ABSTRACT
The oscillatory beating motion of epithelial cilia is generated through the activity of the motor protein, dynein, located in the ciliary axonemes [I ,2]. The dynein forms two rows of projections, usually termed the outer and inner arms, located along one side of each of the 9 doublet microtubules of the axoneme where they form crossbridges with the adjacent microtubules. In beating cilia, these dynein crossbridges repeatedly detach and reattach to successive tubulin subunits on the adjacent microtubule of the axoneme in a cyclic stepping process driven by the dephosphorylation of ATP. The localized sliding displacement of microtubules produced by this activity of the dyne in arms is converted into transverse bending moments by other resistive structures and results in the formation and propagation of the ciliary bending waves. The axonemal dyneins that are responsible for ciliary motility were first discovered and characterized about 30 years ago [3]. They are closely related to cytoplasmic dynein, that was discovered much more recently, and is involved in diverse intracellular activities including intracellular transport, nuclear migration, and the orientation of the cell spindle at mitosis [4]. Both axonemal and cytoplasmic dyneins are characterized by possession of exceptionally large heavy-chain subunits of molecular weight > 500,000 Da, by their sensitivity to inhibition by vanadate, and by their function as minus-end directed motors in assays of motility on microtubules in vitro.
