Bovine thymus nuclei contain a species of protein phosphatase-1 (PP-1N alpha) that can be partially activated by phosphorylation of an associated inhibitory polypeptide, NIPP-1, with protein kinase A [Beullens, Van Eynde, Bollen and Stalmans (1993) J. Biol. Chem. 268, 13172-13177]. Here it is shown that PP-1N alpha can also be activated 4-fold by phosphorylation of NIPP-1 with casein kinase-2. The effects of protein kinase A and casein kinase-2 were additive, yielding an enzyme with an activity close to that of the free catalytic subunit. Casein kinase-2 introduced up to 1.2 phosphate groups into purified NIPP-1 on serine and threonine residues. This phosphorylation was associated with a 14-fold increase in the concentration of NIPP-1 required for 50% inhibition of the type-1 catalytic subunit. The kinase-mediated inactivation of NIPP-1 could be reversed by incubation with the catalytic subunit of protein phosphatase-2A.

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