Function and regulation of kinesin motors in cells

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Microtubule-based kinesin and dynein motors drive a plethora of cellular processes, including intracellular transport of cellular cargo, assembly and function of the mitotic spindle, and ciliary function. While the chemical and physical properties of kinesins are well studied in vitro, much less is known about the specific function and regulation of kinesin motors in cells. The KIF3A/KIF3B/KAP motor, subsequently referred to as kinesin-2, drives intracellular transport of various cargos and is also essential for intraflagellar transport (IFT), a specialized transport inside eukaryotic cilia. Cilia are protrusions of the plasma membrane that are supported by a specialized microtubule structure called the axoneme. Primary cilia are solitary and immotile cilia that sense various stimuli in a tissue-specific manner. They can, for instance, sense the presence of morphogens during development, odorants in the nasal cavity, or the strength of urine flow in kidney tubules. Given these essential sensory functions, it is not surprising that ciliary malfunction underlies many diseases that are collectively classified as ciliopathies. During IFT, large protein assemblies called IFT trains are continuously transported within cilia. The IFT trains are loaded with specific cargo at the ciliary base and subsequently recruit kinesin-2 motors for transport along the axonemal microtubules to the tip of the cilium. There, the kinesin-2 motors are released, specific cargo is unloaded, and the trains are remodeled for subsequent transport back to the ciliary base by dynein-2. It is well established that the loss of any subunit of the kinesin-2 motor leads to the complete absence of cilia, and interference with IFT leads to the disappearance of already established cilia. From experiments with the single-celled flagellate Chlamydomonas we know that tubulin influx into cilia via IFT is modulated as a function of cilium length. Based on this finding several recent models aimed at explaining the impact of IFT on cilium length and cilium maintenance attribute high importance to the ciliary tubulin concentration. However, the change in tubulin concentration in these models cannot explain all experimental findings and it is likely that other aspects of IFT in addition to tubulin import are important for ciliary length and structure. Thus, the importance of IFT for the ciliary structure and the regulation of kinesin-2 motor for IFT is only incompletely understood, especially in mammalian systems. In this proposal, we will use a combination of biochemical & cellular assays, protein & genome engineering, and high-resolution microscopy to study how kinesin-2 is regulated for IFT and to delineate the impact of kinesin-2 driven IFT on the structure of mammalian cilia. At the center of our approach are engineered kinesin proteins whose activity can be precisely regulated in time and space externally by the investigator. The work laid out in this proposal will shed light on the function and regulation of kinesin motors in mammalian cilia and thereby promote the development of therapies aimed at alleviating or curing motor protein-associated human diseases.