Partial purification of a tyrosine kinase activity associated with a 28-kDa phosphoprotein from the particulate fraction of rat spleen.

A phosphoprotein with associated tyrosine kinase activity has been purified from a 26,000 x g particulate fraction of rat spleen. The molecular weight of the native kinase is 85-90 kDa as estimated by gel permeation chromatography on Superdex 200 while SDS/PAGE of an autophosphorylated sample indicated a single 28-kDa band in which the radiolabel was alkali-stable. The SDS/PAGE following the incubation with 5'-p-fluorosulfonylbenzoyl[8-14C]adenosine reveals a single labeled band migrating at a molecular weight of approximately 28 kDa. The enzyme shows very restricted substrate specificity. Casein and, to a lesser degree, the random polypeptide poly(Tyr1,Glu4) were phosphorylated. The enzyme exhibits a preference for Mn2+ over Mg2+ as an activator.