UME6 is a crucial downstream target of other transcriptional regulators of true hyphal development in Candida albicans.

True hyphal growth of Candida albicans can be induced by several environmental conditions and contributes significantly to the high virulence of this pathogenic fungus. The transcriptional network that governs hyphal morphogenesis is complex, depends on several regulators and is not completely understood. Recently, CaUME6, a homolog of the Saccharomyces cerevisiae UME6 gene, has been shown to be required for hyphal elongation. In the present study, the C. albicans ume6Delta strain showed a complete defect in hyphae formation under all the growth conditions tested. UME6 was repressed by the Nrg1-Tup1 repressor in yeast-form cells but NRG1 was not repressed by Ume6p under hyphal growth conditions. Wild-type UME6 expression depended on each hyphal regulator tested, and ectopic UME6 expression in efg1Delta, cph1Delta and ras1Delta cells rescued the hyphal defects of these mutants under some hyphal growth conditions. Thus, UME6 is a common downstream target of regulators promoting hyphal development. Ectopic UME6 expression promoted both germ tube formation and hyphal elongation. The expression of all hyphae-specific genes investigated depended on UME6 expression. A model for transcriptional regulation of hyphal development and the role of Ume6p is proposed.

Candida albicans AGE3, the ortholog of the S. cerevisiae ARF-GAP-encoding gene GCS1, is required for hyphal growth and drug resistance.

BACKGROUND: Hyphal growth and multidrug resistance of C. albicans are important features for virulence and antifungal therapy of this pathogenic fungus. METHODOLOGY/PRINCIPAL FINDINGS: Here we show by phenotypic complementation analysis that the C. albicans gene AGE3 is the functional ortholog of the yeast ARF-GAP-encoding gene GCS1. The finding that the gene is required for efficient endocytosis points to an important functional role of Age3p in endosomal compartments. Most C. albicans age3Delta mutant cells which grew as cell clusters under yeast growth conditions showed defects in filamentation under different hyphal growth conditions and were almost completely disabled for invasive filamentous growth. Under hyphal growth conditions only a fraction of age3Delta cells shows a wild-type-like polarization pattern of the actin cytoskeleton and lipid rafts. Moreover, age3Delta cells were highly susceptible to several unrelated toxic compounds including antifungal azole drugs. Irrespective of the AGE3 genotype, C-terminal fusions of GFP to the drug efflux pumps Cdr1p and Mdr1p were predominantly localized in the plasma membrane. Moreover, the plasma membranes of wild-type and age3Delta mutant cells contained similar amounts of Cdr1p, Cdr2p and Mdr1p. CONCLUSIONS/SIGNIFICANCE: The results indicate that the defect in sustaining filament elongation is probably caused by the failure of age3Delta cells to polarize the actin cytoskeleton and possibly of inefficient endocytosis. The high susceptibility of age3Delta cells to azoles is not caused by inefficient transport of efflux pumps to the cell membrane. A possible role of a vacuolar defect of age3Delta cells in drug susceptibility is proposed and discussed. In conclusion, our study shows that the ARF-GAP Age3p is required for hyphal growth which is an important virulence factor of C. albicans and essential for detoxification of azole drugs which are routinely used for antifungal therapy. Thus, it represents a promising antifungal drug target.