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) Inhibited by L- and D-pipecolic acid, nipecotic acid, isonipecotic acid, L- and D-cycloserine, and L-2-azetidine-carboxylate. Expressed in lung and spleen, and to a lower extent in brain, heart, kidney and skeletal muscle. Belongs to the amino acid/polyamine transporter 2 family.