Myosin light chain and membrane protein phosphorylation in various muscles

Phosphorylation of myosin light chain was compared in various muscles. In 32P-labeled chicken anterior latissimus dorsi and posterior latissimus dorsi the [32P]phosphate content of the 19,000-dalton and 18,000-dalton light chains, respectively, was 1.8-fold higher in muscles tetanized for 5 or 15 s than in the contralateral resting muscles. Similar results had been previously obtained with frog sartorius and semitendinosus muscles. In addition to the radioactive technique, the extent of light chain phosphorylation was also measured by densitometry after separating the phospho and dephospho forms of the light chain with two-dimensional gel electrophoresis. The extent of light chain phosphorylation in tetanized chicken muscles and caffeine-treated frog muscles was not greater than 50%. The extent of phosphorylation of the 19,000-dalton light chain in hearts from several animals (turtle, rat, frog, chicken, dog, and cat) showed major differences. At the extremes it was 76% in turtle and 10% in chicken. the polymorphism of heart light chain was also demonstrated. Not only myosin light chains but also membrane proteins were detected to be phosphorylated. Some of the membrane proteins exhibited increases in phosphorylation during muscle contraction.