The inner dynein arm complex: compatible images from freeze-etch and thin section methods of microscopy

A study has been made of the inner dynein arm complex in the sperm flagellum of Gallus domesticus. It has been found that images of the complex made from highly contrasted, approx. 20 nm, sections are very largely compatible with images made from replicas of rapidly cryofixed material. This suggests that neither technique seriously distorts the native state. From both types of image, we conclude that the most proximal group of inner dynein arm heads (IDA 1) is related to spoke S1 and consists of 3 heads with fine connections to the B-tubule. IDA 2 consists of 2 such heads and is related to spoke S2. IDA 3 is apparently single headed and lies close to the A-attachment of spoke S3. This arrangement of IDAs repeats every 96 nm. Between the IDAs and outer dynein arms (ODAs), lying close to IDAs 2 and 3, is a strap-like linkage to the next B-tubule; it is argued that this represents the circumferential link, nexin. In sections, but not consistently in replicas, IDA2 lies closer to the plane of the nexin link than does IDA 1. In the presence of ATP and vanadate, little change is seen in the IDA complex except for an ill-defined alteration to IDA 1. It is speculated that the apparently smaller size of the inner arm heads (compared with the ODA major subunit) may have functional implications in relation to maximal sliding velocity.