In vitro reconstitution defines the mechanistic basis of HSET motor activity regulation by IntraFlagellar Transport proteins

HSET is a mitotic kinesin essential for centrosome clustering in cells harboring supernumerary centrosomes. Work in cellulo revealed that IntraFlagellar Transport proteins (IFT) interact with the kinesin HSET to promote efficient extra centrosome clustering and subsequent cancer cell proliferation. However, whether and how IFT proteins regulate HSET activity is unknown. Using a reconstituted in vitro system combining purified HSET and IFT proteins coupled to TIRF microscopy approaches, we identified a minimal subcomplex made of IFT52/IFT70 directly binding to HSET. We demonstrate that this binding induces HSET oligomerization allowing its activation and the formation of processive HSET complexes. We also show that HSET activation by IFT proteins accounts for an increased ability to slide microtubules and to organize dynamic microtubule networks in vitro . Overall, this work shows that IFT proteins can directly activate a mitotic kinesin and provides mechanistic explanation for the contribution of IFT proteins to efficient centrosome clustering.