Multisite phosphorylation of the glycogen‐binding subunit of protein phosphatase‐1G by cyclic AMP‐dependent protein kinase and glycogen synthase kinase‐3

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Abstract
The glycogen‐binding (G) subunit of protein phosphatase‐1G is phosphorylated stoichiometrically by glycogen synthase kinase‐3 (GSK3), and with a greater catalytic efficiency than glycogen synthase, but only after prior phosphorylation by cyclic AMP‐dependent protein kinase (A‐kinase) at site 1. The residues phosphorylated are the first two serines in the sequence AIFKPGFSPQPSRRGS‐, while the C‐terminal serine (site 1) is one of the two residues phosphorylated by A‐kinase. These findings demonstrate that (i) the G subunit undergoes multisite phosphorylation in vitro; (ii) phosphorylation by GSK3 requires the presence of a C‐terminal phosphoserine residue; (iii) GSK3 can synergise with protein kinases other than casein kinase‐2.