The Role of Fa2p in Ciliary and Cell Cycle Regulation

Cilia are microtubule based organelles with roles in motility and sensory perception. An emerging pattern suggests that various human diseases are caused by defects in the assembly, maintenance or function of cilia. Some ciliopathies, such as the polycystic kidney diseases and Bardet-Biedl syndrome, involve aberrant cell proliferation in conjunction with ciliary defects. Recent data suggests that the cilium serves as a highly conserved organizing center for early steps in signal transduction pathways that control cell growth and division. As such, signaling molecules important for growth, mitosis or differentiation have been localized to cilia. The relationship between cilia and cell cycle progression is poorly defined, but may involve regulation by the NIMA-family of kinases (Neks). Our discovery that the Nek Fa2p is important for ciliary function and cell cycle progression in Chlamydomonas provides a direct link between these two processes. Fa2p was originally identified from a screen for deflagellation-defective mutants in Chlamydomonas and shown to be defective in calcium-induced severing of the axonemal microtubules. We subsequently showed that fa2 mutants are delayed in transit through at least two points in the cell cycle: (1) G2/M transition; (2) assembly of flagella after exit from mitosis. In this study, we show that Fa2p localizes to a unique site at the proximal end of cilia in Chlamydomonas and kidney epithelial cells, suggesting a high level of conservation of this signaling complex. In both cell types, Fa2p localization is dynamic; when cells enter the cell cycle, Fa2p becomes reduced in the cilium and accumulates at the base of the basal bodies/centrioles. It remains associated with the spindle poles throughout the cell cycle and is assembled on cilia when they begin to regenerate after exit from mitosis. Importantly, Fa2p kinase activity is required for deflagellation, but does not appear to be essential for localization and efficient cell cycle progression. Furthermore, we show that two mammalian Nek homologs of Fa2p (mNek1 and mNek8), which are defective in murine models of polycystic kidney diseases, localize to primary cilia and centrosomes. Finally, biochemical analysis reveals the interaction of two proteins (~20 and ~60 kDa) with Fa2p in situ.