PHOSPHORYLATION AND MODULATION OF BRAIN GLUTAMATE TRANSPORTERS BY PROTEIN-KINASE-C

High affinity sodium- and potassium-coupled L-glutamate transport into presynaptic nerve terminals and fine glial processes removes the neur otransmitter from the synaptic cleft, thereby terminating glutamergic transmission. This report describes that the purified L-glutamate tran sporter from pig brain is phosphorylated by protein kinase C, predomin antly at serine residues. Upon exposure of C6 cells, a cell line of gl ial origin, to 12-O-tetradecanoylphorbol-13-acetate, about a 2-fold st imulation of L-glutamate transport is observed within 30 min. Concomit antly, the level of phosphorylation increases with similar kinetics. T he phorbol ester also stimulates L-glutamate transport in HeLa cells i nfected with a recombinant vaccinia virus expressing T7 RNA polymerase and transfected with pT7-GLT-1. The latter is a recently cloned rat b rain glutamate transporter of glial origin. Mutation of serine 113 to asparagine does not affect the levels of expressed transport but aboli shes its stimulation by the phorbol ester. To our knowledge, this is t he first direct demonstration of the regulation of a neurotransmitter transporter by phosphorylation.