Serine/threonine‐specific protein kinase activity associated with viral pp60src protein

Three different types of experiments are presented in this paper, the results of which converge to indicate that the viral src protein associates with and modulates the activity and/or the specificity of a serine/threonine protein kinase. Firstly, a 60‐kDa protein from extracts of FR3T3 rat fibroblasts transformed by wild‐type Rous sarcoma virus (SRD‐FR3T3) is shown to be immunoprecipitated with a monoclonal antibody (mAb) raised against bacterially produced pp60v‐src, the mAb327 [Lipsich, L. A., Lewis, A. J. & Brugge, J. S. (1983) J. Virol. 48, 352–360] and to be phosphorylated in vitro at serine/threonine/tyrosine residues, in the ratio 25:53:22. Under the same experimental conditions, the pp60c‐src protein immunoprecipitated with mAb327 from extracts of NIH c‐src overexpresser cells is phosphorylated exclusively on tyrosine residues. Secondly, the results of immunoprecipitation experiments using a tumor‐bearing rabbit (TBR) serum and reported in an earlier work [David‐Pfeuty, T. & Hovanessian, A. (1984) Eur. J. Biochem. 140, 325–342], together with those reported here, suggest that the TBR‐immunoprecipitated pp60v‐src coprecipitates with a cellular protein related to the 60‐kDa subunit of the Ca2+/calmodulin protein kinase II from brain. Finally, partially purified preparations of pp60v‐src, but not of pp60c‐src, are shown to contain a Ca2+/calmodulin‐dependent protein kinase activity that phosphorylates a 52‐kDa protein substrate.