Neuronal tyrosine phosphorylation in growth cone glycoproteins

Neuronal glycoproteins derived from growth cones are enriched in protein tyrosine kinase activity. In contrast, glycoproteins obtained from mature synaptosomes are relatively lacking in this activity. At least three types of protein tyrosine kinases were detected in association with growth cone glycoproteins: 1) a Mn2(+)-dependent activity which phosphorylates several endogenous substrates; 2) a Mn2+/Mg2(+)-dependent activity which phosphorylates synthetic substrate and is stimulated by acidic brain extracts; and 3) protein tyrosine kinases corresponding to the insulin and insulin-like growth factor I receptors. These enzymes may be particularly important during earlier stages of neuronal maturation.