Identification of a 58,000 Daltons phosphoprotein with tyrosine protein kinase activity in a murine lymphoma cell line

A very high level of tyrosine protein kinase (TPK) activity has been recently detected in a murine lymphoma, induced by Moloney murine leukemia virus. A major endogenous substrate for tyrosine phosphorylation in vitro is a protein of Mr 55-60,000 (p58) associated with the detergent insoluble matrix of LSTRA cells ; in the present work p58 was solubilized, isolated by anion exchange chromatography and then precipitated by antiphosphotyrosine antibodies. Through these steps of isolation, TPK activity was measured by the use of a simplified gastrin phosphorylation assay. It is demonstrated that the TPK activity copurifies with p58, which leads to the conclusion that p58 bears itself the enzymatic activity. Although functionnally similar to other enzymes of this group, this newly characterized TPK does not seem to be closely related to one of the previously documented TPK. This suggests either that this protein is the product of a so far unrecognized cellular TPK gene or that it derives from a rearrangement of one of the previously described TPK genes.