Abstract
The Aspergillus nidulans proteome possesses a single formin, SepA, which is required for actin ring formation at septation sites and also plays a role in polarized morphogenesis. Previous observations imply that complex regulatory mechanisms control the function of SepA and ensure its correct localization within hyphal tip cells. To characterize these mechanisms, we undertook a screen for mutations that enhance sepA defects. Of the mutants recovered, mesA1 causes the most dramatic defect in polarity establishment when SepA function is compromised. In a wild-type background, mesA1 mutants undergo aberrant hyphal morphogenesis, whereas septum formation remains unaffected. Molecular characterization revealed that MesA is a novel fungal protein that contains predicted transmembrane domains and localizes to hyphal tips. We show that MesA promotes the localized assembly of actin cables at polarization sites by facilitating the stable recruitment of SepA. We also provide evidence that MesA may regulate the formation or distribution of sterol-rich membrane domains. Our results suggest that these domains may be part of novel mechanism that directs SepA to hyphal tips.