AbstractThe tyrosine phosphorylation of glycoproteins in the adult and developing rat brain was investigated. Immunoblotting with anti‐tyr(P) antibodies identified a glycoprotein with an apparent Mr of 180,000 (GP180) as the major tyrosine‐phosphorylated protein in the concanavalin A (con A)‐binding fraction prepared from forebrain homogenates. This glycoprotein had the same electrophoretic mobility as the postysynaptic density (PSD)‐associated glycoprotein PSD‐GP180. Tyrosine‐phosphorylated GP180 was enriched 24‐fold in isolated PSDs relative tohomogenates. Digestion with endoglycosidase F/N‐glycosidase F demonstrated that GP180 present in total homogenates and PSD‐GP180 present in total homogenates and PSD‐GP180 present in isolated PSDs contained similar amounts of N‐linked oligosaccharide suggesting that they are the same glycoprotein. The tyrosine phosphorylation of GP180 in homogenates varied between brain regions with the highest levels occurring in cortical areas and the amygdala and low or undetectable amounts 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 on tyrosine. GP180 was not phosphorylated on tyrosine following the incubation 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. © 1994 Wiley‐Liss, Inc.