A balance of KIF1A-like kinesin and dynein organizes early endosomes in the fungus Ustilago maydis

In Ustilago maydis, bidirectional transport of early endosomes is microtubule dependent and supports growth and cell separation. During early budding, endosomes accumulate at putative microtubule organizers within the bud, whereas in medium-budded cells, endosome clusters appear at the growing ends of microtubules at the distal cell pole. This suggests that motors of opposing transport direction organize endosomes in budding cells. Here we set out to identify these motors and elucidate the molecular mechanism of endosome reorganization. By PCR we isolated kin3, which encodes an UNC-104/KIF1-like kinesin from U.maydis. Recombinant Kin3 binds microtubules and has ATPase activity. Kin3-green fluorescent protein moves along microtubules in vivo, accumulates at sites of growth and localizes to endosomes. Deletion of kin3 reduces endosome motility to approximately 33%, and abolishes endosome clustering at the distal cell pole and at septa. This results in a transition from bipolar to monopolar budding and cell separation defects. Double mutant analysis indicates that the remaining motility in Deltakin3-mutants depends on dynein, and that dynein and Kin3 counteract on the endosomes to arrange them at opposing cell poles.