Solute Carrier Family 36, Member 4 (SLC36A4)

SLC36A4 or hPAT4, which when expressed in Xenopus laevis oocytes also encodes a plasma membrane amino acid transporter, but one which is not proton-coupled and has a very high substrate affinity for the amino acids proline and tryptophan. hPAT4 in Xenopus oocytes mediated sodium-independent, electroneutral uptake of [(3)H]-proline, with the highest rate of uptake when the uptake medium pH was 7.4 and an affinity of 3.13μM. Tryptophan was also an excellently transported substrate with a similarly high affinity (1.72μM). This characterisation of hPAT4 as a very high affinity / low capacity non-proton coupled amino acid transporter raises questions about its physiological role, especially as the transport characteristics of hPAT4 are very similar to the Drosophila orthologue PATH, an amino acid 'transceptor' which plays a role in nutrient sensing.