We raised antibodies in rabbits against the amino-terminal portion of the viral yes protein produced in bacteria with the use of an expression vector based on the lac operon. The anti-yes serum thus obtained precipitated p9Wgag-yes from Yamaguchi 73 virus-transformed chicken embryo fibroblasts, and this immunoprecipitation was blocked by the purified antigen. The anti-yes serum did not recognize viral src, fps, or fgr proteins. Affinity-purified anti-yes immunoglobulin G (IgG) precipitated two proteins of 59 and 62 kilodaltons from lysates of normal chicken embryo fibroblasts. Two-dimensional tryptic peptide mapping showed that these proteins are closely related to p9A09-yes and that they are different from pp60c-S. Similar to P90919-y", the 59-and 62-kilodalton proteins were phosphorylated exclusively on tyrosine in an in vitro kinase reaction, whereas in vivo they were phosphorylated on serine and, to a lesser extent, on tyrosine as well. Expression of the 59-and 62-kilodalton proteins, determined by the immune complex kinase assay, was relatively high in brain, retina, kidney, and liver. The presence in normal chicken embryo fibroblasts and in chicken kidney of two transcripts, 3.7 and 3.9 kilobases in length, that hybridize with a yes-specific DNA probe, as well as the two proteins recognized by anti-yes IgG, suggests either differential splicing of cellular yes gene transcripts or the existence of another yes-related gene.Yamaguchi 73 (Y73) and Esh avian sarcoma viruses are replication-defective retroviruses that were independently isolated from spontaneous tumors in chickens (16,42). Both viruses induce sarcomas in vivo and transform chicken embryo fibroblasts (CEF) in vitro (11, 18). The transforming properties of these viruses are directed by cell-derived sequences called yes (33,43). The yes transforming genes of Y73 and Esh avian sarcoma virus encode pg9yag-yes and P80Iag-Yes phosphoproteins, respectively (10, 18). The amino acid sequence of the p90yag-yes fusion protein predicted from the DNA sequence of the Y73 genome (19) shows a high degree of homology with pp60v-rc, the transforming protein of Rous sarcoma virus (31, 40). The greatest extent of homology exists in the carboxy-terminal domains, whereas the amino-terminal portions of these two oncogene proteins are different (19).Our interest in cellular yes (c-yes) proteins(s) stems from two initial observations. Studies at the RNA level have shown that c-yes expression in various chicken tissues is one to two orders of magnitude higher than that of other known c-onc sequences from the avian sarcoma virus family, and that the pattern of tissue-specific expression of c-yes shows elevated levels of c-yes mRNA in kidney and brain (33).In this report, we describe the characterization of two cellular proteins of 59 and 62 kilodaltons (kDa) immunoprecipitated from normal chicken cells with antibodies raised against the amino-terminal portion of the viral yes (v-yes) protein expressed in bacteria. By a number of criteria we show that these proteins are clo...