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Human Skeletal Muscle Expresses a Glycogen-Targeting Subunit of PP1 That Is Identical to the Insulin-Sensitive Glycogen-Targeting Subunit G_L of Liver

¹ Medical Research Council Protein Phosphorylation Unit, School of Life Sciences,University of Dundee, Dundee, Scotland, U.K.
² Clinical Investigation Unit, Ninewells Hospital Medical School, University of Dundee, Dundee, Scotland, U.K.
³ Department of Human Genetics (Cytogenetics), Ninewells Hospital Medical School, University of Dundee, Dundee, Scotland, U.K.

Insulin has been previously shown to regulate the expression of the hepatic glycogen-targeting subunit, G_L, of protein phosphatase 1 (PP1) and is believed to control the activity of the PP1-G_L complex by modulation of the level of phosphorylase a, which allosterically inhibits the activity of PP1-G_L. These mechanisms contribute to the ability of insulin to increase hepatic glycogen synthesis. Human G_L shows >88% amino acid identity to its rat and mouse homologs, with complete conservation of the phosphorylase a binding site. G_L is highly expressed in the liver and present at appreciable levels in heart tissue of all three species. Surprisingly, G_L is highly expressed in human skeletal muscle while only being detected at very low levels in rat, mouse, and rabbit skeletal muscle. The amino acid sequence of G_L predicted from the cDNA is identical in human liver and skeletal muscle and encoded by a gene on chromosome 8 at p23.1. The species-specific difference in the level of expression of G_L mRNA and protein in skeletal muscle has important implications for understanding the mechanisms by which insulin regulates glycogen synthesis in human skeletal muscle and for questions regarding whether rodents are appropriate models for this purpose.

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