The differential expression of the cellular src-gene product pp60src and its phosphokinase activity in normal chicken...

Chick embryos of different ages and adult chickens were examined for the expression of pp60c -src, the normal cellular homolog of the transforming protein of Rous sarcoma virus. In any brain extract, the pp60c -src kinase activity was always high, whereas muscle extracts of embryos show an age-dependent decrease in kinase activity. Adult animals show either no or barely measurable activity in muscle tissue. In contrast, liver cell extracts of embryos show an age-dependent increase in pp60c -src kinase activity, with adult chickens displaying the highest activity, very similar to that found in brain extracts. This demonstrates that increased expression of c-src is not necessarily correlated with cell proliferation, but suggests that, at an early stage of differentiation of mesenchymal cells, the relatively high expression of c-src could be responsible, at least in part, for the control of cell metabolism and proliferation.