Phosphorylation of the skeletal muscle glycogen-targetting subunit of protein phosphatase 1 in response to adrenaline in vivo

Abstract

The protein G M, which targets protein phosphatase 1 (PP1) to the glycogen particles and sarcoplasmic reticulum (SR) of striated muscles, is known to be phosphorylated at Ser48 and Ser67 in vitro by adenosine 3′,5′ cyclic monophosphate-dependent protein kinase (PKA) and at Ser48 by MAP kinase-activated protein kinase-1 (MAPKAP-K1, also called p90 RSK). The phosphorylation of Ser48 increases the rate at which the glycogen-associated PP1.G M complex dephosphorylates (activates) glycogen synthase, but the phosphorylation of Ser67 has the opposite effect, suppressing the activity of PP1 toward glycogen-bound substrates. The phosphorylation of Ser67 overrides the activating effect of Ser48 phosphorylation because it dissociates PP1 from G M. Here, we use two phospho-specific antibodies to demonstrate that the SR-associated form of G M, as well as the glycogen-associated form of G M, becomes phosphorylated at Ser48 and Ser67 in response to adrenaline, supporting the view that the PKA-mediated regulation of the PP1.G M complex plays a role in the adrenergic control of glycogen metabolism and SR function. In contrast, Ser48 is not phosphorylated significantly in response to insulin, and neither is Ser67. Thus the phosphorylation of G M at Ser48 by MAPKAP-K1 or other insulin-stimulated protein kinases is not involved in the activation of glycogen synthase by insulin.