VPS26A (O75436) Curation Notes

Overview & Complex Architecture

• VPS26A (O75436, ~38 kDa, 327 aa) is the arrestin-fold cargo-recognition and membrane-docking subunit of the mammalian retromer cargo-selective complex (CSC) (VPS35:VPS29:VPS26A) (PMID:11102511; [PMID:16732284, "The structure of human Vps26A at 2.1-A resolution reveals two curved beta-sandwich domains connected by a polar core and a flexible linker. Vps26 has an unpredicted structural relationship to arrestins. The Vps35-binding site on Vps26 maps to a mobile loop spanning residues 235-246, near the tip of the C-terminal domain."]; PMID:17891154).
• Paralog Specialization (VPS26A vs VPS26B vs VPS26C):
• Vertebrates express two mutually exclusive retromer CSC paralogs, VPS26A and VPS26B (Q4G0F5), which compete for the same N-terminal PRLYL binding site on VPS35 to form distinct VPS35:VPS29:VPS26A and VPS35:VPS29:VPS26B retromer complexes ([PMID:37172566, "Retriever, a VPS26C:VPS35L:VPS29 trimer, shares distant homology to Retromer, itself a trimer of VPS29, VPS35, and either VPS26A or VPS26B (paralogues with VPS26C)."]).
• A third distant arrestin-fold paralog, VPS26C (DSCR3), does not bind VPS35 and instead pairs with VPS35L (C16orf62) and VPS29 to form the Retriever complex (PMID:28892079).

Core Biological Functions

1. Structural Assembly of the Retromer Cargo-Selective Complex & Membrane Docking:
2. VPS26A binds the N-terminal tip of the VPS35 alpha-solenoid via its C-terminal arrestin-like subdomain (mobile loop residues 235–246) (PMID:16732284).
3. On endosomal membranes, VPS26A interacts with the N-terminal extension and PX domain of SNX3 to form the SNX3-retromer coat, positioning a conserved hydrophobic pocket at the interface of VPS26A and SNX3 to recognize canonical Øx[L/M/V] sorting signals ([PMID:27889239, "Here, we present an X-ray crystallographic analysis of a four-component complex comprising the VPS26 and VPS35 subunits of retromer, the sorting nexin SNX3, and a recycling signal from the divalent cation transporter DMT1-II. This analysis identifies a binding site for canonical recycling signals at the interface between VPS26 and SNX3."]; PMID:21725319).
4. Direct Cargo & Adaptor Recognition (GO:0140312 / GO:0042147 / GO:0032456):
5. Direct cargo tail binding: VPS26A and SNX3 directly recognize cytoplasmic Øx[L/M] sorting signals on cargoes such as DMT1-II (SLC11A2), CI-MPR (IGF2R), sortilin (SORT1), and Wntless (WLS) (PMID:27889239; PMID:15078903; PMID:18193037).
6. SNX27 adaptor binding: VPS26A directly binds an exposed beta-hairpin loop in the PDZ domain of the endosomal sorting nexin SNX27, increasing SNX27 affinity for PDZ-binding motifs by an order of magnitude and coupling PDZ-motif cargoes (such as beta-2 adrenergic receptor ADRB2, glucose transporter GLUT1/SLC2A1, and metal/ion transporters) to the retromer CSC for endosome-to-plasma membrane recycling ([PMID:23563491, "Furthermore, we establish that direct interaction of the SNX27 PDZ domain with the retromer subunit VPS26 is necessary and sufficient to prevent lysosomal entry of SNX27 cargo."]; [PMID:25136126, "Crystal structures and NMR experiments reveal that an exposed β-hairpin in the SNX27 PDZ domain engages a groove in the arrestin-like structure of the vacuolar protein sorting 26A (VPS26A) retromer subunit."]; PMID:21602791).
7. Endosome-to-TGN Retrieval:
8. Required as an essential subunit of the retromer CSC for endosome-to-TGN retrieval of CI-MPR (IGF2R), sortilin (SORT1), and Wntless (WLS) (PMID:15078903; PMID:18193037; PMID:22431521).

Key Curation & GO Annotation Decisions

• Core MF (NEW):
• GO:0140312 (cargo adaptor activity, contributes_to): Proposed as NEW (IDA from PMID:27889239, supported by PMID:27889239, PMID:23563491, and PMID:25136126) because X-ray crystal structures demonstrate that VPS26A directly forms the canonical Øx[L/M] cargo-recognition pocket with SNX3 and binds the SNX27 PDZ domain; the other two retromer CSC subunits (VPS35 and VPS29) already carry GO:0140312 in GOA.
• GO:0005515 (protein binding): All 31 unique IPI entries in existing_annotations (corresponding to 32 raw GO:0005515 IPI rows in VPS26A-goa.tsv, where rows 11 and 71 are duplicate IntAct/UniProtKB:Q9UBQ0 rows) are REMOVE per standard GO protein-binding policy.
• Core BP (ACCEPT and MODIFY):
• GO:0042147 (retrograde transport, endosome to Golgi): ACCEPT across IBA, IMP (PMID:15078903), and NAS (PMID:11102511, PMID:26220253) rows.
• GO:0032456 (endocytic recycling): ACCEPT across IMP (PMID:21602791) and NAS (PMID:30072228) rows, supported by direct SNX27-VPS26A structural and proteomic studies (PMID:23563491, PMID:25136126).
• GO:0006886 (intracellular protein transport, IBA and IEA): MODIFY to GO:0042147 (retrograde transport, endosome to Golgi) and GO:0032456 (endocytic recycling).
• Core CC (ACCEPT and MODIFY):
• GO:0030906 (retromer, cargo-selective complex) and GO:0030904 (retromer complex): ACCEPT.
• GO:0005768 (endosome), GO:0005769 (early endosome), GO:0010008 (endosome membrane), GO:0097422 (tubular endosome), and GO:0005829 (cytosol): ACCEPT.
• GO:0005737 (cytoplasm): MODIFY to GO:0005829 (cytosol) and GO:0010008 (endosome membrane).
• GO:0031982 (vesicle, IDA PMID:20682791): MODIFY to GO:0005769 (early endosome) and GO:0097422 (tubular endosome).
• GO:0005764 (lysosome, IDA GO_REF:0000052): MODIFY (lateral refinement of HPA endolysosomal puncta) to GO:0005768 (endosome).
• Non-Core / Indirect BP (KEEP_AS_NON_CORE):
• GO:0016241 (regulation of macroautophagy, IEA and TAS PMID:26172538): KEEP_AS_NON_CORE — in Vps26/Vps35-deficient cells, autophagosome formation and autophagy initiation are intact (consistent with the curated NOT|involved_in GO:0010506 annotation on VPS29), whereas late-stage autophagic cargo degradation is impaired indirectly due to defective CI-MPR-mediated lysosomal hydrolase delivery.