Hierarchical regulation by casein kinases I and II of the activation of protein phosphatase-1i by glycogen synthase kinase-3 is ionic strength dependent.

The roles of casein kinases I and II in the activation of protein phosphatase-1i (PP-1i) by glycogen synthase kinase-3 (GSK-3) were studied using enzyme preparations from porcine heart. PP-1i was activated by GSK-3 and the levels of activation achieved decreased by increasing the ionic strength (0-0.2 M KCl) in the incubation mixtures. At low ionic strength (no KCl added) casein kinase II increased the rate of activation of PP-1i by GSK-3 and the activation proceeded to a slightly greater extent (110-120%) than that obtained by GSK-3 alone. In the presence of 0.14 M KCl only a partial activation of PP-1i by GSK-3 was observed, but when casein kinase II was also added activation was restored to levels observed when PP-1i was activated by GSK-3 in the absence of salt. This effect was shown to be dependent on the concentration of casein kinase II. These results would imply that at low ionic strength casein kinase II and GSK-3 synergistically activate PP-1i as has been previously reported for the rabbit skeletal muscle enzyme (DePaoli-Roach, A. A., J. Biol. Chem. 259, 12144-12152, 1984), whereas, at physiological ionic strength, casein kinase II action may be obligatory for GSK-3 activity. Similar results were obtained when casein kinase I replaced casein kinase II.