Rac1 Dynamics in the Human Opportunistic Fungal Pathogen Candida albicans

The small Rho G-protein Rac1 is highly conserved from fungi to humans, with approximately 65% overall sequence identity in Candida albicans. As observed with human Rac1, we show that C. albicans Rac1 can accumulate in the nucleus, and fluorescence recovery after photobleaching (FRAP) together with fluorescence loss in photobleaching (FLIP) studies indicate that this Rho G-protein undergoes nucleo-cytoplasmic shuttling. Analyses of different chimeras revealed that nuclear accumulation of C. albicans Rac1 requires the NLS-motifs at its carboxyl-terminus, which are blocked by prenylation of the adjacent cysteine residue. Furthermore, we show that C. albicans Rac1 dynamics, both at the plasma membrane and in the nucleus, are dependent on its activation state and in particular that the inactive form accumulates faster in the nucleus. Heterologous expression of human Rac1 in C. albicans also results in nuclear accumulation, yet accumulation is more rapid than that of C. albicans Rac1. Taken together our results indicate that Rac1 nuclear accumulation is an inherent property of this G-protein and suggest that the requirements for its nucleo-cytoplasmic shuttling are conserved from fungi to humans

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

Phosphorylation of Rga2, a Cdc42 GAP, by CDK/Hgc1 is crucial for Candida albicans hyphal growth

Cyclin-dependent kinases (CDKs) control yeast morphogenesis, although how they regulate the polarity machinery remains unclear. The dimorphic fungus Candida albicans uses Cdc28/Hgc1, a CDK/cyclin complex, to promote persistent actin polarization for hyphal growth. Here, we report that Rga2, a GTPase-activating protein (GAP) of the central polarity regulator Cdc42, undergoes Hgc1-dependent hyperphosphorylation. Using the analog-sensitive Cdc28as mutant, we confirmed that Cdc28 controls Rga2 phosphorylation in vitro and in vivo. Deleting RGA2 produced elongated yeast cells without apparent effect on hyphal morphogenesis. However, deleting it or inactivating its GAP activity restored hyphal growth in hgc1Δ mutants, suggesting that Rga2 represses hyphal development and Cdc28/Hgc1 inactivates it upon hyphal induction. We provide evidence that Cdc28/Hgc1 may act to prevent Rga2 from localizing to hyphal tips, leading to localized Cdc42 activation for hyphal extension. Rga2 also undergoes transient Cdc28-dependent hyperphosphorylation at bud emergence, suggesting that regulating a GAP(s) of Cdc42 by CDKs may play an important role in governing different forms of polarized morphogenesis in yeast. This study reveals a direct molecular link between CDKs and the polarity machinery