Processing of Candida albicans Ece1p Is Critical for Candidalysin Maturation and Fungal Virulence

Candida albicans is an opportunistic fungal pathogen responsible for superficial and life-threatening infections in humans. During mucosal infection, C. albicans undergoes a morphological transition from yeast to invasive filamentous hyphae that secrete candidalysin, a 31-amino-acid peptide toxin required for virulence. Candidalysin damages epithelial cell plasma membranes and stimulates the activating protein 1 (AP-1) transcription factor c-Fos (via p38-mitogen-activated protein kinase [MAPK]), and the MAPK phosphatase MKP1 (via extracellular signal-regulated kinases 1 and 2 [ERK1/2]-MAPK), which trigger and regulate proinflammatory cytokine responses, respectively. The candidalysin toxin resides as a discrete cryptic sequence within a larger 271-amino-acid parental preproprotein, Ece1p. Here, we demonstrate that kexin-like proteinases, but not secreted aspartyl proteinases, initiate a two-step posttranslational processing of Ece1p to produce candidalysin. Kex2p-mediated proteolysis of Ece1p after Arg61 and Arg93, but not after other processing sites within Ece1p, is required to generate immature candidalysin from Ece1p, followed by Kex1p-mediated removal of a carboxyl arginine residue to generate mature candidalysin. C. albicans strains harboring mutations of Arg61 and/or Arg93 did not secrete candidalysin, were unable to induce epithelial damage and inflammatory responses in vitro, and showed attenuated virulence in vivo in a murine model of oropharyngeal candidiasis. These observations identify enzymatic processing of C. albicans Ece1p by kexin-like proteinases as crucial steps required for candidalysin production and fungal pathogenicity.IMPORTANCECandida albicans is an opportunistic fungal pathogen that causes mucosal infection in millions of individuals worldwide. Successful infection requires the secretion of candidalysin, the first cytolytic peptide toxin identified in any human fungal pathogen. Candidalysin is derived from its parent protein Ece1p. Here, we identify two key amino acids within Ece1p vital for processing and production of candidalysin. Mutations of these residues render C. albicans incapable of causing epithelial damage and markedly reduce mucosal infection in vivo Importantly, candidalysin production requires two individual enzymatic events. The first involves processing of Ece1p by Kex2p, yielding immature candidalysin, which is then further processed by Kex1p to produce the mature toxin. These observations identify important steps for C. albicans pathogenicity at mucosal surfaces.

The Candida albicans transcription factor Cas5 couples stress responses, drug resistance and cell cycle regulation

The capacity to coordinate environmental sensing with initiation of cellular responses underpins microbial survival and is crucial for virulence and stress responses in microbial pathogens. Here we define circuitry that enables the fungal pathogen Candida albicans to couple cell cycle dynamics with responses to cell wall stress induced by echinocandins, a front-line class of antifungal drugs. We discover that the C. albicans transcription factor Cas5 is crucial for proper cell cycle dynamics and responses to echinocandins, which inhibit β-1,3-glucan synthesis. Cas5 has distinct transcriptional targets under basal and stress conditions, is activated by the phosphatase Glc7, and can regulate the expression of target genes in concert with the transcriptional regulators Swi4 and Swi6. Thus, we illuminate a mechanism of transcriptional control that couples cell wall integrity with cell cycle regulation, and uncover circuitry governing antifungal drug resistance.Cas5 is a transcriptional regulator of responses to cell wall stress in the fungal pathogen Candida albicans. Here, Xie et al. show that Cas5 also modulates cell cycle dynamics and responses to antifungal drugs.

Complex Haploinsufficiency-Based Genetic Analysis of the NDR/Lats Kinase Cbk1 Provides Insight into Its Multiple Functions in Candida albicans

Although the analysis of genetic interactions and networks is a powerful approach to understanding biology, it has not been applied widely to the pathogenic yeast Candida albicans Here, we describe the use of both screening and directed genetic interaction studies based on complex haploinsufficiency to probe the function of the R: egulation of A: ce2 and M: orphogenesis (RAM) pathway in C. albicans A library of 5200 Tn7-mutagenized derivatives of a parental strain heterozygous at CBK1, the key kinase in the RAM pathway, was screened for alterations in serum-induced filamentation. Following confirmation of phenotypes and identification of insertion sites by sequencing, a set of 36 unique double heterozygous strains showing complex haploinsufficiency was obtained. In addition to a large set of genes regulated by the RAM transcription factor Ace2, genes related to cell wall biosynthesis, cell cycle, polarity, oxidative stress, and nitrogen utilization were identified. Follow-up analysis led to the first demonstration that the RAM pathway is required for oxidative stress tolerance in a manner related to the two-component-regulated kinase Chk1 and revealed a potential direct connection between the RAM pathway and the essential Mps1 spindle pole-related kinase. In addition, genetic interactions with CDC42-related genes MSB1, a putative scaffold protein, and RGD3, a putative Rho GTPase-activating protein (GAP) were identified. We also provide evidence that Rgd3 is a GAP for Cdc42 and show that its localization and phosphorylation are dependent on Cbk1.