Identification of three in vivo phosphorylation sites on the glycogen-binding subunit of protein phosphatase 1 from rabbit skeletal muscle, and their response to adrenaline

The in vivo phosphorylation stoichiometries of 4 serines on the glycogen-binding (G)-subunit of protein phosphatase 1 (PP1) have been determined. In fed rabbits injected with propranolol stoichiometries (molmol) were: site 1 (0.67 ± 0.09), site 2 (0.20 ± 0.07), site 3a (0.23 ± 0.01) and site 3b (0). After injection with adrenalin they became: site 1 (0.90±0.02), site 2 (0.72 ± 0.01), site 3a (0.23 ± 0.02) and site 3b (0). These results, together with other data, establish that site 2 phosphorylation by cyclic AMP-dependent protein kinase triggers dissociation of PP1 from the G-subunit in vivo. They also demonstrate that a residue phosphorylated in vitro by glycogen synthase kinase 3 (site 3a) is phosphorylated in vivo.