Several microbial pathogens augment their invasive potential by binding and activating human plasminogen to generate the proteolytic enzyme plasmin. Yeast cells and cell wall proteins (CWP) of the human pathogenic fungus Candida albicans bound plasminogen with a K(d) of 70 +/- 11 nM and 112 +/- 20 nM respectively. Bound plasminogen could be activated to plasmin by mammalian plasminogen activators; no C. albicans plasminogen activator was detected. Binding of plasminogen to CWP and whole cells was inhibited by epsilon ACA, indicating that binding was predominantly to lysine residues. Candida albicans mutant strains defective in protein glycosylation did not show altered plasminogen binding, suggesting that binding was not mediated via a surface lectin. Binding was sensitive to digestion by basic carboxypeptidase, implicating C-terminal lysine residues in binding. Proteomic analysis identified eight major plasminogen-binding proteins in isolated CWP. Five of these (phosphoglycerate mutase, alcohol dehydrogenase, thioredoxin peroxidase, catalase, transcription elongation factor) had C-terminal lysine residues and three (glyceraldehyde-3-phosphate dehydrogenase, phosphoglycerate kinase and fructose bisphosphate aldolase) did not. Activation of plasminogen could potentially increase the capacity of this pathogenic fungus for tissue invasion and necrosis. Although surface-bound plasmin(ogen) degraded fibrin, no direct evidence for a role in invasion of endothelial matrix or in penetration and damage of endothelial cells was found.

A better understanding of the molecular basis of programmed cell death (PCD) in fungi could provide information that is useful in the design of antifungal drugs that combat life-threatening fungal infections. Harsh environmental stresses, such as acetic acid or hydrogen peroxide, have been shown to induce PCD in the pathogenic fungus Candida albicans. In this study, we show that dying cells progress from an apoptotic state to a secondary necrotic state and that the rate at which this change occurs is proportional to the intensity of the stimulus. Also, we found that the temporal response is modulated by Ras-cAMP-PKA signals. Mutations that block Ras-cAMP-PKA signaling (ras1⌬, cdc35⌬, tpk1⌬, and tpk2⌬) suppress or delay the apoptotic response, whereas mutations that stimulate signaling (RAS1 val13 and pde2⌬) accelerate the rate of entry of cells into apoptosis. Pharmacological stimulation or inhibition of Ras signaling reinforces these findings. Transient increases in endogenous cAMP occur under conditions that stimulate apoptosis but not growth arrest. Death-specific changes in the abundance of different isoforms of the PKA regulatory subunit, Bcy1p, are also observed. Activation of Ras signals may regulate PCD of C. albicans, either by inhibiting antiapoptotic functions (such as stress responses) or by activating proapoptotic functions.apoptosis ͉ cAMP ͉ necrosis

MNL1, the Candida albicans homologue of an orphan Msn2-like gene (YER130c in Saccharomyces cerevisiae) has no known function. Here we report that MNL1 regulates weak acid stress responses. Deletion of MNL1 prevents the long-term adaptation of C. albicans cells to weak acid stresses and compromises their global transcriptional response under these conditions. The promoters of Mnl1-dependent genes contain a novel STRE-like element (SLE) that imposes Mnl1-dependent, weak acid stress-induced transcription upon a lacZ reporter in C. albicans. The SLE (HHYYCCCCT-TYTY) is related to the Nrg1 response element (NRE) element recognized by the transcriptional repressor Nrg1. Deletion of NRG1 partially restores the ability of C. albicans mnl1 cells to adapt to weak acid stress, indicating that Mnl1 and Nrg1 act antagonistically to regulate this response. Molecular, microarray, and proteomic analyses revealed that Mnl1-dependent adaptation does not occur in cells exposed to proapoptotic or pronecrotic doses of weak acid, suggesting that Ras-pathway activation might suppress the Mnl1-dependent weak acid response in dying cells. Our work defines a role for this YER130c orthologue in stress adaptation and cell death. INTRODUCTIONAll organisms must respond and adapt to environmental stresses if they are to survive adverse conditions. Microbes elicit a combination of specific and general stress responses that repair the damage generated by environmental stresses and restore cellular and metabolic homeostasis under the hostile conditions. These responses are particularly important in pathogenic microbes which have evolved molecular mechanisms to counteract the defenses of their host.In the benign model yeast Saccharomyces cerevisiae, the so-called general stress response or environmental stress response, confers resistance to heat shock, pro-oxidants, osmotic shock, nutrient deprivation, alcohol, and weak acids (Gasch et al., 2000;Causton et al., 2001). This general stress response is largely coordinated by the transcription factors Msn2 and Msn4 (Estruch and Carlson 1993, Marchler et al., 1993). Under stress conditions Msn2 and Msn4 accumulate in the nucleus (Gorner et al., 1998;Jacquet et al., 2003) where they activate the transcription of stress genes containing STRE elements (CCCCT) in their promoters (Martinez-Pastor et al., 1996). This response is down-regulated by the Ras-cAMP-PKA pathway (Garreau et al., 2000). Protein kinase A (PKA)-mediated phosphorylation of Msn2 and Msn4 results in their cytoplasmic accumulation, thereby decreasing the expression of their target stress genes (Gorner et al., 1998(Gorner et al., , 2002.The pathogenic yeast Candida albicans causes frequent infections of the oral and vaginal mucosa and potentially lethal systemic infections in severely immunocompromised individuals, including patients receiving transplants or chemotherapy (Odds, 1988). C. albicans occupies a variety of niches within the human body, encountering a range of stressful conditions as it interacts with its host and counteracts ...