Kinesin and dynein mutants provide novel insights into the roles of vesicle traffic during cell morphogenesis in Neurospora

Background: Kinesin and cytoplasmic dynein are force-generating molecules t hat move in opposite directions along microtubules. They have been implicat ed in the directed transport of a wide variety of cellular organelles, but it is unclear whether they have overlapping or largely independent function s, Results: We analyzed organelle transport in kinesin and dynein single mutan ts, and in a kinesin and dynein double mutant of Neurospora crassa, Remarka bly, the simultaneous mutation of kinesin and dynein was not lethal and res ulted in an additive phenotype that combined the features of the single mut ants. The mutation of kinesin and dynein had opposite effects on the apical and retrograde transport, respectively, of vesicular organelles. In the ki nesin mutant, apical movement of submicroscopic, secretory vesicles to the Spitzenkorper- an organelle in the hyphal apex - was defective, whereas the predominantly retrograde movement of microscopic organelles was only sligh tly reduced. In contrast, the dynein mutant still had a prominent Spitzenko rper, demonstrating that apical transport was intact, but retrograde transp ort was essentially inhibited completely. A major defect in vacuole formati on and dynamics was also evident. In agreement with the observations on api cal transport, protein secretion into the medium was markedly inhibited in the kinesin mutant but not in the dynein mutant. Conclusions: Transport of secretory vesicles is necessary but not sufficien t for normal apical extension. A component of retrograde transport, presuma bly precursors of the vacuole system, is also essential. Our findings provi de new information on the role microtubule motors play in cell morphogenesi s and suggest that kinesin and cytoplasmic dynein have largely independent functions within separate pathways.