Intraflagellar transport and the generation of dynamic, structurally and functionally diverse cilia

Cilia are organelles that project from most eukaryotic organisms and cell types. Their pervasiveness stems from having remarkably versatile propulsive and sensory functions, which in humans are recognized to have essential roles in physiology and development. Under-appreciated, however, are their diverse ultrastructures and typically bipartite organization consisting of doublet and singlet microtubules. Moreover, the overall shapes of the membrane-ensheathed cilia are varied, as exemplified by differences between hair-like olfactory cilia and rod- or cone-shaped photoreceptor connecting cilia-outer segments. Although cell-specific transcriptional programs are evidently crucial in establishing ciliary morphological specialization, few players directly involved in generating such diversity are known. Recent findings suggest that at least two molecular motors (kinesin-II and OSM-3/KIF17) can differentially mobilize the intraflagellar transport machinery required for ciliogenesis and, presumably, different cargo to help generate dynamic, structurally and functionally distinct cilia.