A FUNGAL KINESIN REQUIRED FOR ORGANELLE MOTILITY, HYPHAL GROWTH, AND MORPHOGENESIS

A gene (NhKIN1) encoding a kinesin was cloned from Nectria haematococc a genomic DNA by polymerase chain reaction amplification, using primer s corresponding to conserved regions of known kinesin-encoding genes. Sequence analysis showed that NhKIN1 belongs to the subfamily of conve ntional kinesins and is distinct from any of the currently designated kinesin-related protein subfamilies. Deletion of NhKIN1 by transformat ion-mediated homologous recombination caused several dramatic phenotyp es: a 50% reduction in colony growth rate, helical or wavy hyphae with reduced diameter, and subcellular abnormalities including withdrawal of mitochondria from the growing hyphal apex and reduction in the size of the Spitzenkorper, an apical aggregate of secretory vesicles. The effects on mitochondria and Spitzenkorper were not due to altered micr otubule distribution, as microtubules were abundant throughout the len gth of hyphal tip cells of the mutant. The rate of spindle elongation during anaphase B of mitosis was reduced 11%, but the rate was not sig nificantly different from that of wild type. This lack of a substantia l mitotic phenotype is consistent with the primary role of the convent ional kinesins in organelle motility rather than mitosis. Our results provide further evidence that the microtubule-based motility mechanism has a direct role in apical transport of secretory vesicles and the f irst evidence for its role in apical transport of mitochondria in a fi lamentous fungus. They also include a unique demonstration that a micr otubule-based motor protein is essential for normal positioning of the Spitzenkorper, thus providing a new insight into the cellular basis f or the aberrant hyphal morphology.