Cloning and functional identification of a neuronal glutamine transporter

Glutamine is the preferred precursor for the neurotransmitter pool of gluta mate, the major excitatory transmitter in the mammalian central nervous sys tem. We have isolated a complementary DNA clone (designated GlnT) encoding a plasma membrane glutamine transporter from glutamatergic neurons in cultu re, and its properties have been examined using the T7 vaccinia system in f ibroblasts. When GlnT is transfected into CV-1 cells, L-glutamine is the pr eferred substrate. Transport is Na+-dependent and inhibited by alpha-methyl amino-isobutyric acid, a specific inhibitor of neutral amino acid transport system A. Kinetic analysis of glutamine uptake by GlnT is saturable, with a Michaelis constant (K-m) of 489 +/- 88 mu M at pH 7.4. Glutamine uptake m ediated by GlnT is pH-sensitive with a 5-fold greater efficiency of uptake at pH 8.2 than at pH 6.6. Only the maximal velocity of transport increases without a significant change in K-m. The distribution of GlnT mRNA and prot ein in the central nervous system is widespread and is expressed on neurons that use glutamate as their neurotransmitter. In cultured cerebellar granu le cells, GlnT is expressed only on neurons and is absent from astrocytes. GlnT expression increases concomitantly with the morphologic and functional differentiation of these cells in vitro, consistent with its role of suppl ying glutamatergic neurons with their neurotransmitter precursor, GlnT is t he first member of the system A family of neutral amino acid transporters w ith 11 putative membrane-spanning domains and is a potential target to modu late presynaptic glutamatergic function.