Ciliary entry of the kinesin-2 motor KIF17 is regulated by importin-beta2 and RanGTP

The biogenesis, maintenance, and function of primary cilia are controlled through intraflagellar transport (IFT) driven by two kinesin-2 family members, the heterotrimeric KIF3A/KIF3B/KAP complex and the homodimeric KIF17 motor1,2. How these motors and their cargoes gain access to the ciliary compartment is poorly understood. We identify a ciliary localization signal (CLS) in the KIF17 tail domain that is necessary and sufficient for ciliary targeting. Similarities between the CLS and classic nuclear localization signals (NLS) suggests that similar mechanisms regulate nuclear and ciliary import. We hypothesize that ciliary targeting of KIF17 is regulated by a Ran-GTP gradient across the ciliary base. Consistent with this, cytoplasmic expression of GTP-locked Ran(G19V) disrupts the gradient and abolishes ciliary entry of KIF17. Furthermore, KIF17 interacts with importin-β2 in a manner dependent on the CLS and inhibited by Ran-GTP. We propose that Ran plays a global role in regulating cellular compartmentalization by controlling the shuttling of cytoplasmic proteins into nuclear and ciliary compartments

Mechanisms of Ciliary Entry

Cilia are microtubule-based projections that extend from the surface of all mammalian cells. These specialized organelles function in motility and in sensing extracellular signals during development and homeostasis. Mutations in proteins that localize to and function within cilia result in a range of human diseases, collectively termed ciliopathies. How ciliary proteins gain access to and are compartmentalized in the ciliary compartment is not well characterized. I demonstrated for the first time that the cilium utilizes a permeability barrier to exclude soluble cytoplasmic molecules in a size-dependent manner, similar to the passive mechanisms that restrict entry into the nucleus. This demonstrates that the cilium is maintained as a privileged domain, accessible to specific components. I investigated the molecular mechanisms regulating active trafficking of the kinesin-2 motor KIF17 to cilia in mammalian cells. KIF17 is essential for photoreceptor cell formation and regulates the ciliary trafficking of select membrane proteins including opsin and olfactory CNG channels (Insinna and Besharse, 2008; Jenkins et al., 2006). We determined that ciliary entry of KIF17 requires components that regulate active uptake into the nuclear compartment: a ciliary localization sequence similar to a nuclear localization sequence, the importin-β2 transport receptor, and the GTPase Ran (Dishinger et al., 2010). Finally, I found that specific nucleoporins, key components of the nuclear pore complex (NPC) in the nuclear membrane, localize to the base of cilia and function to regulate ciliary entry of KIF17. I thus propose a model in which nuclei and cilia use similar active and passive mechanisms to regulate transport into the compartment. Further studies will be focused on determining mechanisms that distinguishes ciliary trafficking from nuclear transport.PhDCell and Developmental BiologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/96014/1/lynnkee_1.pd