Phosphorylase kinase (EC 188.8.131.52) from rabbit skeletal muscle catalyzed the phosphorylation of κ-casein (Mr = 19,000) to a stoichiometry of approximately one mole of phosphate per mole of κ-casein. The reaction rate was modified by several factors known to influence phosphorylase kinase activity: (1) stimulation by Ca2+, (2) further stimulation by added calmodulin in the presence of Ca2+, (3) higher activity at pH 8.2 than at pH 6.8, and (4) activation, measured at pH 6.8, following partial proteolysis of the kinase by trypsin. The maximal rate of phosphorylation of κ-casein by nonactivated phosphorylase kinase in the presence of Ca2+ at pH 8.2 was 16 nmol/min/mg, and the κ-casein concentration for half-maximal activity was 80 μm. The phosphorylation of other components of whole casein, αs1-casein and β-casein, by phosphorylase kinase was detectable but much slower than the reaction with κ-casein. When whole casein was used as a substrate, κ-casein was identified by electrophoresis as a major phosphorylated species. The amino acid residues in κ-casein modified by phosphorylase kinase were shown to be serines. The present work extends the known substrates of phosphorylase kinase to include a well-characterized protein that may prove an interesting model substrate. Furthermore, this report emphasizes that whole casein, because of its heterogeneity, is a poor substrate to use in characterizing the substrate specificities of protein kinases, since different "casein kinases" may be specific for different components of whole casein.
ASJC Scopus subject areas
- Molecular Biology