ISOLATION AND IDENTIFICATION OF 2 PROTOONCOGENE PRODUCTS RELATED TO C-FGR AND FYN IN A TYROSINE-PROTEIN-KINASE FRACTION OF RAT SPLEEN

TPK-III, a tyrosine-protein-kinase fraction previously isolated from rat spleen [Brunati, A. M. & Pinna, L. A. (1988) Eur J. Biochem. 172, 451-457] has been further purified and resolved by Mono-Q FPLC into two homogeneous compounds, Ql and Q2, with molecular mass approximately 52 kDa, exhibiting high specific activities for phosphate incorporation into the phosphoacceptor substrate poly(Glu80Tyr20) (1194 nmol.min-1.mg-1 and 579 nmol.min-1.mg-1, respectively). Both Q1 and Q2 appear to be scr-related enzymes since they are readily recognised by anti-SEEP serum raised against the highly conserved segment at positions 330-345 of p60c-scr. Q1, but not Q2, interacts with a specific antibody raised against the N-terminal segment of p55c-fgr. Microsequence analysis of tryptic fragments generated from Ql revealed five peptides which exactly overlap the expected segments of p55c-fgr. Two of these peptides were not entirely conserved in any of the other src-related tyrosine protein kinases. A sixth fragment is very similar, albeit not identical, to the C-terminus of p55c-fgr. Microsequence analysis of two tryptic fragments from the other TPK-III fraction, Q2, provided the sequences FQILNSSE and LTTQETGYIPSNY, which are identical to two segments of fyn not entirely conserved in any of the other src-related tyrosine protein kinases. These results provide evidence that the spleen tyrosine-protein-kinase fraction, conventionally designated TPK-III, is composed of products from two proto-oncogenes, one of which corresponds to fyn, while the other is either identical or very closely related to c-fgr.