ABSTRACT
The key structure in a eukaryotic flagellum or cilium is the axoneme; the axoneme consists of a cylindrical array of nine doublet microtubules surrounding a pair of singlet microtubules, together with a variety of microtubule linkages and projections.1 The mechanism responsible for the movement of these organelles is associated with actively sliding microtubules,2 in which the dynein arms on one peripheral doublet of the axoneme exert a tipward force on the neighboring doublet.3•4 (For convenience, the two types of organelle are collectively referred to as cilia.) The shearing forces produced by the dynein-tubulin interaction are converted into bending moments by basal structures and interdoublet linkages that oppose free sliding, and the coordinated activity of the dynein-microtubule interaction results in the bending patterns seen on the intact organelles. While there is considerable evidence to support the sliding microtubule model, the mechanism by which bends are initiated and propagated has yet to be described in detail, although many of the physical constraints required can be specified.
