Abstract
Protein tyrosine kinase (PTK) activity associated with synaptosomal membrane glycoprotein (SMGP) fractions of rat brain was examined. The synthetic substrate poly(Glu4-Tyr) was phosphorylated by SMGP in the presence of Mg2+ and Mn2+, whose stimulatory effects were additive. In contrast, endogenous tyrosine phosphorylation in SMGPs was strictly dependent on Mn2+. Anti-phosphotyrosine antibodies (PY20) immunoprecipitated two polypeptides in SMGPs of Mr 170K and 60K. Upon preincubation with IGF-I, 97/90K polypeptides were phosphorylated, corresponding to the IGF-I receptor beta-subunits, and were immunoprecipitated with both PY20 and anti-IGF-I-receptor antibodies. Immunoblot analysis using anti-src antibody revealed that there was src protein associated with the glycoprotein fractions of solubilized synaptosomal membranes. Additional experiments revealed that the 60K tyrosine-phosphorylated polypeptide present in the PY20 precipitates was indeed pp60c-src. This was confirmed by subjecting the PY20 immunoprecipitates to immunoblotting using anti-src antibodies. In addition, src protein was directly immunoprecipitated by anti-src antibodies from the SMGP preparations. Hence, IGF-I receptors and glycoprotein-associated PTKs including pp60c-src may play an important role in synaptic transmembrane signalling, plasticity, and neuronal survival.
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