Isolation and identification of two proto‐oncogene products related to c‐fgr and fyn in a tyrosine‐protein‐kinase fraction of rat spleen

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Abstract
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, Q1 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 x mg-1 and 579 nmol.min-1 x 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 Q1 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.