SNX31

Model summary

A soluble endosomal adaptor that uses an N-terminal lipid-binding module to target PI3P-enriched early endosomes and a C-terminal FERM/PH-like scaffold to oligomerize and assemble sorting platforms. By forming homotrimers and recruiting trafficking factors, it organizes protein complexes that regulate cargo flow between early endosomes and the Golgi apparatus. It cycles between the cytosol and endosomal membranes, where it coordinates receptor recycling and endosome–Golgi transport through multivalent protein interactions.

Predicted Gene Ontology terms

1. 01

   I begin with the domain architecture.

   The N-terminus carries IPR001683 (Phox homology, residues 1–109) embedded within IPR036871 (PX domain superfamily, residues 2–110). PX domains create a curved β-sandwich that binds phosphoinositides—most characteristically phosphatidylinositol-3-phosphate (PI3P)—and thereby targets soluble factors to PI3P-enriched early endosomal membranes. Immediately downstream, the polypeptide assembles a tripartite FERM module: IPR048763 (Sorting nexin 17/31, FERM domain, F1 lobe, residues 119–206) followed by IPR048767 (FERM domain, F2 lobe, residues 209–267). These lobes together form IPR040842 (Sorting nexin-17/31, FERM domain, residues 270–380). FERM domains act as membrane–protein adaptors that bind acidic membrane surfaces and short linear motifs in partner proteins, frequently stabilizing higher-order assemblies. Overlapping with the FERM region is IPR011993 (PH-like domain superfamily, residues 271–384), reinforcing that the C-terminal half adopts a PH/FERM-like fold that enhances membrane association and multivalent protein binding rather than catalysis.

2. 02

   This ordered layout—an N-terminal PX module for PI3P-directed membrane targeting followed by a FERM/PH-like scaffold—causes a soluble adaptor behavior: it recruits to PI3P-positive early endosomes and organizes protein complexes through multivalent contacts.

   Such scaffolds typically lack enzymatic motifs and instead function through GO:0005515 protein binding as their primary molecular function. The presence of a FERM platform that nucleates higher-order assemblies naturally explains GO:0070207 protein homotrimerization; homotrimerization increases avidity for membranes and cargo, stabilizing endosomal sorting platforms.

3. 03

   From molecular function to process, PX-driven endosomal recruitment and FERM-mediated oligomerization position the protein squarely in endosomal sorting and Golgi–endosome trafficking.

   The PX domain’s affinity for early endosome lipids aligns with localization to the early endosome and endosome membrane, while the FERM/PH-like region supports assembly of larger protein-containing complexes. Dynamic exchange between membranes and the cytosol is expected for a soluble adaptor, consistent with cytosolic residence and transient membrane association. The trafficking axis that interconverts endosomes and the Golgi apparatus accounts for observed enrichment at the Golgi apparatus.

4. 04

   Cellular localization follows directly from this architecture and mechanism.

   PX-mediated PI3P binding and FERM-driven assembly place the protein at the early endosome and endosome membrane, where it forms and joins protein-containing complexes. Its soluble nature and cycling behavior support a cytosolic pool, and its role in bidirectional endosome–Golgi traffic rationalizes presence at the Golgi apparatus.

5. 05

   Mechanistically, I hypothesize that the PX domain first captures the protein at PI3P-rich early endosomes; homotrimerization via the FERM core then builds a platform that concentrates cargo receptors and sorting nexins.

   This platform likely cooperates with retromer and accessory adaptors to regulate receptor recycling and Golgi-directed transport. Candidate partners include PX-only and FERM-containing trafficking factors and small GTPases that define early endosomal identity. Together, these interactions produce a membrane-proximal scaffold that times and positions cargo flow between early endosomes and the Golgi.