Rac1 and Cdc42 Have Different Roles in Candida albicans Development

Abstract

We investigated the role of the highly conserved G protein Rac1 in the opportunistic pathogen Candida albicans. We identified and disrupted RAC1 and show here that, in contrast to CDC42, it is not necessary for viability or serum-induced hyphal growth but is essential for filamentous growth when cells are embedded in a matrix. Rac1 is localized to the plasma membrane, yet its distribution is more homogenous than that of Cdc42, with no enrichment at the tips of either buds or hyphae. In addition, fluorescence recovery after photobleaching results indicate that Rac1 and Cdc42 have different dynamics at the membrane. Furthermore, overexpression of Rac1 does not complement Cdc42 function, and conversely, overexpression of Cdc42 does not complement Rac1 function. Thus, Rac1 and Cdc42, although highly similar to one another, have different roles in C. albicans development.Candida albicans is an opportunistic human fungal pathogen which causes essentially superficial mycoses of skin or the mucosal epithelia (20) but also can be life-threatening to immunodeficient patients (15). C. albicans is a dimorphic fungus, i.e., it can switch from budding to filamentous growth, and this dimorphic transition is important for its pathogenicity (10,26). In order to restrict the polarized growth necessary for invasive C. albicans filamentous development, cells need to reorganize their cytoskeleton.Rho-type G proteins are required for cytoskeleton organization in virtually all eukaryotic cells (13). They are divided into three subfamilies, Rho, Rac, and Cdc42, within the Ras superfamily. In mammals, Rac and Cdc42, in particular, play overlapping and complementary functions in cell morphogenesis, division, and migration (13). In fungi, the functions of Cdc42 and Rac1 appear to vary. For example, in Yarrowia lipolytica, Rac1 is not essential for cell viability or actin organization but is required for hyphal growth (12). In contrast, in Penicillium marneffei, the Rac homolog CflB plays a crucial role in actin cytoskeleton organization and vegetative growth (5). In Cryptococcus neoformans, Rac1 is not required for actin cytoskeleton organization but is required for haploid filamentation and mating (22). In the ascomycete Saccharomyces cerevisiae, Cdc42 is required for viability (14), mating (19), and pseudohyphal development (18); however, no Rac1 ortholog exists.In C. albicans, Cdc42 and its exchange factor, Cdc24, have critical roles in viability, the bud-to-hypha transition, hyphal growth maintenance, and pathogenicity in a murine model of systemic candidiasis (3,4,17,21,23). Here we identify a Rac1 protein in C. albicans which has 60% overall sequence identity with its human counterpart and 57% identity with C. albicans Cdc42. We have disrupted RAC1 and show here that this G protein is not necessary for viability or actin cytoskeleton organization. Rac1 is essential for filamentous growth when cells are embedded in an agar matrix yet is not required for seruminduced hyphal growth, in contrast to Cdc42. We also show that Rac1 is local...

Rho GTPase activity zones and transient contractile arrays

“…(18) Meanwhile, a new Rho zone appears just as rapidly as the old one disappears. Third, photobleaching experiments in budding yeast (45) and Candida albicans (46) indicate that Cdc42 at the bud site is turned over within ~5 seconds; photobleaching experiments in echinoderm embryos likewise indicate rapid turnover within cytokinetic Rho zones (10-30 seconds; unpublished results).…”

Fungal Morphogenesis

“…Moreover, despite high sequence conservation, overexpression of Rac or Cdc42-encoding genes does not cross-complement mutations of the other GTPase. Therefore, these two highly related proteins appear to serve distinct functions in the physiology of C. albicans (Bassilana and Arkowitz 2006).…”

“…In other fungi, especially those with more prominent hyphal morphological forms, the related Rac and Cdc42 proteins together direct cell polarity (Hurtado et al 2000;Boyce et al 2003;Chen and Dickman 2004;Vallim et al 2005;Bassilana and Arkowitz 2006). C. neoformans is unique in that it possesses duplicated genes for Ras, Rac, and Cdc42 proteins (Ballou et al 2013a).…”

Regulation of polarised growth in fungi

“…The C. albicans genome also contains a member of the Rac family of GTPases. Mutants lacking Rac1p have a normal phenotype except they are unable to form hyphae when embedded in a matrix (Bassilana and Arkowitz, 2006). Thus, Rac1p plays a minor role compared to Cdc42p in C. albicans.…”