Architecture of the outer arm dynein ATPase in an avian sperm flagellum, with further evidence for the B-link.

Demembranated sperm flagella from Gallus domesticus have been prepared by the rapid-freeze, deep-etch, rotary replica technique in order to study the three-dimensional morphology of the outer dynein arm (ODA)-ATPase complex. In general, the ODAs resemble most closely those from the sea urchin Strongylocentrotus described by W. S. Sale et al. In the 'rigor' condition, the ODA consists of a major subunit (the head), from which a slender link extends to the adjacent B-tubule (the B-link). A smaller, intermediate subunit lies adjacent to the head, distally, and a further extension of the complex, the minor subunit, continues distally beneath the head domain of the next arm complex, where it attaches to the A-tubule. In the presence of ATP and vanadate ('relaxed' condition), the attachment point of the B-link to the head is shifted to a more proximal position, and the minor subunit is no longer visible. This is interpreted as resulting from a rotation of the head. These features are demonstrated stereoscopically, and from several viewpoints. Image enhancement has been used to clarify and define the repetitive features of the dynein arrays. In addition, some of the axonemes have been imaged from highly contrasted 20 nm thin sections; the detection of B-links in such sections means that these slender structures cannot be considered artefacts of the rapid-freeze, deep-etch protocol.