Electrogenic proton/amino acid symporter with a high selectivity for the small side chains amino acids glycine, alanine and proline, where both L- and D-enantiomers are transported. Extension of the backbone length, as in beta-alanine and 4-aminobutanoate or methylation of the amino group, as in sarcosine and N,N-dimethylglycine, are also tolerated but decrease transport efficiency. A free carboxyl group is preferred. glycine(in) + H(+)(in) = glycine(out) + H(+)(out) H(+)(in) + L-alanine(in) = H(+)(out) + L-alanine(out) D-alanine(in) + H(+)(in) = D-alanine(out) + H(+)(out) H(+)(out) + L-proline(out) = H(+)(in) + L-proline(in) D-proline(out) + H(+)(out) = D-proline(in) + H(+)(in) 4-hydroxy-L-proline(in) + H(+)(in) = 4-hydroxy-L-proline(out) + H(+)(out) H(+)(in) + L-serine(in) = H(+)(out) + L-serine(out) D-serine(out) + H(+)(out) = D-serine(in) + H(+)(in) beta-alanine(in) + H(+)(in) = beta-alanine(out) + H(+)(out) 4-aminobutanoate(in) + H(+)(in) = 4-aminobutanoate(out) + H(+)(out) H(+)(in) + sarcosine(in) = H(+)(out) + sarcosine(out) H(+)(in) + N,N-dimethylglycine(in) = H(+)(out) + N,N-dimethylglycine(out) Expressed in spinal cord, brain, testis, lung, heart, colon, spleen, kidney and muscle. Found in neuronal cell bodies in the anterior horn, in spinal cord brain stem, cerebellum, hippocampus, hypothalamus, rhinencephalon, cerebral cortex, and olfactory bulb in the brain. Also expressed in bone and fat tissues. Belongs to the amino acid/polyamine transporter 2 family.