Figure 1. Some key features of ciliary kinesins. (A) An electron micrograph of a cross section through a cilium in zebrafish (left, from ref. 64) and a schematic representation of ciliary microtubules (right). Inside the ciliary shaft, microtubules form nine doublets that point their plus ends toward the tip of the cilium. These doublets frequently continue as single microtubules in the distal region, sometimes referred to as the “distal segment” of the cilium (d. s.). The portion of the cilium occupied by single microtubules is highly variable in different cells. (B and C) Kinesins are microtubule-dependent motors that transport cargo toward plus ends of microtubules. In all species examined so far, the heterotrimeric kinesin-2 (also known as kinesin-II) plays a major role in ciliary transport. It consists of two distinct motor subunits and an accessory subunit (KAP3). In C. elegans, two genes encode kinesin-II motor subunits (B). In vertebrates, on the other hand, these motor subunits are encoded by at least three loci (C). Their protein products are thought to assemble into complexes containing the Kif3a subunit, and either Kif3b or Kif3c. Thus at least two distinct Kinesin-II complexes appear to function in vertebrates. In addition to the heterotrimeric kinesin-2, the homodimeric kinesin-2, in vertebrates encoded by the kif-17 gene, contributes to ciliary transport. In C. elegans, this kinesin is frequently redundant with Kinesin-II and plays a major role in cilia formation. In at least one group of cells, the homodimeric kinesin-2, but not the heterotrimeric one, is necessary for the formation of the distal segment. In vertebrates, the homodimeric kinesin-2 appears to play a less significant role. It may transport unique cargos and/or kinesin-II cargos. There is no evidence that the formation of single microtubules in the distal portion of vertebrate cilia requires kinesins that differ from these that function in the formation of microtubule doubles. Kinesins of other families also contribute to ciliary transport (not shown). Verterbrate and C. elegans nomenclature is provided at the bottom of the figure.