Intraflagellar transport: it's not just for cilia anymore

Abstract

Recently published information on the role of intraflagellar transport (IFT) polypeptides in vesicle exocytosis is reviewed, describing the formation of the immune synapse in nonciliated cells as an example. A hypothesis is detailed suggesting that all polypeptides which enter the cilium, both membrane and axonemal, do so in association, first, with cytoplasmic vesicles which exocytose adjacent to the ciliary basal body, and then with the ciliary membrane. Axonemal proteins are moved to the ciliary tip by peripheral association with the inner aspects of the ciliary membrane by cannonical ciliary IFT. At the tip, some polypeptides are released for axonemal assembly, and others are budded off as part of vesicular exosomes into the environment. It is proposed that the cilium, in addition to being a sensory and motile organelle, is also a secretory organelle.

Figure 1

These diagrams illustrate similarities between (A) cilia and the (B) immune synapse. IFT20 is associated with the Golgi and vesicles destined for the cilia and is also involved in formation of the immune synapse. Although the entire complement of IFT proteins has not been examined, at least IFT57, IFT88 and the IFT motor Kif3a are also involved in formation of the immune synapse [26]. The centriole, which nucleates the microtubules of the cilium also moves near the cell surface during formation of the immune synapse, positioned between Golgi and the plasma membrane, which sometimes is seen to form a bulge in this area. Figure modified after [53].