A Rac Homolog Is Required for Induction of Hyphal Growth in the Dimorphic Yeast Yarrowia lipolytica

Abstract

Dimorphism in fungi is believed to constitute a mechanism of response to adverse conditions and represents an important attribute for the development of virulence by a number of pathogenic fungal species. We have isolated YlRAC1, a gene encoding a 192-amino-acid protein that is essential for hyphal growth in the dimorphic yeast Yarrowia lipolytica and which represents the first Rac homolog described for fungi. YlRAC1 is not an essential gene, and its deletion does not affect the ability to mate or impair actin polarization in Y. lipolytica. However, strains lacking functional YlRAC1 show alterations in cell morphology, suggesting that the function of YlRAC1 may be related to some aspect of the polarization of cell growth. Northern blot analysis showed that transcription of YlRAC1 increases steadily during the yeast-to-hypha transition, while Southern blot analysis of genomic DNA suggested the presence of several RAC family members in Y. lipolytica. Interestingly, strains lacking functional YlRAC1 are still able to grow as the pseudohyphal form and to invade agar, thus pointing to a function for YlRAC1 downstream of MHY1, a previously isolated gene encoding a C 2 H 2 -type zinc finger protein with the ability to bind putative stress response elements and whose activity is essential for both hyphal and pseudohyphal growth in Y. lipolytica.

Fungal Morphogenesis

“…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).…”

Sustained cell polarity and virulence in the phytopathogenic fungusUstilago maydisdepends on an essential cyclin-dependent kinase from the Cdk5/Pho85 family

“…In dimorphic fungi, the Rho-like proteins Rac1 and Cdc42 play essential roles in cytoskeleton organization and polarization (Hurtado et al, 2000;Ushinsky et al, 2002;Boyce et al, 2003;Vallim et al, 2005;Bassilana and Arkowitz, 2006;Mahlert et al, 2006). In U. maydis, Rac1 is required to sustain polar growth, whereas Cdc42 has a role in cell separation (Mahlert et al, 2006).…”

Morphological analysis of Yarrowia lipolytica under stress conditions through image processing

“…In wild strains, such as the one employed in this work, the hyphae growing ability can provide a selective advantage when facing conditions of stress [9,10]. It is known that, specifically in Y. lipolytica, the carbon source employed has no influence on cell morphology and has a medium pH.…”

“…It is known that, specifically in Y. lipolytica, the carbon source employed has no influence on cell morphology and has a medium pH. However, their morphology is considered to be strongly dependent on the nitrogen source and, if the mitochondrial respiration is inhibited, no hyphae formation is achieved [7,10].…”

“…It is known that, specifically in Y. lipolytica, the carbon source employed has no influence on cell morphology and has a medium pH. However, their morphology is considered to be strongly dependent on the nitrogen source and, if the mitochondrial respiration is inhibited, no hyphae formation is achieved [7,10].Fungal dimorphism is generally characterized by the presence of many intermediate morphological forms, displaying a broad distribution of cell sizes and shapes. Nevertheless, under certain conditions, one particular morphology may dominate [11].…”