Primary cilia are microtubule-based sensory organelles whose assembly and function rely on the conserved bidirectional intraflagellar transport (IFT) system, which is powered by anterograde kinesin-2 and retrograde cytoplasmic dynein 2 motors. Nematodes additionally employ a cell type-specific kinesin-3 motor, KLP-6, which moves within cilia independently of IFT and regulates ciliary content and function. Here we provide evidence that a KLP-6 homolog, KIF13B, undergoes bursts of bidirectional movement within primary cilia of cultured immortalized human retinal pigment epithelial (hTERT-RPE1) cells. Anterograde and retrograde intraciliary velocities of KIF13B were similar to those of IFT (IFT172-eGFP), but intraciliary movement of KIF13B required its own motor domain and appeared to be cell-type specific. Our work provides the first demonstration of motor-driven, intraciliary movement by a vertebrate kinesin other than kinesin-2 motors.

Primary cilia are microtubule-based sensory organelles whose assembly and function rely on the conserved bidirectional intraflagellar transport (IFT) system, which is powered by anterograde kinesin-2 and retrograde cytoplasmic dynein 2 motors. Nematodes additionally employ a male-specific kinesin-3 motor, KLP-6, which regulates ciliary content and function by promoting release of bioactive extracellular vesicles (EVs) from cilia. Here we show by live cell imaging that a KLP-6 homolog, KIF13B, undergoes bursts of bidirectional movement within primary cilia of cultured mammalian cells at 0.64 ± 0.07 μm/s in the anterograde direction and at 0.39 ± 0.06 μm/s in the retrograde direction, reminiscent of conventional IFT. In addition, we found that KIF13B undergoes EV-like release from the ciliary tip whereas a ciliary membrane marker, SMO-tRFP, remains stably associated with cilia during such EV release. Our results suggest that KIF13B, similar to KLP-6, regulates ciliary membrane content by promoting ciliary EV release, possibly in coordination with conventional IFT.