The structure and disposition of crossbridges in deep-etched fish muscle

Deep-etching and rotary-shadowing techniques were used to describe crossbridges in fish (Chanda ranga) muscle, relaxed and in iodoacetate rigor conditions. Three major fracture planes from rigor muscle were studied using stereomicroscopy and Fourier image analysis. The 1,0 plane reveals alternating thick and thin filaments with the thick filaments frontmost in the fracture and the thin filaments in the recessed plane. All crossbridges coming from the frontmost thick filaments are visible on actin filaments in the 1,0 plane. Fourier transforms of digitized images from these fracture planes exhibit axial periodicities of 14 and 36 nm. The actin layer, a fracture plane just below the myosin filaments in the 1,0 plane, shows end-on views of crossbridges projecting out of the fracture plane and limited transverse alignment of crossbridges. Actin layer Fourier transforms demonstrate a 14 nm reflection associated with the attachment of crossbridges with a mean axial periodicity determined by their myosin origins. The 1,1 lattice direction shows pairs of thin filaments alternated with single thick filaments. In this view, all crossbridges coming from three adjacent myosins are visible. In all fracture planes, decoration of individual thin filaments by crossbridges is variable, but usually one (singlet) or two (doublet) closely spaced crossbridges mark each actin target zone, at intervals of 35-38 nm. Counts of crossbridges decorating actin filaments give an average of four every three target zones. The anticipated stagger of target zones for crossbridges from two adjacent myosin filaments is observed. Alignment of actin target zones across the sarcomere is good. We can distinguish two distinct shapes for rigor crossbridges: a narrow, straight bridge and a wider bridge with a triangular shape. We interpret these as being the appearance of crossbridges with one or two S1 subfragments (single and double headed) respectively. Comparison between rigor and relaxed structures indicates attachment of all crossbridges in rigor.