A novel electrogenic amino acid transporter is activated by K+ or Na+, is alkaline pH-dependent, and is Cl--independent

Abstract

A new eukaryotic nutrient amino acid transporter has been cloned from an epithelium that is exposed to high voltages and alkaline pH. The full-length cDNA encoding this novel CAATCH1 (cation-anion-activated Amino acid transporter/channel) was isolated using a polymerase chain reaction-based strategy, and its expression product in Xenopus oocytes displayed a combination of several unique, unanticipated functional properties. CAATCH1 electrophysiologlcal properties resembled those of Na+,Cl--coupled neurotransmitter amine transporters, although CAATCH1 was cloned from a gut absorptive epithelium rather than from an excitable tissue. Amino acids such as L-proline, L-threonine, and L-methionine elicited complex current-voltage relationships in alkaline pH-dependent CAATCH1 that were reminiscent of the behavior of the dopamine, serotonin, and norepinephrine transporters (DAT, SERT, NET) in the presence of their substrates and pharmacological inhibitors such as cocaine or antidepressants. These I-V relationships indicated a combination of substrate-associated carrier current plus an independent CAATCH1-associated leakage current that could be blocked by certain amino acids. However, unlike all structurally related proteins, CAATCH1 activity is absolutely independent of Cl-. Unlike related KAAT1, CAATCH1 possesses a methionine-inhibitable constitutive leakage current and is able to switch its narrow substrate selectivity, preferring threonine in the presence of K+ but preferring proline in the presence of Na+.