Substructure of the thick filament of vertebrate striated muscle

Abstract

The number of myosin molecules per 143 Å cross bridge repeat in the thick filaments of rabbit psoas muscle was determined by two independent methods. First the molecular weight of isolated thick filaments was evaluated using a modification of the conventional particle counting technique. Isolated thick filaments, fixed with 2% formaldehyde, were sedimented onto electron microscope grids and the number per unit volume estimated. From this information and the weight concentration we find the molecular weight of the vertebrate thick filament to be 204 ± 11 × 10 6. When the weight fraction of C-protein is taken into account, the number of myosin molecules per axial repeat is 4.3 ± 0.2. The second determination is based on the number ratio of myosin cross bridge repeats to actin monomer repeats in the myofibrillar structure. A weight ratio for myosin:actin of 2.65 ± 0.04 was obtained from analysis of the sodium dodecyl sulfate gel electrophoresis patterns of glycerinated muscle giving 3.9 ± 0.3 myosin molecules per axial repeat. Thus our combined results strongly favor four myosin molecules per 143 Å repeat in vertebrate striated muscle. We have also estimated a myosin: C-protein weight ratio of 27.2 ± 1.1 from sodium dodecyl sulfate gel electrophoresis experiments of glycerinated muscle. This value, coupled with the molecular weight of the thick filament, indicates that 2.9 ± 0.2 C-protein molecules are associated with each of the 18 C-protein stripes flanking the M-line region of the thick filament.