TYROSINE PHOSPHORYLATION OF GLYCOPROTEINS IN THE ADULT AND DEVELOPINGRAT-BRAIN

The tyrosine phosphorylation of glycoproteins in the adult and develop ing rat brain was investigated. Immunoblotting with anti-tyr(P) antibo dies identified a glycoprotein with an apparent Mr of 180,000 (GP180) as the major tyrosine-phosphorylated protein in the concanavalin A (co n A)-binding fraction prepared from forebrain homogenates. This glycop rotein had the same electrophoretic mobility as the postsynaptic densi ty (PSD)-associated glycoprotein PSD-GP180. Tyrosine-phosphorylated GP 180 was enriched 24-fold in isolated PSDs relative to homogenates. Dig estion with endoglycosidase F/N-glycosidase F demonstrated that GP180 present in total homogenates and PSD-GP180 present in isolated PSDs co ntained similar amounts of N-linked oligosaccharide suggesting that th ey are the same glycoprotein. The tyrosine phosphorylation of GP180 in homogenates varied between brain regions with the highest levels occu rring in cortical areas and the amygdala and low or undetectable amoun ts being present in hindbrain regions. Incubation of homogenates with adenosine triphosphate (ATP) resulted in the tyrosine phosphorylation of GP180 in all regions examined except the cerebellum and identified a second con A-binding glycoprotein, GP110, which was phosphorylated o n tyrosine. GP180 was not phosphorylated on tyrosine following the inc ubation of cerebellar homogenate, synaptic membranes, or PSDs with ATP . Tyr(P)-GP180 was not detected prior to the onset of synaptogenesis, increased in parallel with the formation of synapses during the first 4 weeks of postnatal development of the frontal cortex and hippocampus , and then decreased 50-60% to adult levels. The results suggest that GP180 corresponds to the PSD glycoprotein PSD-GP180 and are consistent with a role for this glycoprotein in synaptic development and signal transduction at the synapse. (C) 1993 Wiley-Liss, Inc.