CPAP is required for cilia formation in neuronal cells.

The primary cilium is a microtubule-based structure protruded from the basal body analogous to the centriole. CPAP (centrosomal P4.1-associated protein) has previously been reported to be a cell cycle-regulated protein that controls centriole length. Mutations in CPAP cause primary microcephaly (MCPH) in humans. Here, using a cell-based system that we established to monitor cilia formation in neuronal CAD (Cath.a-differentiated) cells and hippocampal neurons, we found that CPAP is required for cilia biogenesis. Overexpression of wild-type CPAP promoted cilia formation and induced longer cilia. In contrast, an exogenously expressed CPAP-377EE mutant that lacks tubulin-dimer binding significantly inhibited cilia formation and caused cilia shortening. Furthermore, depletion of CPAP inhibited ciliogenesis and such effect was effectively rescued by expression of wild-type CPAP, but not by the CPAP-377EE mutant. Taken together, our results suggest that CPAP is a positive regulator of ciliogenesis whose intrinsic tubulin-dimer binding activity is required for cilia formation in neuronal cells.