Abstract
In the filamentous fungus Aspergillus nidulans, both microtubules and actin filaments are important for polarized growth at the hyphal tip. Less clear is how different microtubule-based and actin-based motors work together to support this growth. Here we examined the role of myosin-V (MYOV) in hyphal growth. MYOV-depleted cells form elongated hyphae, but the rate of hyphal elongation is significantly reduced. In addition, although wild type cells without microtubules still undergo polarized growth, microtubule disassembly abolishes polarized growth in MYOV-depleted cells. Thus, MYOV is essential for polarized growth in the absence of microtubules. Moreover, while a triple kinesin null mutant lacking kinesin-1 (KINA) and two kinesin-3s (UNCA and UNCB) undergoes hyphal elongation and forms a colony, depleting MYOV in this triple mutant results in lethality due to a severe defect in polarized growth. These results argue that MYOV, through its ability to transport secretory cargo, can support a significant amount of polarized hyphal tip growth in the absence of any microtubule-based transport. Finally, our genetic analyses also indicate that KINA (kinesin-1) rather than UNCA (kinesin-3) is the major kinesin motor that supports polarized growth in the absence of MYOV.
Highlights
Type V myosins have been implicated in organelle transport in numerous organisms [1]
On non-repressive glycerol medium, the alcA-GFP-myoV strain grew as well as the wild-type strain (Figure 1D). This observation argues that the GFP-MYOV fusion protein is functional and that the phenotype on glucose-containing YUU medium is due to the depletion of MYOV rather than any dominant negative effect caused by the expression of the N-terminal portion of MYOV from the native promoter
We analyzed the function of myosin-V in the filamentous fungus A. nidulans, where both microtubules and the actin cytoskeleton contribute to the transport of organelles and vesicles required for polarized growth at the hyphal tip [13,23,40,41,42]
Summary
Type V myosins have been implicated in organelle transport in numerous organisms [1]. In the budding yeast Saccharomyces cerevisiae, where cellular transport of vesicles depends solely on the actin cytoskeleton, class V myosins transport most if not all of the cell’s organelles, including late Golgi elements, post-Golgi secretory vesicles, peroxisomes, mitochondria, and the endoplasmic reticulum [2,3,4,5,6,7]. In mouse cerebellar Purkinje neurons, the endoplasmic reticulum (ER) is first distributed throughout dendrites by microtubule motors, and transported into dendritic spines by myosin-Va [9]. While the recent characterization of myosin Va-dependent ER transport in Purkinje neurons provided strong support for the idea that myosin-V functions as a point-to-point cargo transporter [9,12], the extent to which this is true in other cellular contexts remains to be seen
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