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Abstract

Analysis of the rigor complex of muscle thin filaments decorated with myosin subfragment-1 (S-1) by three-dimensional image reconstruction from electron micrographs1 shows S-1 to be a comma-shaped molecule, the broad head of which interacts with F-actin near the groove between the two strands of actin monomers. Increased resolution near the filament axis has allowed us to distinguish separate components more reliably than in an earlier analysis2. Using X-ray diffraction data from striated muscle decorated with S-1 molecules3 as a reference, we have now refined the electron microscope data to show the interaction in more detail. We show here that individual S-1 molecules apparently interact with both strands of a filament, straddling the regulatory protein tropomyosin in the long-pitch groove. This double-sited interaction agrees with chemical cross-linking data4 and suggests some interesting possibilities for cross-bridge cycling in muscle.