A gene encoding the heavy chain of conventional kinesin (kin2) has recently been identified in the dimorphic fungus Ustilago maydis (Lehmler et al., 1997). From the phenotype of kin2 null-mutants it was concluded that Kin2 might be involved in vesicle traffic towards the tip. However, this model did not explain why kin2-null mutant hyphae were unable to create empty cell compartments that are normally left behind the growing tip cell. Here we present a re-investigation of the function of Kin2 in hyphae and sporidia. We provide evidence that suggests a different and unexpected role of this kinesin motor in hyphal growth of Ustilago maydis. In addition, Kin2 was partially purified from U. maydis and in vitro properties were investigated. Isolated kinesin supported in vitro microtubule gliding at speeds of up to 1.8 micron/second, and showed motility properties and hydrodynamic behavior similar to those described for kinesin from N. crassa. It appears to be the product of the kin2 gene. Compared with wild-type sporidia, the kin2-null mutant sporidia grew normally but were defective in accumulation of Lucifer Yellow in their vacuoles, which were smaller than normal and often misplaced. The dikaryotic hyphae, produced by the fusion of two kin2-null sporidia, showed tip growth, but unlike wild-type hyphae, these structures lacked the large, basal vacuole and contain significantly more 200–400 nm vesicles scattered over the hole hypha. This defect was accompanied by a failure to generate regular empty cell compartments that are left behind in wild-type tip cells as the hyphae grow longer. These results suggest that Kin2 is a microtubule-dependent motor enzyme which is involved in the formation of vacuoles. The accumulation of these vacuoles at the basal end of the tip cell might be crucial for the formation of the empty sections and supports cytoplasmic migration during the growth of dikaryotic hyphae.

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