PROTEIN PHOSPHATASE 2A1 IS THE MAJOR ENZYME IN VERTEBRATE CELL-EXTRACTS THAT DEPHOSPHORYLATES SEVERAL PHYSIOLOGICAL SUBSTRATES FOR CYCLIN-DEPENDENT PROTEIN-KINASES

Okadaic acid (2 nM) inhibited by 80-90% the protein phosphatase activi ties in diluted extracts of rat liver, human fibroblasts, and Xenopus eggs acting on three substrates (high mobility group protein-I(Y), cal desmon and histone H1) phosphorylated by a cyclin-dependent protein ki nase (CDK) suggesting that a type-2A phosphatase was responsible for d ephosphorylating each protein. This result was confirmed by anion exch ange chromatography of rat liver and Xenopus extracts, which demonstra ted that the phosphatases acting on these substrates coeluted with the two major species of protein phosphatase 2A, termed PP2A1 and PP2A2. When matched for activity toward glycogen phosphorylase, PP2A1 was fiv e- to sevenfold more active than PP2A2 and 35-fold to 70-fold more act ive than the free catalytic subunit (PP2A(C)) toward the three CDK-lab eled substrates. Protein phosphatases 1, 2B, and 2C accounted for a ne gligible proportion of the activity toward each substrate under the as say conditions examined. The results suggest that PP2A, is the phospha tase that dephosphorylates a number of CDK substrates in vivo and indi cate that the A and B subunits that are associated with PP2A(C) in PP2 A1 accelerate the dephosphorylation of CDK substrates, while suppressi ng the dephosphorylation of most other proteins. The possibility that PP2A, activity is regulated during the cell cycle is discussed.