Phosphorylation of rat ascites tumor non-histone chromatin proteins. Differential phosphorylation by two cyclic nucleotide-independent protein kinases and comparison to in vivo phosphorylation.

The substrate specificity of two cyclic nucleotide-independent protein kinases purified from both rat ascites tumor cells and calf thymus has been examined. The protein kinases, designated casein kinase I and II, were purified to near homogeneity and are free of contaminating protein substrates. Non-histone chromatin proteins, purified from Novikoff ascites tumor cells and free of protein kinase and activity, were phosphorylated with casein kinases I and II and the spectrum of labeled polypeptides determined. Phosphorylation of non-histone chromatin proteins with casein kinase I from either source, resulted in the same pattern of 32P-labeled polypeptides when analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis. Similarly, casein kinase II from either source phosphorylated an identical set of polypeptides. The distribution of 32P-labeled non-histone chromatin proteins phosphorylated by casein kinase I differed from that labeled by casein kinase II suggesting that each protein kinase phosphorylates a specific subset of non-histone chromatin proteins. The differences in protein substrate specificity of casein kinase I and casein kinase II was confirmed by two-dimensional polyacrylamide gel electrophoresis. Non-histone chromatin proteins, labeled with 32P in vivo, were analyzed by the same methods and the distribution of labeled polypeptides compared to in vitro phosphorylation patterns. A number of prominent in vitro phosphorylated polypeptides appear identical with non-histone chromatin proteins labeled with 32P in vivo. These results suggest that casein kinases I and II may function in vivo in the phosphorylation of non-histone chromatin proteins.