Symmetry and three-dimensional arrangement of filaments in vertebrate striated muscle

Abstract

A theoretical analysis is given of the three-dimensional geometry of the arrangement of actin and myosin filaments in vertebrate striated muscle. Three notable points arise out of this analysis: 1. (i) Only one of the possible models for the symmetry of the myosin filaments, that with a three-stranded helical arrangement of myosin heads, seems to provide an obvious explanation for the occurrence of the observed myosin filament superlattice. 2. (ii) In the almost square Z-line lattice only two alternative regular arrangements of equivalent actin filaments are possible. One of these, that with all the actin filaments identically oriented, seems the more likely to be correct. The same two arrangements are also the only two actin arrangements which could fit regularly into the myosin superlattice. 3. (iii) Taking the myosin filaments to be three-stranded for reasons given in (i) and taking the possible actin arrangements to be as in (ii), then the geometry of myosin cross-bridge interactions with actin can be analysed. On this basis, by analogy with previous results from rigor insect flight muscle, it is likely that in rigor vertebrate striated muscle, only about two thirds of the myosin cross-bridges are attached to actin.