The glycogen‐binding subunit of protein phosphatase‐1g from rabbit skeletal muscle: Further characterisation of its structure and glycogen‐binding properties

Abstract

The glycogen‐bound form of protein phosphatase‐1 (PP‐1g) was previously purified as a heterodimer composed of a 37‐kDa catalytic (C) subunit and a proteolytically sensitive 103‐kDa glycogen‐binding (G) subunit [Stråhlfors, P., Hiraga, A. & Cohen, P. (1985) Eur. J. Biochem. 149, 295–303]. In this paper we demonstrate by a variety of criteria that the intact G subunit is a 161‐kDa protein, and that the 103‐kDa species (now termed G′) is itself a product of proteolysis. A second phosphorylation site for cAMP‐dependent protein kinase (termed site 2) was identified on the G subunit. The site 2 serine was phosphorylated at a comparable rate to site 1, and near stoichiometric phosphorylation could ..