Molecular and functional characterization of the intestinal Na+-dependent multivitamin transporter

Abstract

We have cloned a Na+-dependent multivitamin transporter from rabbit intestine (riSMVT). The cDNA codes for a protein of 636 amino acids with 12 putative transmembrane domains. When expressed in mammalian cells, the cDNA induces Na+-dependent uptake of the vitamins pantothenate and biotin. Lipoate is also a substrate for the cDNA-induced uptake process. The affinity constant for the cDNA-specific transport of pantothenate and biotin is approximately 2 and approximately 8 microM, respectively. The Na+:vitamin stoichiometry is greater than 1, indicating that the transport process is electrogenic. The SMVT-specific transcripts of 3.2 kbp are equally distributed throughout the small intestine. We have also cloned SMVT from the human intestinal cell line Caco-2. The Caco-2 SMVT cDNA codes for a protein of 635 amino acids which is homologous to riSMVT and is identical to the SMVT expressed in the human choriocarcinoma cell line JAR. Caco-2 SMVT also catalyzes Na+-dependent uptake of pantothenate, biotin, and lipoate. In oocytes expressing Caco-2 SMVT, all three vitamins evoke inward currents, confirming the electrogenicity of the transport process.