Cell polarity and hyphal morphogenesis are controlled by multiple rho-protein modules in the filamentous ascomycete Ashbya gossypii

Polarized cell growth requires a polarized organization of the actin cytosk eleton. Small GTP-binding proteins of the Rho-family have been shown to be involved in the regulation of actin polarization as well as other processes . Hyphal growth in filamentous fungi represents an ideal model to investiga te mechanisms involved in generating cell polarity and establishing polariz ed cell growth. Since a potential role of Rho-proteins has not been studied so far in filamentous fungi we isolated and characterized the Ashbya gossy pii homologs of the Saccharomyces cerevisiae CDC42, CDC24, RHO1, and RHO3 g enes. The AgCDC42 and AgCDC24 genes can both complement conditional mutatio ns in the S. cerevisiae CDC42 and CDC24 genes and both proteins are require d for the establishment of actin polarization in A. gossypii germ cells. Ag rho1 mutants show a cell lysis phenotype. Null mutant strains of Agrho3 sho w periodic swelling of hyphal tips that is overcome by repolarization and p olar hyphal growth in a manner resembling the germination pattern of spores . Thus different Rho-protein modules are required for distinct steps during polarized hyphal growth of A. gossypii.