Bai G, Zhang Z J, Werner R, Nuttall F Q, Tan A W, Lee E Y
Department of Biochemistry and Molecular Biology, University of Miami School of Medicine, Florida 33101.
J Biol Chem. 1990 May 15;265(14):7843-8.
The cDNA for rat liver glycogen synthase was isolated by screening a rat liver cDNA library constructed in lambda gt11. 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.
通过筛选构建于λgt11中的大鼠肝脏cDNA文库，分离得到了大鼠肝脏糖原合酶的cDNA。该cDNA长度为2.4千碱基，编码一个由703个氨基酸残基组成、分子量为80.5 kDa的蛋白质。大鼠肝脏和人类肌肉序列的比较表明，与具有80%同一性的内部序列相比，氨基末端和羧基末端区域差异较大。大鼠肝脏的羧基末端区域截短了33个残基，与肌肉序列的同一性仅为46%，但保留了疏水氨基酸含量低（13%）这一共同特征。肝脏序列中不存在作为cAMP依赖性蛋白激酶磷酸化主要靶点的磷酸化位点1a和1b。肝脏和肌肉糖原合酶中这些不同的、结构异常的羧基末端区域的存在表明，它们不需要拥有与蛋白质核心结构完整的三级结构。提出了一个模型，其中该区域与催化核心相互作用以维持I状态，并且磷酸化作用使这种相互作用解偶联。
