Different susceptibility of whole casein components to enzymatic phosphorylation by two forms of rat liver ‘casein kinase’

Abstract

The phosphorylation of the single casein subfractions occurring when whole casein is incubated with [γ- 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 β-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 α s1-casein together with minor α s fractions, following α s1-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 α s casein fractions with the exclusion of both the “[ 32P]Thr-rich fraction” and α s1-casein itself. Therefore, of the major casein components, β- and κ-caseins apparently play a quite unimportant role in the overall phosphorylation of whole casein by both the protein kinases tested, while α S1-casein itself, unlabeled by casein kinase S, accounts for no more than 20–30% of 32P incorporated in the presence of casein kinase TS.