Isolation of a 50 000 dalton cAMP binding protein and its characterization as a regulatory subunit of protein kinase II

An unusual cAMP binding protein of 50 000 Da previously found in human tumors was isolated from HeLa cells in the presence of protease inhibitors. The protein was neutralized by anti-bovine R_II antibodies but not by anti-R_I. It was able to form a dimer, and to inhibit HeLa C kinase in a dose-dependent manner. The HeLa R_II 50 000 was also subject to limited proteolysis and it could be phosphorylated by C kinase. HeLa cells contain two R_I proteins, a predominant 49 000 Da and a minor 51 000 Da isoprotein. In addition, large amounts of a protein consisting of 19 000 and 20 000 Da subunits were isolated by 8-thio-CAMP affinity chromatography that was not immunologically related to the R proteins.