Although almost every cell possesses a primary cilium, we know little about the mechanisms that cells use to regulate the protein compositions of the ciliary membrane.
During ciliogenesis, ciliary membrane proteins, along with structural and signaling proteins, are carried through a complex ciliary diffusion barrier at the base of the organelle by intraflagellar transport (IFT). Disruption of the barrier leads to several human diseases of cilia, the ciliopathies. A favored model is that signaling-triggered accumulation of previosuly excluded membrane proteins in fully formed cilia also requires IFT, but direct evidence is lacking.
We recently developed a new system to study entry of a membrance protein into the flagellum of Chlamydomonas during flagellar signaling. In resting cells, a flagellar signaling component, the integral membrane polypeptide SAG1-C65, is uniformly distributed over the plasma membrane and excluded from the flagellar membrane. Flagellar adhesion-induced signaling triggers rapid redistribution of the protein to the apical ends of the cells concomitantly with entry into the flagella. This massive protein redistribution and flagellar enrichment are facilitated by cytoplasmic microtubles.
Using a conditional anterograde IFT mutant, we found that the IFT machinery is not required for regulated SAG1-C65 entry into flagella. Our working model is that integral membrane proteins can negotiate passage through the ciliary diffusion barrier without the need for a motor. (Belzile et al. Current Biology).