The substructure of heavy meromyosin. The effect of Ca2+ and Mg2+ on the tryptic fragmentation of heavy meromyosin.

Heavy meromyosin, obtained by tryptic digestion of myosin, containing two main polypeptides whose masses were estimated as 81,000 and 74,000 dlatons from Na dodecyl-SO4 polyacrylamide gel electrophoresis, was further digested with trypsin. The Ca2+-activated ATPase activity remainded unchanged and the K+-EDTA activity increased while various smaller fragments were formed. The formation of some of these fragments is affected by Ca2+ or Mg2+ as first shown by Bálint et al. (Bálint, M., Schaefer, A., Biro, N. A., Menczel, L., AND Fejes, E. (1971) J. Physiol. Chem. Phys. 3, 455). On the basis of the time course of the appearance of fragments the following relationship emerges: see article. The 64K leads to 60K step is inhibited by divalent cations, while the breakdown of the 74K fragment is accelerated. The effect of Ca2+ was maximal at 0 similar to 0.1 muM, that of Mg2+ at 10 muM. The original light chains of myosin are not present in the heavy meromyosin serving as the starting material, but peptide material appears on electrophoresis in positions starting material, but peptide material appears on electrophoresis in positions where the light chains would be found. The fragments marked by an asterisk are considered to ba alpha-helical on the basis of their solubility at low ionic strength after precipitation with ethanol (Bálint et al.). The fact that alpha helical fragments are derived from the 60,000-dalton fragment indicateds that it is adjacent to the light meromyosin in the intact myosin while the 74,000- dalton fragment would be part of heavy meromysoin subfragment 1. Chromatography of Sephadex G-200 separates fractions with ATPase activity corresponding to heavy meromyosin and heavy meromyosin subfragment 1. Electrophoresis of these Sephadex fractions suggests that the main peptide constituting heavy meromysoin subfragment 1 is connected by noncobalent forces to a portion of the rod that is not immediately adjacent to it in the primary sequence. The significance of this finding is discussed in terms of the flexibility of the myosin head.