X-ray scattering by myosin S-1: implications for the steric blocking model of muscle control

Abstract

Steric blocking and unblocking^1–3 of the myosin interaction sites in muscle thin filaments by tropomyosin provides a simple yet elegant method of regulating contraction in skeletal muscle. Recently, this model was challenged⁴ by the finding that tropomyosin is located on the opposite side of the thin filament helix axis from where Moore et al. reported that the myosin cross-bridge, S-1, bound⁵. However, new three-dimensional image reconstructions of electron micrographs of thin filaments decorated with S-1 were obtained by Taylor and Amos⁶, who proposed that the prior assignment of the S-1 binding site of Moore et al.⁵ was incorrect, and that consequently tropomyosin and S-1 are located on the same side of the actin helix. These past difficulties and the current lack of agreement⁷ on the location of the S-1 binding site suggested that another technique would be useful. We report here that the shape of S-1, derived from reconstructions using the Taylor and Amos actomyosin interpretation, gives a better fit to the X-ray scattering pattern observed from S-1 in solution than alternative interpretations.