A novel system A isoform mediating Na+/neutral amino acid cotransport

A cDNA clone encoding a plasma membrane alanine-preferring transporter (SAT 2) has been isolated from glutamatergic neurons in culture and represents t he second member of the system A family of neutral amino acid transporters. SAT2 displays a widespread distribution and is expressed in most tissues, including heart, adrenal gland, skeletal muscle, stomach, fat, brain, spina l cord, colon, and lung, with lower levels detected in spleen. No signal is detected in liver or testis. In the central nervous system, SAT2 is expres sed in neurons. SAT2 is significantly up-regulated during differentiation o f cerebellar granule cells and is absent from astrocytes in primary culture . The functional properties of SATE, examined using transfected fibroblasts and in cRNA-injected voltage-clamped Xenopus oocytes, show that small alip hatic neutral amino acids are preferred substrates and that transport is vo ltage- and Na+-dependent (1:1 stoichiometry), pH-sensitive, and inhibited b y alpha-(methylamino)isobutyric acid (MeAIB), a specific inhibitor of syste m A. Kinetic analyses of alanine and MeAIB uptake by SAT2 are saturable, wi th Michaelis constants (K-m) of 200-500 mu M. In addition to its ubiquitous role as a substrate for oxidative metabolism and a major vehicle of nitrog en transport, SAT2 may provide alanine to function as the amino group donor to alpha-keto-glutarate to provide an alternative source for neurotransmit ter synthesis in glutamatergic neurons.