Identification of mammalian proline transporter SIT1 (SLC6A20) with characteristics of classical system imino

Amino acid homeostasis depends on specific amino acid transport systems, many of which have been characterized at the molecular level. However, the classical System IMINO, defined as the Na+-dependent proline transport activity that escapes inhibition by alanine, had not been identified at the molecular level. We report here the functional characteristics and tissue distribution of Sodium/Imino-acid Transporter 1 (SIT1), which exhibits the properties of classical System IMINO. SIT1, the product of the slc6a20 gene, is a member of the SLC6 Na+- and Cl--dependent neurotransmitter transporter family whose function has remained unknown. When expressed in Xenopus oocytes, rat SIT1 mediated the uptake of imino acids such as proline (K0.5 approximately 0.2 mM) and pipecolate, as well as N-methylated amino acids (e.g. MeAIB, sarcosine). SIT1-mediated proline transport was pH-independent and insensitive to inhibition by alanine or lysine. Proline transport was Na+-dependent, Cl--stimulated, and voltage-dependent. Li+, but not H+, could substitute for Na+. Human SIT1 also functioned as a Na+-dependent proline transporter. Rat SIT1 mRNA was expressed in epithelial cells of duodenum, jejunum, ileum, stomach, cecum, colon, and kidney proximal tubule S 3 segments. SIT1 mRNA was also expressed in the choroid plexus, microglia, and meninges of the brain and in the ovary. Previous reports have documented the marked urinary hyperexcretion of proline in newborn rodents and man. We found that SIT1 was dramatically up-regulated in the kidneys of 3-day-old mice, accounting for the maturation of proline reabsorption in the mouse. The human slc6a20 gene coding SIT1 is an appropriate target for investigation of hereditary forms of iminoaciduria in man.