Structure, function and immunolocalization of a proton-coupled amino acid transporter (hPAT1) in the human intestinal cell line Caco-2.

The human orthologue of the H(+)-coupled amino acid transporter (hPAT1) was cloned from the human intestinal cell line Caco-2 and its functional characteristics evaluated in a mammalian cell heterologous expression system. The cloned hPAT1 consists of 476 amino acids and exhibits 85 % identity with rat PAT1. Among the various human tissues examined by Northern blot, PAT1 mRNA was expressed most predominantly in the intestinal tract. When expressed heterologously in mammalian cells, hPAT1 mediated the transport of alpha-(methylamino)isobutyric acid (MeAIB). The cDNA-induced transport was Na(+)-independent, but was energized by an inwardly directed H(+) gradient. hPAT1 interacted with glycine, L-alanine, L-proline, alpha-aminoisobutyrate (AIB) and gamma-aminobutyrate (GABA), as evidenced from direct transport measurements and from competition experiments with MeAIB as a transport substrate. hPAT1 also recognized the D-isomers of alanine and proline. With serine and cysteine, though the L-isomers did not interact with hPAT1 to any significant extent, the corresponding D-isomers were recognized as substrates. With proline and alanine, the affinity was similar for L- and D-isomers. However, with cysteine and serine, the D-isomers showed 6- to 8-fold higher affinity for hPAT1 than the corresponding L-isomers. These functional characteristics of hPAT1 closely resemble those that have been described previously for the H(+)-coupled amino acid transport system in Caco-2 cells. Furthermore, there was a high degree of correlation (r(2) = 0.93) between the relative potencies of various amino acids to inhibit the H(+)-coupled MeAIB transport measured with native Caco-2 cells and with hPAT1 in the heterologous expression system. Immunolocalization studies showed that PAT1 was expressed exclusively in the apical membrane of Caco-2 cells. These data suggest that hPAT1 is responsible for the H(+)-coupled amino acid transport expressed in the apical membrane of Caco-2 cells.