Determination of cell polarity in germinated spores and hyphal tips of thefilamentous ascomycete Ashbya gossypii requires a rhoGAP homolog

In the filamentous ascomycete Ashbya gossypii, like in other filamentous fu ngi onset of growth in dormant spores occurs as an isotropic growth phase g enerating spherical germ cells. Thereafter, a switch to polarized growth re sults in the formation of the first hyphal tip. The initial steps of hyphal tip formation in filamentous fungi, therefore, resemble processes taking p lace prior to and during bud emergence of unicellular yeast-like fungi. We investigated whether phenotypic similarities between these distinct events extended to the molecular level. To this end we isolated and characterized the A. gossypii homolog of the Saccharomyces cerevisiae BEM2 gene which is part of a network of rho-GTPases and their regulators required for bud emer gence and bud growth in yeast. Here we show that the AgBem2 protein contain s a GAP- (GTPase activating protein) domain for rho-like GTPases at its car boxy terminus, and that this part of AgBem2p is required for complementatio n of an Agbem2 null strain. Germination of spores resulted in enlarged Agbe m2 germ cells that were unable to generate the bipolar branching pattern fo und in wild-type germ cells. In addition, mutant hyphae were swollen due to defects in polarized cell growth indicated by the delocalized distribution of chitin and cortical actin patches. Surprisingly, the complete loss of c ell polarity which lead to spherical hyphal tips was overcome by the establ ishment of new cell polarities and the formation of multiple new hyphal tip s. In conclusion these results and other findings demonstrate that establis hment of cell polarity, maintenance of cell polarity, and polarized hyphal growth in filamentous fungi require members of rho-GTPase modules.