TUBULIN IS PHOSPHORYLATED AT TYROSINE BY PP60C-SRC IN NERVE GROWTH CONE MEMBRANES

We show here that tubulin is the major in vivo substrate of the tyrosine-specific protein kinase pp60(c-src) in nerve growth cone membranes. Phosphotyrosine antibodies were used to demonstrate phosphotyrosyl residues in a subpopulation of α- and β-tubulin that was highly enriched in a subcellular fraction of growth cone membranes from fetal rat brain. The presence of phosphotyrosine-modified isoforms of α- and β-tubulin in vivo was confirmed by 32P labeling of rat cortical neurons in culture. Tubulin in growth cone membranes was phosphorylated at tyrosine in endogenous membrane phosphorylation reactions (0.068 mol phosphotyrosine/mol α-tubulin and 0.045 mol phosphotyrosine/mol β-tubulin), and phosphorylation was specifically inhibited by antibodies directed against pp60(c-src), which is localized in the growth cone membranes. pp60(c-src) was capable of directly phosphorylating tubulin as shown in immune complex kinase assays with purified brain tubulin. Phosphopeptide mapping revealed a limited number of sites of tyrosine phosphorylation in α- and β-tubulin, with similar phosphopeptides observed in vivo and in vitro. These results reveal a novel posttranslational modification of tubulin that could regulate microtubule dynamics at the growth cone.