Coordinated action of RAB11, Sec4, myosin V, and the exocyst governs Aspergillus nidulans tip growth

Abstract

Summary Aspergillus nidulans hyphae grow by apical extension fueled by apex-directed exocytosis. Vesicles arriving at the tip are concentrated by MyoE (myosin V) in a membraneless organelle, the Spitzenkörper, before undergoing a second round of transport to the plasma membrane. Sec4, which is recruited to the vesicles after reaching the tip, in turn recruits the exocyst, targeting vesicles to an apical arc-shaped stripe. However, vesicles spread across the apical dome in myoEΔ or sec4Δ cells, indicating that transport between the Spitzenkörper and the membrane is Sec4- and MyoE-mediated. The exocyst is essential for viability, whereas Sec4 or MyoE are not, although losing both is highly detrimental. Sec15, a component of the exocyst, binds MyoE, and substitutions affecting a conserved epitope mediating this interaction severely impair growth if Sec4 is absent. Therefore, Sec4 and MyoE transport vesicles between the Spitzenkörper and the membrane and cooperate to recruit the exocyst to mediate the fusion step.