Intraflagellar Transport: It's Not Just for Cilia Anymore

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.