The protein phosphatases involved in cellular regulation. Evidence that dephosphorylation of glycogen phosphorylase and glycogen synthase in the glycogen and microsomal fractions of rat liver are catalysed by the same enzyme: protein phosphatase-1

Abstract

Glycogen synthase (labelled in sites‐3) and glycogen phosphorylase from rabbit skeletal muscle were used as substrates to investigate the nature of the protein phosphatases that act on these proteins in the glycogen and microsomal fractions of rat liver. Under the assay conditions employed, glycogen synthase phosphatase and phosphorylase phosphatase activities in both subcellular fractions could be inhibited 80–90% by inhibitor‐1 or inhibitor‐2, and the concentrations required for half‐maximal inhibition were similar. Glycogen synthase phosphatase and phosphorylase phosphatase activities coeluted from Sephadex G‐100 as broad peaks, stretching from the void volume to an apparent molecular mass of about 50 kDa. Incubation with trypsin decreased the apparent molecular mass of both activities to about 35 kDa, and decreased their I₅₀ for inhibitors‐1 and ‐2 in an identical manner. After tryptic digestion, the I₅₀ values for inhibitors‐1 and ‐2 were very similar to those of the catalytic subunit of protein phosphatase‐1 from rabbit skeletal muscle. The glycogen and microsomal fractions of rat liver dephosphorylated the β‐subunit of phosphorylase kinase much faster than the α‐subunit and dephosphorylation of the β‐subunit was prevented by the same concentrations of inhibitor‐1 and inhibitor‐2 that were required to inhibit the dephosphorylation of phosphorylase. The same experiments performed with the glycogen plus microsomal fraction from rabbit skeletal muscle revealed that the properties of glycogen synthase phosphatase and phosphorylase phosphatase were very similar to the corresponding activities in the hepatic glycogen fraction, except that the two activities coeluted as sharp peaks near the void volume of Sephadex G‐100 (before tryptic digestion). Tryptic digestion of the hepatic glycogen and microsomal fractions increased phosphorylase phosphatase about threefold, but decreased glycogen synthase phosphatase activity. Similar results were obtained with the glycogen plus microsomal fraction from rabbit skeletal muscle or the glycogen‐bound form of protein phosphatase‐1 purified to homogeneity from the same tissue. Therefore the divergent effects of trypsin on glycogen synthase phosphatase and phosphorylase phosphatase activities are an intrinsic property of protein phosphatase‐1. It is concluded that the major protein phosphatase in both the glycogen and microsomal fractions of rat liver is a form of protein phosphatase‐1, and that this enzyme accounts for virtually all the glycogen synthase phosphatase and phosphorylase phosphatase activity associated with these subcellular fractions.