Dyneins are molecular motors responsible for many different types of microtubule-based motility. Within cilla and flagella, dyneins are contained in the inner and outer arms and generate the force required for ciliary and flagellar bending. The outer arm dynein is probably the most extensively studied of all dynein isoforms.
Examination of the cross-sections of sperm flagella and cilia in various animals, including mammals, tunicates, echinoderms, molluses, annelids, arthropods, flatworms and sea anemones, indicates that their outer arms are hook-or fist-like, in contrast to the pistol-like shape in Chlamydomonas, Paramecium, and Tetrahymena. Thus, it may be concluded that the outer arm dynein molecules of flagella and cilia in Protozoa has a three-headed structure, whereas that of sperm flagella in Metazoa exhibits a two-headed structure.
The outer arm dynein of sea urchin sperm axoneme contains three intermediate chains (IC1, IC2, and IC3 ; Mr 128,000,98,000, and 74,000, respectively). In the present study, we cloned all the intermediate chains. IC2 and IC3 are members of the WD family ; the WD motif is responsible for a protein-protein interaction. IC1 has a unique primary structure, the N-terminal part is homologous to the sequence of thioredoxin, the middle part consists of three repetitive sequences homologous to the sequence of nucleoside diphosphate kinase, and the C-terminal part contains a high proportion of negatively charged glutamic acid residues. Thus, IC1 is a novel dynein intermediate chain distinct from IC2 and IC3 and may be a multifunctional protein.
In addition, a piece of evidence was obtained to indicate bidirectional power stroke of the dynein arms in sea urchin sperm axonemes.