Rigor crossbridge structure in tilted single filament layers and flared-X formations from insect flight muscle.
The averaged structure of rigor crossbridges in insect flight muscle has been studied in filtered images. Their three-dimensional structure has been deduced by relating tilt views of single filament layers in 25 nm longitudinal sections (myac layers and actin layers) to the flared-X appearance in 15 nm cross-sections showing single crossbridge levels. Tilting myac or actin layers around the filament axis makes crossbridges show one of two patterns. Beadlike densities appear either singly over thin filaments ("center-beading") or doubled and flanking thin filaments ("straddle-beading"). These express two different projections from the crossbridge-actin complexes as seen end-on in flared-X formations. Tannic acid/glutaraldehyde fixation gave improved actin preservation, showing, in 15 nm cross-sections, the long-pitch helical strands as "two-dot" profiles of consistent azimuth in the gaps between double chevrons. The azimuth in the flared-X arms was then inferred from lattice relationships, since it was not seen directly. The tangential attachment of comma-shaped crossbridges to the inferred actin dyad fits the binding geometry in recent actin-subfragment 1 complex reconstructions. However, averaged crossbridge structure differs between lead and rear members of double chevrons, unlike the uniform heads on decorated actin. In filtered images of myac layers, the lead bridges are dense and steeply angled; the rear chevron is seen as a dense bead over the thin filament with faint, less angled bars extending laterally. Actin layer images also suggest that rear and lead bridges differ in angle. Left and right flared-X arms are end-on views of lead and rear chevron bridges, respectively, and differ in shape. Improved fixation with tannic acid/glutaraldehyde allows us to distinguish three crossbridge domains in flared-X arms: (1) a dense bulb-like head merged into the thin filament; (2) a dense but thinner neck tangential to actin; and (3) a faint thin stem joining the necks to myosin filaments. Shape differences in lead and rear members between the head-neck-actin complexes are indicated by the names "L sigmoid" and "R dogleg". Within crossbridges, internal angles between the head-neck axis and the head-actin-head axis differ between sigmoid and dogleg by about 30 degrees, implying a flexible junction between bridge-head and bridge-neck. Lead and rear bridges are axially at least 13 nm apart on actin; the expected 60 degrees difference in azimuth is expressed by head-neck portions, but the head-actin-head axis rotates by only 30 degrees.(ABSTRACT TRUNCATED AT 400 WORDS)
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