Ras1-Induced Hyphal Development in Candida albicans Requires the Formin Bni1

Formins are downstream effector proteins of Rho-type GTPases and are involved in the organization of the actin cytoskeleton and actin cable assembly at sites of polarized cell growth. Here we show using in vivo time-lapse microscopy that deletion of theCandida albicans formin homologBNI1 results in polarity defects during yeast growth and hyphal stages. Deletion of the secondC. albicans formin,BNR1 , resulted in elongated yeast cells with cell separation defects but did not interfere with the ability ofbnr1 cells to initiate and maintain polarized hyphal growth. Yeastbni1 cells were swollen, showed an increased random budding pattern, and had a severe defect in cytokinesis, with enlarged bud necks. Induction of hyphal development inbni1 cells resulted in germ tube formation but was halted at the step of polarity maintenance. Bni1-green fluorescent protein is found persistently at the hyphal tip and colocalizes with a structure resembling the Spitzenkörper of true filamentous fungi. Introduction of constitutively activeras1 G13V in thebni1 strain or addition of cyclic AMP to the growth medium did not bypassbni1 hyphal growth defects. Similarly, these agents were not able to suppress hyphal growth defects in thewal1 mutant which is lacking the Wiskott-Aldrich syndrome protein (WASP) homolog. These results suggest that the maintenance of polarized hyphal growth inC. albicans requires coordinated regulation of two actin cytoskeletal pathways, including formin-mediated secretion and WASP-dependent endocytosis.