Characterization of a mouse colonic system B0+ amino acid transporter related to amino acid absorption in colon

Previous experiments have shown that an amino acid transport system BO+ transporter in cultured colonic epithelial cells mediates amino acid absorption. Here we describe the cloning and functional characterization of a system BO+ transporter selectively expressed in the colon. Using the combination of an expressed sequence tag database search and RT-PCR approaches, we cloned a mouse colonic amino acid transporter, designated mCATB(O+). Northern blot analysis revealed that mCATB(O+) was selectively expressed in the large intestine. In situ hybridization showed the mCATB(O+) mRNA to be localized in absorptive epithelial cells. When expressed in Xenopus oocytes, mCATB(O+) exhibited a Na+-dependent stereoselective uptake and a broad specificity for neutral and cationic amino acids, which is characteristic of amino acid transport system BO+. In vivo [ H-3] glycine uptake assay demonstrated that a system BO+-like transporter protein was expressed on the apical surface of the colonic absorptive cells. Our data suggest that a mouse colonic amino acid transporter mCATB(O+) may absorb amino acids from the intestinal contents in the colon.