The proteolytic substructure of light meromyosin. Localization of a region responsible for the low ionic strength insolubility of myosin.

Light meromyosin (LMM), prepared by limited tryptic digestion of myosin, usually contains several polypeptide chains, LMM-A, LMM-B, and LMM-C in decreasing order of molecular weight estimated from sodium dodecyl sulfate-gel electrophoresis. Further limited tryptic digestion of LMM produces well defined fragments (Balint, M., Szilagyi, L., Fekete, Gy., Blazso, M., and Biro, E. N. A. J. Mol. Biol. (1968) 37, 317-330). Fragments LF-1, LMM-D, LF-2, and LF-3, with chain masses equal to 63, 56, 47, and 30 kDa, respectively, have been isolated by column chromatography. Based on the time course of the changes in the sodium dodecyl sulfate-gel pattern of the digests, chain masses estimated from sodium dodecyl sulfate-gel electrophoresis, and the NH2- and COOH-terminal sequences of the isolated peptides, the following scheme can be deduced. Formula; see text. C and N over the arrows indicate removal of residues from the COOH and NH2 terminus, respectively. The positions of the peptides along the myosin heavy chain have been established by comparison with the published primary structures of rabbit skeletal (Elzinga, M., Behar, K., Walton, G., and Trus, B. L. (1980) Fed. Proc. 33, 1579) and nematode myosin (McLachlan, A. D., and Karn, J. (1982) Nature (Lond.) 299, 226-231). LMM and fragment LMM-D are insoluble, whereas LF-1, LF-2, and LF-3 are soluble at low ionic strength. Their solubility properties, in conjunction with their locations along the myosin heavy chain, suggest that a relatively small stretch of peptide (chain weight, 5,000 Da) located about 100 residues from the COOH terminus of myosin heavy chain is responsible for the insolubility of LMM at low ionic strength.