KINESIN FROM THE PLANT-PATHOGENIC FUNGUS USTILAGO-MAYDIS IS INVOLVED IN VACUOLE FORMATION AND CYTOPLASMIC MIGRATION

A gene encoding the heavy chain of conventional kinesin (kin2) has rec ently been identified in the dimorphic:fungus Ustilago maydis (Lehmler et al., 1997), From the phenotype of kin2 null-mutants it was conclud ed that Kin2 might be involved in vesicle traffic towards the tip. How ever, this model did not explain why kin2-null mutant hyphae were unab le 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 di fferent and unexpected role of this kinesin motor in hyphal growth of Ustilago maydis. In addition, Kin2 was partially purified from U. mayd is and in vitro properties were investigated. Isolated kinesin support ed in vitro microtubule gliding at speeds of up to 1.8 mu m/second, an d showed motility properties and hydrodynamic behavior similar to thos e described for kinesin from N. crassa. It appears to be the product o f the kin2 gene. Compared with wild-type sporidia, the kin2-null mutan t sporidia grew normally but were defective in accumulation of Lucifer Yellow in their vacuoles, which were smaller than normal and often mi splaced. The dikaryotic hyphae, produced by the fusion of two kin2-nul l sporidia, showed tip growth, but unlike wild-type hyphae, these stru ctures lacked the large, basal vacuole and contain significantly more 200-400 nm vesicles scattered over the hole hypha. This defect was acc ompanied by a failure to generate regular empty cell compartments that are left behind in wild-type tip cells as the hyphae grow longer. The se results suggest that Kin2 is a microtubule-dependent motor enzyme w hich is involved in the formation of vacuoles, The accumulation of the se vacuoles at the basal end of the tip cell might be crucial for the formation of the empty sections and supports cytoplasmic migration dur ing the growth of dikaryotic hyphae.