Primary cilia are nonmotile organelles implicated in signaling and sensory 
functions. Understanding how primary cilia assemble could shed light on the many 
human diseases caused by mutations in ciliary proteins. The centrosomal protein 
CP110 is known to suppress ciliogenesis through an unknown mechanism. Here, we 
report that CP110 interacts with CEP290--a protein whose deficiency is 
implicated in human ciliary disease--in a discrete complex separable from other 
CP110 complexes involved in regulating the centrosome cycle. Ablation of CEP290 
prevents ciliogenesis without affecting centrosome function or cell-cycle 
progression. Interaction with CEP290 is absolutely required for the ability of 
CP110 to suppress primary cilia formation. Furthermore, CEP290 and CP110 
interact with Rab8a, a small GTPase required for cilia assembly. Depletion of 
CEP290 interferes with localization of Rab8a to centrosomes and cilia. Our 
results suggest that CEP290 cooperates with Rab8a to promote ciliogenesis and 
that this function is antagonized by CP110.