The c-src protein isolated from neuronal cells (pp6Oc-src) displays a higher level of protein kinase activity than does pp6O-src from nonneural tissues. There are two structural alterations present in the amino-terminal half of pp60`rC expressed in neurons which could contribute to the enhanced activity of this form of pp6Oc-src (i) a hexapeptide insert located at amino acid 114 of avian pp60c-src and (ii) a novel site(s) of serine phosphorylation. We characterized pp6oc-src+ expressed in a nonneuronal cell type to identify factors that regulate the activity of the c-src+ protein and the importance of the neuronal environment on this regulation.The c-src+ protein overexpressed in chicken embryo fibroblasts (CEFs) displayed higher kinase activity than did pp60c-src. The major sites of phosphorylation of the c-src+ protein were Ser-17 and Tyr-527. The unique site(s) of serine phosphorylation originally identified in pp60c-src expressed in neurons was not detected in the c-src+ protein overexpressed in CEFs. Therefore, the hexapeptide insert is sufficient to cause an elevation in the tyrosine protein kinase activity of pp6Ocsrc+. Our data also indicate that CEFs infected with the Rous sarcoma virus (RSV)c-src+ display phenotypic changes that distinguish them from cultures producing pp60csrc and that pp6oc-src -expressing cells are better able to grow in an anchorage-independent manner. The level of total cellular tyrosine phosphorylation in RSVc-src+-infected cultures was moderately higher than the level observed in cultures infected with RSVc-src. This level was not as pronounced as that observed in cells infected with RSVv-src or oncogenic variants of RSVc-src. Thus, pp60csrc could be considered a partially activated c-src variant protein much like other c-src proteins that contain mutations in the amino-terminal domain.The c-src gene has served as a model system in investigations of the function and importance of a large family of proto-oncogene products, the protein tyrosine kinases. High levels of the c-src protein (pp60c-src) have been identified in neural tissues (5,14,17,31,47,49), platelets (21), peripheral blood lymphocytes (21), monocytes (1, 18), and chromaffin cells (41). The majority of the c-src gene product expressed in neural tissues, however, is structurally distinct from the protein produced in nonneural tissues (4, 5) and is encoded by a uniquely processed c-src mRNA (4, 32, 37). Neuralspecific c-src cDNA clones isolated from both chicken brain (32) and mouse brain (37) cDNA libraries contain an 18-base-pair insertion located at the splice junction between exons 3 and 4 of the c-src gene. The additional six amino acids encoded by this miniexon are identical in avian and mammalian species, and this insertion is responsible for the altered electrophoretic mobility of the c-src protein in neuronal cells (designated pp6Oc-src ) (32, 37).The hexapeptide insert, located at amino acid 114 of avian pp60c-src (amino acid 117 of mammalian pp6OCs'C), lies outside the catalytic domain of pp6Ocsr', within a ...