Different susceptibility of whole casein components to enzymatic phosphorylation by two forms of rat liver 'casein kinase'.

The phosphorylation of the single casein subfractions occurring when whole casein is incubated with [gamma-32P]ATP in the presence of two different rat liver 'casein kinases', both cyclic AMP-insensitive, has been studied. "Casein kinase TS", active on both threonine and serine residues of whole casein, was found to be active towards a minor protein fraction, running slightly ahead of beta-casein during gel electrophoresis, and accounting for most, if not all, of the [32P]Thr residues labeled in whole casein ("[32P]Thr-rich fraction"). The [32P]Ser residues labeled by this enzyme were recovered in an heterogeneous "[32P]Ser-rich fraction" including alphas1-casein together with minor alphas fractions, following alphas1-casein during gel electrophoresis. "Casein kinase S", on the other hand, active only towards serine residues of whole casein, is active almost exclusively towards the minor alphas casein fractions, with the exclusion of both the "[32P]Thr-rich fraction" and alphas1-casein itself. Therefore, of the major casein components, beta- and K-caseins apparently play a quite unimportant role in the overall phosphorylation of whole casein by both the protein kinases tested, while alphas1-casein itself, unlabeled by casein kinase S, accounts for no more than 20--30% of 32P incorporated in the presence of casein kinase TS.