science 327 (5970): 1261–4. doi:10.1126/science.1184840. PMID 20133524.
- Sadigh-Eteghad S, Askari-Nejad MS, Mahmoudi J, Majdi A (Oct 2015). "Cargo trafficking in Alzheimer's disease: the possible role of retromer". Neurological Sciences. doi:10.1007/s10072-015-2399-3. PMID 26482054.
- Muhammad A, Flores I, Zhang H, Yu R, Staniszewski A, Planel E, Herman M, Ho L, Kreber R, Honig LS, Ganetzky B, Duff K, Arancio O, Small SA (May 2008). "Retromer deficiency observed in Alzheimer's disease causes hippocampal dysfunction, neurodegeneration, and Abeta accumulation". Proceedings of the National Academy of Sciences of the United States of America 105 (20): 7327–32. doi:10.1073/pnas.0802545105. PMC 2386077. PMID 18480253.
- Zimprich A, Benet-Pagès A, Struhal W, Graf E, Eck SH, Offman MN, Haubenberger D, Spielberger S, Schulte EC, Lichtner P, Rossle SC, Klopp N, Wolf E, Seppi K, Pirker W, Presslauer S, Mollenhauer B, Katzenschlager R, Foki T, Hotzy C, Reinthaler E, Harutyunyan A, Kralovics R, Peters A, Zimprich F, Brücke T, Poewe W, Auff E, Trenkwalder C, Rost B, Ransmayr G, Winkelmann J, Meitinger T, Strom TM (Jul 2011). "A mutation in VPS35, encoding a subunit of the retromer complex, causes late-onset Parkinson disease". American Journal of Human Genetics 89 (1): 168–75. doi:10.1016/j.ajhg.2011.06.008. PMC 3135812. PMID 21763483.
- Yin P, Hong Z, Yang X, Chung RT, Zhang L (Aug 2015). "A role for retromer in hepatitis C virus replication". Cellular and Molecular Life Sciences. doi:10.1007/s00018-015-2027-7. PMID 26298293.
- Nothwehr SF, Ha SA, Bruinsma P (Oct 2000). "Sorting of yeast membrane proteins into an endosome-to-Golgi pathway involves direct interaction of their cytosolic domains with Vps35p". The Journal of Cell Biology 151 (2): 297–310. doi:10.1083/jcb.151.2.297. PMC 2192648. PMID 11038177.
- Rojas R, van Vlijmen T, Mardones GA, Prabhu Y, Rojas AL, Mohammed S, Heck AJ, Raposo G, van der Sluijs P, Bonifacino JS (Nov 2008). "Regulation of retromer recruitment to endosomes by sequential action of Rab5 and Rab7". The Journal of Cell Biology 183 (3): 513–26. doi:10.1083/jcb.200804048. PMC 2575791. PMID 18981234.
- McGough IJ, Cullen PJ (Aug 2011). "Recent advances in retromer biology". Traffic 12 (8): 963–71. doi:10.1111/j.1600-0854.2011.01201.x. PMID 21463457.
- Shimada A, Niwa H, Tsujita K, Suetsugu S, Nitta K, Hanawa-Suetsugu K, Akasaka R, Nishino Y, Toyama M, Chen L, Liu ZJ, Wang BC, Yamamoto M, Terada T, Miyazawa A, Tanaka A, Sugano S, Shirouzu M, Nagayama K, Takenawa T, Yokoyama S (May 2007). "Curved EFC/F-BAR-domain dimers are joined end to end into a filament for membrane invagination in endocytosis". Cell 129 (4): 761–72. doi:10.1016/j.cell.2007.03.040. PMID 17512409.
- Bhatia VK, Madsen KL, Bolinger PY, Kunding A, Hedegård P, Gether U, Stamou D (Nov 2009). "Amphipathic motifs in BAR domains are essential for membrane curvature sensing". The EMBO Journal 28 (21): 3303–14. doi:10.1038/emboj.2009.261. PMC 2776096. PMID 19816406.
- Walseng E, Bakke O, Roche PA (May 2008). "Major histocompatibility complex class II-peptide complexes internalize using a clathrin- and dynamin-independent endocytosis pathway". The Journal of Biological Chemistry 283 (21): 14717–27. doi:10.1074/jbc.M801070200. PMC 2386912. PMID 18378669.