SLC45A1

Model summary

A multi-pass membrane transporter that uses the proton gradient to drive sugar uptake and handling within the endolysosomal system. It resides predominantly on late endosome and lysosome membranes and can also be found on melanosomes and chitosomes. By coupling proton binding to alternating-access conformational changes, it mediates transmembrane movement of monosaccharides and related carbohydrates, supporting glucose and broader sugar fluxes that feed cytosolic metabolism and organelle homeostasis.

Predicted Gene Ontology terms

1. 01

   I begin with the InterPro architecture.

   The polypeptide carries IPR036259 (MFS transporter superfamily, homologous_superfamily) spanning residues 85–746, encompassing nearly the entire core, and an internal IPR011701 (Major facilitator superfamily, family) from residues 99–311. This layered arrangement is characteristic of a canonical major facilitator superfamily (MFS) transporter: a 12-helix, multi-pass membrane fold that uses alternating-access conformational changes to move small solutes across membranes. The broad MFS superfamily signature (IPR036259) establishes the membrane-embedded scaffold and energy-coupling logic, while the embedded MFS family signature (IPR011701) refines the fold to the sugar/monosaccharide transporter clade. Such an architecture causes proton-coupled uptake of polyhydroxy substrates by harnessing the proton motive force through titratable residues that gate access to a central substrate cavity.

2. 02

   From this scaffold, I infer molecular function.

   The MFS fold dictates secondary active transport—consistent with GO:0005351 carbohydrate:proton symporter activity—where proton binding and release drive conformational cycling and sugar translocation. The substrate cavity and sugar-porter lineage support GO:0015145 monosaccharide transmembrane transporter activity and GO:0055056 D-glucose transmembrane transporter activity. The presence of a defined oligosaccharide-binding track in some MFS sugar porters, together with the alternating-access mechanism, supports GO:0015157 oligosaccharide transmembrane transporter activity. The large cytosolic loops typical of MFS proteins often mediate regulatory and scaffolding contacts, aligning with GO:0005515 protein binding as an auxiliary function that stabilizes complexes and tunes transport.

3. 03

   These activities feed directly into biological process.

   Proton-coupled sugar transport across intracellular membranes implements GO:1904659 glucose transmembrane transport and more broadly GO:0015749 monosaccharide transmembrane transport. In acidic organelles, this chemistry is especially efficient: the lumenal proton gradient energizes retrieval or fine-tuning of sugars and related metabolites.

4. 04

   Cellular location follows from both the fold and trafficking cues.

   A multi-pass MFS core mandates membrane embedding, and the experimentally supported endolysosomal itinerary points to late endosome and lysosome membranes. This aligns with GO:0010008 endosome membrane and GO:0005765 lysosomal membrane. The transporter’s presence in specialized pigment-cell organelles extends its distribution to GO:0042470 melanosome. Its participation in endolysosome biogenesis and cargo sorting rationalizes association with GO:0045009 chitosome, a chaperone-rich compartment along the endocytic route.

5. 05

   Mechanistically, I propose a proton-symport cycle tuned to acidic organelles: luminal H+ binds to conserved acidic residues, shifting the MFS core into an outward-open state toward the lumen to load sugars; transition to inward-open releases sugar to the cytosolic face of endosomes and lysosomes, where it can be metabolized or recycled.

   Protein-binding surfaces likely recruit trafficking and acidification machinery to position the transporter precisely on late endosomes and lysosomes.

6. 06

   This model suggests interaction partners.

   To maintain acidic luminal pH and proton gradients, the transporter likely associates with the V-ATPase and endolysosomal proton pumps. Trafficking and retention are plausibly mediated by adaptor complexes and small GTPases that define endosome identity and maturation. In pigment cells, associations with melanosome biogenesis factors and chaperone assemblies would stabilize its presence on melanosomes. Together, these contacts situate the transporter as a proton-coupled sugar porter that supports intracellular sugar handling within the endolysosomal system.