Isolation and identification of mesA1, a gene involved in polarized growth

Date of Completion

January 2004


Biology, Molecular




Cell polarity is critical for the proper function of many cell types. The filamentous fungus Aspergillus nidulans is an excellent model system for studying cell polarity because it grows in a highly polar manner with growth restricted to the hyphal tip and the formation of a septum, a cross wall of chitin. The formin homology family of proteins is important in establishing cell polarity in a variety of cell types by nucleating unbranched actin filaments. The A. nidulans formin, SepA was isolated in a screen that identified mutants that failed to septate. SepA has previously been shown to have a role in septation and polarized growth. The sepA null allele, fails to septate and produce conidia. When SepA is compromised the hyphae have dichotomous branching (tip splitting) instead of forming a branch in a sub-apical compartment of the hyphae, the hyphae are wider than wild-type hyphae, and have a severe delay in septation. SepA localization is dynamic at the septation site where it forms a contractile ring and the hyphal tip where the movement of SepA::GFP correlates with hyphal extension. ^ The A. nidulans genome encode a single formin, SepA, which is required for actin ring formation at septation sites and plays a role in polarized morphogenesis. However, sepA mutants are still able to form hyphae that undergo polarized extension, unlike the yeasts, which need one functional formin for viability. To understand how polarity establishment occurs in the absence of formin function, a screen was undertaken for mutations that enhance sepA defects. The screen recovered four mes (A–D, m&barbelow;orphological e&barbelow;nhancer of S&barbelow;epA) mutants. 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 display defects in hyphal morphogenesis, while septum formation remains unaffected. Molecular characterization revealed that MesA is a novel fungal protein that contains predicted transmembrane domains and localizes to hyphal tips. Microscopy indicates that MesA promotes the localized assembly of actin cables at the site of polarization by facilitating the stable recruitment of SepA. ^