Survival Strategies of Yeast and Filamentous Fungi against the Antifungal Protein AFP

Abstract

The activities of signaling pathways are critical for fungi to survive antifungal attack and to maintain cell integrity. However, little is known about how fungi respond to antifungals, particularly if these interact with multiple cellular targets. The antifungal protein AFP is a very potent inhibitor of chitin synthesis and membrane integrity in filamentous fungi and has so far not been reported to interfere with the viability of yeast strains. With the hypothesis that the susceptibility of fungi toward AFP is not merely dependent on the presence of an AFP-specific target at the cell surface but relies also on the cell's capacity to counteract AFP, we used a genetic approach to decipher defense strategies of the naturally AFP-resistant strain Saccharomyces cerevisiae. The screening of selected strains from the yeast genomic deletion collection for AFPsensitive phenotypes revealed that a concerted action of calcium signaling, TOR signaling, cAMP-protein kinase A signaling, and cell wall integrity signaling is likely to safeguard S. cerevisiae against AFP. Our studies uncovered that the yeast cell wall gets fortified with chitin to defend against AFP and that this response is largely dependent on calcium/Crz1p signaling. Most importantly, we observed that stimulation of chitin synthesis is characteristic for AFP-resistant fungi but not for AFP-sensitive fungi, suggesting that this response is a successful strategy to protect against AFP. We finally propose the adoption of the damage-response framework of microbial pathogenesis for the interactions of antimicrobial proteins and microorganisms in order to comprehensively understand the outcome of an antifungal attack.

Impact of the antifungal protein PgAFP from Penicillium chrysogenum on the protein profile in Aspergillus flavus

“…Similarly, reinforcement of chitin synthesis has been described as a distinctive feature of AFP-moderate sensitive and AFPresistant fungi, whereas it is not realized in AFP-sensitive fungi (Ouedraogo et al 2011). This elevation of chitin levels has been interpreted as the necessary response to survive an AFP attack.…”

“…Until now, the effect of antifungal proteins from molds has been explained by signaling processes via G protein and RhoA (Binder et al 2010;Ouedraogo et al 2011). We propose the lower relative abundance of Rho1 and G protein subunit β CpcB to be the main factors in the mode of action of PgAFP.…”

FLO11 Gene Is Involved in the Interaction of Flor Strains of Saccharomyces cerevisiae with a Biofilm-Promoting Synthetic Hexapeptide

“…We should notice that increased CFW staining could be also due to CW changes in cells exposed to PAF26. Previous independent results indicate the involvement of chitin synthase genes and chitin content in the mechanism of action of antifungal proteins (52)(53)(54)(55). Nevertheless, the changes in CFW staining that occur during PAF26 exposure (Fig.…”

Components of the Calcium-Calcineurin Signaling Pathway in Fungal Cells and Their Potential as Antifungal Targets

“…Thus, the specific mechanism of PAF in Ca 2ϩ homeostasis disruption requires more research. Another antifungal protein, AFP NN5353 , which is a defensinlike protein of Aspergillus giganteus, has been examined (99,100). This protein mediates the germination and growth of filamentous ascomycetes, including important human and plant pathogens, as well as the model organisms Aspergillus nidulans and Aspergillus niger, by inducing the rapid influx of extracellular Ca 2ϩ .…”