Effect of bivalent cations on rat liver cytosol casein (glycogen synthase) kinases 1 and 2. Influence of the protein substrate.

Rat liver cytosol casein kinases 1 and 2 were stimulated by free Mg2+, but the optimal concentration of cation varied with both the casein kinase and the protein substrate used. Mn2+, but neither Ca2+ nor Zn2+, could efficiently substitute for Mg2+ in forming the bivalent-cation-ATP complex used as substrate, but free Mn2+ was inhibitory. The magnitude of these effects depended on the type of casein kinase and the protein substrate used. These results support the idea that, besides the effects of Mg2+ as a component of the Mg-ATP complex, or through interaction with the protein substrate, free Mg2+ is an allosteric effector of both casein kinases.