Purification and characterization of a pp60^c-src-related tyrosine kinase that effectively phosphorylates a synthetic peptide derived from p34^cdc2

A protein tyrosine kinase has been purified from the particulate fraction of bovine spleen to a specific activity of 0.217 μmol/min/mg at 100 μM ATP and 3 mM [Val⁵] angiotensin II. Both the angiotensin phosphorylation activity and immunoreactivity towards an antibody preparation raised against a synthetic peptide containing the autophosphorylation site of pp60^c-src, Cys-src(403-421), were monitored during the purification. The purified sample displayed three closely spaced protein bands with molecular weights of 50-55 kDa on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. All bands could be phosphorylated exclusively on tyrosine residues under autophosphorylation conditions. All reacted on immunoblots with an antibody raised against a synthetic peptide corresponding to the consensus autophosphorylation site of members of the pp60^c-src family of tyrosine kinases. Tryptic phosphopeptide maps of the three proteins were essentially indistinguishable. The results suggest that the purified enzyme preparation contained mainly three closely related pp60^c-src-family protein tyrosine kinases or a pp60^src-family protein tyrosine kinase modified posttranslationally to give three closely spaced protein bands on sodium dodecyl sulfate gel. Neither of these proteins appears to be pp60^c-src or p56^lck. The spleen protein tyrosine kinase was found to phosphorylate a p34^cdc2 kinase peptide, Cys-cdc2(8-20), which contained the regulatory tyrosine residue Tyr-15 about 20 times better than [Val⁵]angiotensin II or Cys-src(403-421) peptide at a peptide substrate concentration of 1 mM. In contrast, epidermal growth factor receptor kinase partially purified from A431 cells did not show preference for Cys-cdc2(8-20) as its substrate. Although Cys-cdc2(8-20) contained two tyrosine residues, only the tyrosine corresponding to Tyr-15 in p34^cdc2 was phosphorylated by the spleen tyrosine kinase. The observation suggests that the primary structure surrounding Tyr-15 of p34^cdc2 contains substrate structural determinants specific for the spleen tyrosine kinase.