Three-dimensional image analysis of the complex of thin filaments and myosin molecules from skeletal muscle. II. The multi-domain structure of actin-myosin S1 complex.

A three-dimensional image of the "rigor" complex of actin and chymotryptic myosin subfragment-1 (S1) was reconstituted from electron micrographs of specimens embedded in unbroken and unbacked stain sheets of uranyl acetate over the holes of perforated carbon films to an effective resolution of 20 A radially and 26 A axially. The morphological unit of actin-S1 complex consists of at least three domains and myosin S1 shows a multi-domain submolecular structure. Possible ways to assign actin to one or more of these three domains are discussed. Two candidates for the shape of S1 molecule are also shown. Both candidates have a complex "embryo"-like shape. The solid model of the actin-S1 complex appears to be far less polar than that shown in the original electron micrographs or the projected density map of the reconstituted image shown by Toyoshima and Wakabayashi (1). The conspicuous polarity of the arrowhead pattern is related to the projected image of the spiral shape of the main part of the S1 molecule, which is almost at right angles to the helix axis. As one way to reconcile the non-tilted configuration of S1 in the rigor complex with sliding theory, the possibility of a pivoting mechanism (rotation of S1 head in the horizontal plane normal to the helix axis rather than in the vertical plane parallel to the helix axis) is discussed.