Towards a Subdiffraction View of Motor-Mediated Transport in Primary Cilia

Primary cilia play important sensory and signaling roles, with proteins responsible for functional activities delivered to the ciliary compartment via intraflagellar transport (IFT). IFT is a bidirectional microtubule-based motility mediated by molecular motors. This review aims to survey the course of motor-mediated transport in different stages of IFT and demonstrate the potential of using superresolution imaging in the studies of dynamic activities in primary cilia. Three major superresolution imaging techniques, i.e., STED, PALM/STORM, and SIM, are compared to illustrate their advantages and challenges. We overview the processes of recruitment of molecules to the base of a primary cilium, the assembly at the ciliary base, the passage through the transition zone, the anterograde IFT driven by kinesin, the turnover at the ciliary tip, the retrograde IFT driven by dynein, and the exit from the ciliary compartment. Given the small characteristic lengths of the molecules and structural elements involved in the motor-mediated transport and at the path of IFT, we further examine the advantages of using superresolution microscopy and the current development in subdiffraction studies of primary cilia.