A Novel System A Isoform Mediating Na+/Neutral Amino Acid Cotransport

A cDNA clone encoding a plasma membrane alanine-preferring transporter (SAT2) has been isolated from glutamatergic neurons in culture and represents the 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, spinal 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 expressed in neurons. SAT2 is significantly up-regulated during differentiation of cerebellar granule cells and is absent from astrocytes in primary culture. The functional properties of SAT2, examined using transfected fibroblasts and in cRNA-injected voltage-clamped Xenopus oocytes, show that small aliphatic neutral amino acids are preferred substrates and that transport is voltage- and Na+-dependent (1:1 stoichiometry), pH-sensitive, and inhibited by α-(methylamino)isobutyric acid (MeAIB), a specific inhibitor of system A. Kinetic analyses of alanine and MeAIB uptake by SAT2 are saturable, with Michaelis constants (Km) of 200–500 μm. In addition to its ubiquitous role as a substrate for oxidative metabolism and a major vehicle of nitrogen transport, SAT2 may provide alanine to function as the amino group donor to α-ketoglutarate to provide an alternative source for neurotransmitter synthesis in glutamatergic neurons.