Axial Period of Actin Filaments: X-ray Diffraction Studies

Abstract

ACTIN filaments appear to be an essential part of the contractile apparatus in all types of muscle. Electron microscope investigations of negatively stained material isolated from a variety of muscles have suggested that these filaments (about 80 Å in diameter) consist of two helically wound strands of globular units about 55 Å in diameter, each unit probably representing one monomer of actin¹. This structural picture of actin filaments is in essential agreement with earlier X-ray diffraction studies on whole muscle by Selby and Bear². Their analysis, however, could not specify the length of the long period (that is, the pitch) of each of the helically wound strands. On the assumption that there is an integral number of monomeric units in each turn of this helix, the long period could be either 2 × 350 Å (thirteen units per turn) or 2 × 410 Å (fifteen units per turn). But there is, in fact, no reason to assume that the number of monomeric units in each turn is integral; the pitch of the helix could also lie anywhere near or between 2 × 350 and 2 × 410 Å.