Purification and characterization of a possible protooncogene fjln product, P59^fyn, from a rat brain particulate fraction

Four tyrosine-protein kinases that reacted with antibodies specific to p62^c-yes, p60^c-src, p60^c-src, and p59^fyn, respectively, were solubilized from a rat brain particulate fraction and separated by casein-Toyopearl column chromatography. Possible p59^fyn, with a pI of 6.5, was purified 490-fold as a single 69-kDa protein band on SDS-PAGE. The purified enzyme contained almost no phosphotyrosine residues but was autophosphorylated with Mg²⁺-·ATP exclusively at tyrosine residues, with a concomitant increase in the kinase activity toward tyrosine-glutamate (1 : 4) copolymers. The rate of the copolymer phosphorylation was proportional to the square of the enzyme concentration, suggesting activation through intermolecular catalysis. In the presence of Mn²⁺, however, the reaction showed a firstorder dependence on the enzyme concentration.