Functional characterization, localization, and molecular identification of rabbit intestinal N-amino acid transporter

We have characterized the Na-glutamine cotransporter in the rabbit intestinal crypt cell brush border membrane vesicles (BBMV). Substrate specificity experiments showed that crypt cell glutamine uptake is mediated by system N. Real-time PCR experiments showed that SN2 (SLC38A5) mRNA is more abundant in crypt cells compared with SN1 (SLC38A3), indicating that SN2 is the major glutamine transporter present in the apical membrane of the crypt cells. SN2 cDNA was obtained by screening a rabbit intestinal cDNA library with human SN1 used as probe. Rabbit SN2 cDNA encompassed a 473-amino-acid-long open reading frame. SN2 protein displayed 87% identity and 91% similarity to human SN2. Functional characterization studies of rabbit SN2 were performed by using vaccinia virusmediated transient expression system. Substrate specificity of the cloned transporter was identical to that of SN2 described in the literature and matched well with substrate specificity experiments performed using crypt cell BBMV. Cloned rabbit SN2, analogous to its human counterpart, is Li⁺ tolerant. Hill coefficient for Li⁺ activation of rabbit SN2-mediated uptake was 1. Taken together, functional data from the crypt cell BBMV and the cloned SN2 cDNA indicate that the crypt cell glutamine transport is most likely mediated by SN2.