Ciliary and flagellar motility and the nexin-dynein regulatory complex

Nexin was first identified in electron microscope images of axonemes as a linker that repeats every 96. nm between adjacent outer doublet microtubules. The nexin links are thought to be protease-sensitive structures that limit microtubule sliding during the ciliary beat cycle. The dynein regulatory complex (DRC) was first described in genetic screens for suppressor mutations that restore motility to paralyzed radial spoke (RS) or central pair (CP) mutants. The DRC is thought to identify intermediates in the signaling pathway between the CP, RS, and dynein arms. Cryo-electron tomography of drc mutant axonemes revealed that the DRC is located within the nexin link, in close proximity to RS2 and a calmodulin spoke complex, and also serves as an adaptor that stabilizes the assembly of several inner arm dyneins. Recent studies have identified at least 11 DRC subunits and several other N-DRC-associated proteins. The implications of these findings for future studies of ciliary motility are discussed.