The function of SLC1A5 (solute carrier family 1 member 5, Ensembl gene identifier ENSG00000105281) is as follows. Sodium-coupled antiporter of neutral amino acids. In a tri- substrate transport cycle, exchanges neutral amino acids between the extracellular and intracellular compartments, coupled to the inward cotransport of at least one sodium ion (PubMed:17094966, PubMed:23756778, PubMed:26492990, PubMed:29872227, PubMed:34741534, PubMed:8702519, PubMed:39095408, PubMed:27272177). The preferred substrate is the essential amino acid L-glutamine, a precursor for biosynthesis of proteins, nucleotides and amine sugars as well as an alternative fuel for mitochondrial oxidative phosphorylation. Exchanges L-glutamine with other neutral amino acids such as L-serine, L- threonine and L-asparagine in a bidirectional way. Provides L-glutamine to proliferating stem and activated cells driving the metabolic switch toward cell differentiation (PubMed:23756778, PubMed:24953180). The transport cycle is usually pH-independent, with the exception of L- glutamate. Transports extracellular L-glutamate coupled to the cotransport of one proton and one sodium ion in exchange for intracellular L-glutamine counter-ion. May provide for L-glutamate uptake in glial cells regulating glutamine/glutamate cycle in the nervous system (PubMed:32733894). Part of transport systems for D-amino acids. Mediates D-serine release from the retinal glia potentially affecting NMDA receptor function in retinal neurons. May contribute to D-serine reabsorption at the brush border of renal proximal tubules thus modulating the dynamics of urinary versus blood D-serine levels (PubMed:17094966, PubMed:38650461). Displays sodium- and amino acid- dependent but uncoupled channel-like anion conductance with a preference SCN(-) >> NO3(-) > I(-) > Cl(-) (By similarity). Through binding of the fusogenic protein syncytin-1/ERVW-1 may mediate trophoblasts syncytialization, the spontaneous fusion of their plasma membranes, an essential process in placental development (PubMed:10708449, PubMed:23492904, PubMed:38671230). {ECO:0000250|UniProtKB:D3ZJ25, ECO:0000269|PubMed:10708449, ECO:0000269|PubMed:17094966, ECO:0000269|PubMed:23492904, ECO:0000269|PubMed:23756778, ECO:0000269|PubMed:24953180, ECO:0000269|PubMed:26492990, ECO:0000269|PubMed:27272177, ECO:0000269|PubMed:29872227, ECO:0000269|PubMed:32733894, ECO:0000269|PubMed:34741534, ECO:0000269|PubMed:38650461, ECO:0000269|PubMed:38671230, ECO:0000269|PubMed:39095408, ECO:0000269|PubMed:8702519}. (Microbial infection) Acts as a cell surface receptor for Feline endogenous virus RD114. {ECO:0000269|PubMed:10051606, ECO:0000269|PubMed:10196349}. (Microbial infection) Acts as a cell surface receptor for Baboon M7 endogenous virus. . (Microbial infection) Acts as a cell surface receptor for type D simian retroviruses. .