pp60c-src is expressed in human fetal and adult brain

Human cells contain a tyrosine-specific protein kinase, pp60c-src, that is highly homologous to the oncogene product, pp60v-src, from Rous sarcoma virus but is of unknown function. The expression of human pp60c-src was examined in tissues obtained from human adults and fetuses of 20-32 weeks' gestational age. pp60c-src was quantitated in tissue extracts by measurement of its protein kinase activity by the use of the immune complex protein kinase assay. Brain showed the highest levels of pp60c-src protein kinase activity, but all other human tissues examined had significant levels. Fetal tissues, including brain, showed three- to eight-fold higher levels of pp60c-src kinase activity than the corresponding adult tissues. pp60c-src kinase was found to be uniformly distributed in the adult brain; frontal, occipital, and parietal cortex, and cerebellum expressed equivalent amounts of pp60c-src kinase activity. The protein kinase activity in human tissues exhibited properties characteristic of pp60c-src in other species, namely, tyrosine-specific phosphorylation of specific antibody heavy chains, autophosphorylation of a 60,000 Mr protein following immunoprecipitation with a monoclonal antibody specific for pp60src, and sensitivity to inhibition by P1,P4-di(adenosine-5')tetraphosphate. The high levels of human pp60c-src in fetal tissues, particularly in brain, suggest a possible function in developmental processes.