Autophosphorylation-activated protein kinase phosphorylates and inactivates protein phosphatase 2A.

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Abstract
Purified preparations of a distinct autophosphorylation-activated protein kinase from bovine kidney phosphorylated and inactivated purified preparations of protein phosphatase 2A2 (PP2A2) by about 80% with the autophosphorylation-activated protein kinase, protamine kinase, and 32P-labeled myelin basic protein as substrates. Analysis of incubations performed in the presence of 0.2 mM [gamma-32P]ATP by autoradiography following SDS/PAGE and by FPLC gel permeation chromatography on Superose 12 demonstrated that the catalytic subunit of PP2A2 was phosphorylated in the incubation mixtures containing the kinase and phosphatase. Up to 0.3 mol of phosphate groups was incorporated per mol of the catalytic subunit of PP2A2 following incubation with the kinase. This phosphorylation was enhanced about 5-fold in the presence of 0.4 microM microcystin-LR. In addition, up to 1 mol of phosphate groups was incorporated per mol of the PP2A2 subunit of apparent M(r) approximately 60,000 when microcystin-LR was included. Analysis by thin-layer chromatography indicated that PP2A2 catalyzed an autodephosphorylation reaction which was inhibited by microcystin-LR. Phospho amino acid analysis showed that the catalytic subunit of PP2A2 was phosphorylated on threonine residues by the autophosphorylation-activated protein kinase. Together with previous observations, the results suggest that inactivation of PP2A by phosphorylation catalyzed by the autophosphorylation-activated protein kinase could contribute to the marked increase in the phosphorylation of cellular proteins in response to insulin and other mitogens.