STUDIES ON PROTEINS AND PROTEIN COMPLEXES OF MUSCLE BY MEANS OF PROTEOLYSIS .V. FRAGMENTATION OF LIGHT MEROMYOSIN BY TRYPSIN

The tryptic digestion of light meromysin and of myosin at pH 8.2 in 0.6 m-KCl, in 0.02 m-KCl and in 0.02 m-KCl-0.01 m-CaCl2 respectively was followed by disc electrophoresis. It has been found in our experiments that three kinds of electrophoretically distinct fragments are formed from light meromyosin by prolonged trypsin treatment. Changes of ionic environment brought about changes in the rate of the over-all process, but the fragments formed remained the same. The fragments arise in a consecutive manner and there are no indications of any intermediary forms, as shown by the changes of electrophoregrams with time as well as by digestion experiments carried out with the isolated subfragments. The same three kinds of subfragments are formed from light meromyosin fraction 1. When light meromyosin is digested with chymotrypsin, a fraction comparable in electrophoretic behaviour to the first tryptic subfragment is first formed, but later a large number of components are observed, and there is a very substantial decrease in the protein material present. All these observations can be interpreted by assuming that the fragments are formed by successive cleavage at distinct regions along the length of the light meromyosin molecule. The three fragments were isolated by gel filtration on Sephadex G 200 and some of their molecular parameters were measured. The Cotton effect at 233 mμ indicated that all three subfragments were 75 to 85% helical. Molecular weights of the successive fragments (determined by the Archibald method) were: 112,000, 84,000 and 56,000, with the length of the molecules 750, 580 and 410 Å, respectively, calculated from their intrinsic viscosity. On the basis of these data, taken in conjunction with the results of other authors, we suggest a hypothetical model in which a periodic structure is assumed for the entire shaft of the myosin molecule. © 1968.