The primary structure of rat liver glycogen synthase deduced by cDNA cloning: Absence of phosphorylation sites 1a and 1b

The cDNA for rat liver glycogen synthase was isolated by screening a rat liver cDNA library constructed in λgt11. The cDNA was 2.4 kilobases in length and encoded a protein of 703 amino acid residues with a molecular mass of 80.5 kDa. Comparison of the rat liver and the human muscle sequences show that the amino- and carboxyl-terminal regions are quite divergent as compared to the internal sequences which show an 80% identity. The rat liver carboxyl-terminal region is truncated by 33 residues and has only 46% identity with the muscle sequence but retains the common feature of a low content of hydrophobic amino acids (13%). Phosphorylation sites 1a and 1b, which are the primary targets for phosphorylation by cAMP-dependent protein kinase, are absent in the liver sequence. The presence of these divergent, structurally anomalous carboxyl-terminal regions in liver and muscle glycogen synthase suggests the absence of the requirement that they possess a tertiary structure that is integral to that of the protein core. A model is proposed in which this region interacts with a catalytic core to maintain the I state, and in which phosphorylation serves to uncouple this interaction.