Mechanisms of ciliary motility: An update

Ciliary motility is dependent upon dynein-driven sliding of microtubules. Attention has been focussed on the molecular biology and mechanochemistry of dynein, via techniques involving mutant analysis and in vitro motility measurements. In Chlamydomonas, Tetrahymena and Paramecium, the best studied protistan genera, the outer dynein arm is a compacted three-headed heterotrimeric bouquet. Every bouquet is identical, while each ATPase head of the bouquet is coded by a different dynein heavy (H) chain gene. One H chain (Paramecium α) binds a 29 kDa regulatory light chain whose phosphorylation increases the velocity of sliding of the dynein, which in turn increases ciliary beat frequency. At least seven different H chains within a 96 nm spoke group repeat form the inner dynein arms. The structural organization of the inner arms is complex, and several models have been proposed. The inner dynein arms evidently control bend formation. Their activity also changes upon phosphorylation or dephosphorylation, the signal for which is transduced via the spoke-central sheath complex and a dynein-regulatory complex that lies adjacent to the arms on the doublet. How inner and outer arms operate coordinately is not understood. The outer dynein arms have a strong attachment and force-producing phase that is very short (1%) compared to their cycle time. Only a few dynein steps probably occur per doublet during an effective stroke, which suggests that arm activity on an active doublet is stochastic. The orientation of the central pair within the axoneme determines the plane of bending, probably by dividing the doublets into active and inactive half axonemes, according to a switch point model. Computer reconstructions and analytical techniques such as finite element analysis are useful tools of exploring this model. Although many aspects of the mechanism of bend production and propagation still remain either undefined or controversial, a general outline is beginning to emerge.