Resolution and reassociation of three distinct components from pig heart phosphoprotein phosphatase

Abstract

A partially purified pig heart phosphoprotein phosphatase was dissociated into three distinct components, namely alpha, beta, and gamma, by gel filtration on Sephacryl S-200 followed by chromatography on DEAE-Sephadex in the presence of 6 M urea. Although alpha itself had phosphatase activities toward P-H2B histone, P-H1 histone, phosphorylase a, and glycogen synthase b, beta and gamma had no activity toward these substrates even in the presence of 1 mM Mn2+. The beta component (Mr = 80,000) combined with alpha (Mr = 31,000) in the absence of urea to produce Form 2 (Mr = 123,000) with concomitant increase in P-H1 histone phosphatase activity and Mg2+ requirement for P-H2B histone phosphatase activity (Imazu, M., Imaoka, T., Usui, H., Kinohara, N., and Takeda, M. (1981) J. Biochem. 90, 851-862). The gamma component (Mr = 62,000) reassociated with Form 2 to produce Form 1 (Mr = 199,000) which was similar to the original phosphoprotein phosphatase in substrate specificity and Mg2+ requirement. Binding of gamma to Form 2 strongly suppressed the phosphatase activities toward phosphorylase a and glycogen synthase b with marginal effects on the other phosphatase activities and Mg2+ requirement. However, gamma alone could not associate with alpha. The gamma component was sensitive to treatment with heat (60 degrees C for 2 min) or trypsin and was resistant to treatment with DNase or RNase. The pig heart phosphoprotein phosphatase was further purified to near homogeneity, as judged by polyacrylamide gel electrophoresis. Sodium dodecyl sulfate gel electrophoresis revealed that the purified enzyme (Mr = 171,000) was composed of three polypeptide components, namely alpha', beta', and gamma' with molecular weights of 34,000, 69,000, and 56,000, respectively. The component stoichiometry was determined to be alpha' 1 beta' 1 gamma' 1 by densitometric tracing of the Coomassie blue-stained bands on the acrylamide gel. After dissociation of alpha ' and other components by gel filtration of the purified enzyme on Sephacryl S-200 in the presence of 6 M urea, one alpha ' combined with one beta' to produce Form 2' of Mr = 106,000. Since Form 1 and the purified enzyme as well as Form 2 and Form 2' had similar catalytic properties and s20,w values, respectively, component compositions are suggested to be alpha 1 beta 1 gamma 1 for Form 1 and alpha 1 beta 1 for Form Form 2.