You may think of cilia primarily as structures specialized to confer motility, but nonmotile cells (virtually all vertebrate cells have a primary cilium) also use cilia for sensory functions. Receptors concentrated on the surface of cilia allow cells to efficiently sample their immediate environment, but it is unclear how such receptors function in the absence of structures that usually associate with them at the plasma membrane. Wang et al. addressed this issue in the biflagellated alga Chlamydomonas. These cells use sex-specific adhesion receptors on their flagella (flagella are essentially the same as cilia) during mating. In this system, the authors could take advantage of temperature-sensitive mutations in kinesin 2, an essential motor that functions in the intraflagellar transport (IFT) system. The IFT actively transports material from the cell body to the flagellum, using kinesin and dynein to move particles along microtubules. Interaction of the flagella of mating Chlamydomonas cells leads to activation of a protein tyrosine kinase, and Wang et al. used immunopurification and mass spectrometry to identify a cyclic GMP (cyclic guanosine monophosphate)-dependent protein kinase (CrPKG) as a target of the tyrosine kinase. Although the flagellum represents about 5% of the volume of the cell, it contained almost 50% of the CrPKG. CrPKG was activated by flagellar adhesion, and depletion of CrPKG by RNAi showed that the kinase was required for successful fertilization. Fractionation by centrifugation revealed that CrPKG became associated with a new particulate compartment after flagellar adhesion but that functional IFT was required for this effect. The authors also used antibodies to monitor components of the IFT machinery before and after adhesion, and further fractionation showed that the IFT components became associated, along with CrPKG, with detergent-sensitive complexes in a particulate fraction derived from cell extracts. Given the signal-dependent modification of the IFT components and their association with CrPKG, which behaved similarly, the authors propose that IFT not only functions to move signaling components from the cell body to the flagellum but also actively participates in signaling.
Q. Wang, J. Pan, W. J. Snell, Intraflagellar transport particles participate directly in cilium-generated signaling in Chlamydomonas. Cell 125, 549-562 (2006). [Online Journal]