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Identification of a motor protein required for filamentous growth in Ustilago maydis

The phytopathogenic fungus Ustilago maydis exists in two stages, the yeast‐like haploid form and the filamentous dikaryon. Both pathogenicity and dimorphism are genetically controlled by two mating‐type loci, with only the filamentous stage being pathogenic on corn. We have identified two genes (kin1 and kin2) encoding motor proteins of the kinesin family. Kin1 is most similar to the human CENP‐E gene product, while Kin2 is most closely related to the conventional kinesin Nkin of Neurospora crassa. Deletion mutants of kin1 had no discernible phenotype; Δkin2 mutants, however, were severely affected in hyphal extension and pathogenicity. The wild‐type dikaryon showed rapid tip growth, with all the cytoplasm being moved to the tip compartment. Left behind are septate cell wall tubes devoid of cytoplasm. In Δkin2 mutants, dikaryotic cells were formed after cell fusion, but these hyphal structures remained short and filled with cytoplasm. A functional green fluorescent protein (GFP)–Kin2 fusion was generated and used to determine the localization of the motor protein by fluorescence microscopy. Inspection of the hyphal tips by electron microscopy revealed a characteristic accumulation of darkly stained vesicles which was absent in mutant cells. We suggest that the motor protein Kin2 is involved in organizing this specialized growth zone at the hyphal tip, probably by affecting the vectorial transport of vesicles.