Alterations in pp60c-src accompany differentiation of neurons from rat embryo striatum

Cultured neurons from rat embryo striatum were found to contain two structurally distinct forms of pp60c-src. The 60-kilodalton (kDa) form appeared similar to pp60c-src from cultured rat fibroblasts or astrocytes. The 61-kDa form was specific to neurons and differed in the NH2-terminal 18 kDa of the molecule. In undifferentiated neurons the predominant phosphorylated species of pp60c-src was the fibroblast form. Upon differentiation, a second phosphorylated form of pp60c-src was detected. This form had two or more additional sites of serine phosphorylation within the NH2-terminal 18-kDa region of the molecule, one of which was Ser-12. The specific protein-tyrosine kinase activity of the total pp60c-src population increased 14-fold, as measured by autophosphorylation, or 7-fold, as measured by phosphorylation of an exogenous substrate, as striatal neurons differentiated. This elevation in protein kinase activity occurred without a detectable decrease in Tyr-527 phosphorylation or increase in Tyr-416 phosphorylation. Our results support the idea that the expression of the neuron-specific form of pp60c-src and the increase in specific protein kinase activity may be important for neuronal differentiation.