Phosphorylation of casein by mitochondrial protein kinase(s)

At least two protein kinase activities are bound to the rat liver mitochondrial membranes. Both activities are found to phosphorylate, besides endogenous proteins tightly bound to the membrane structures, also exogenous phosphoproteins such as casein and phosvitin. However one is able to phosphorylate both casein-bound serine and threonine residues, while the other is phosphorylating almost only serine residues.