A Size-Exclusion Permeability Barrier and Nucleoporins Characterize a Ciliary Pore Complex that Regulates Transport into Cilia

The cilium is a microtubule-based organelle that contains a unique complement of proteins for cell motility and signaling functions. Entry into the ciliary compartment is proposed to be regulated at the base of the cilium (1). Recent work demonstrated that components of the nuclear import machinery, including the RanGTPase and importins, regulate ciliary entry (2–4). We hypothesized that the ciliary base contains a ciliary pore complex (CPC) whose molecular nature and selective mechanism are similar to the nuclear pore complex (NPC). By microinjecting fluorescently-labeled dextrans and recombinant proteins of various sizes, we characterize a size-dependent diffusion barrier for the entry of cytoplasmic molecules into primary cilia in mammalian cells. We demonstrate that nucleoporins localize to the base of primary and motile cilia and that microinjection of nucleoporin function-blocking reagents blocks the ciliary entry of kinesin-2 KIF17 motors. Together, this work demonstrates that the physical and molecular nature of the CPC is similar to the NPC, and further extends functional parallels between nuclear and ciliary import.