Abstract
Host defenses expose fungal pathogens to oxidants and antimicrobial chemicals. The fungal cell employs conserved eukaryotic signaling pathways and dedicated transcription factors to program its response to these stresses. The oxidant-sensitive transcription factor of yeast, YAP1, and its orthologs in filamentous fungi, are central to tolerance to oxidative stress. The C-terminal domain of YAP1 contains cysteine residues that, under oxidizing conditions, form an intramolecular disulfide bridge locking the molecule in a conformation where the nuclear export sequence is masked. YAP1 accumulates in the nucleus, promoting transcription of genes that provide the cell with the ability to counteract oxidative stress. Chemicals including xenobiotics and plant signals can also promote YAP1 nuclearization in yeast and filamentous fungi. This could happen via direct or indirect oxidative stress, or by a different biochemical pathway. Plant phenolics are known antioxidants, yet they have been shown to elicit cellular responses that would usually be triggered to counter oxidant stress. Here we will discuss the evidence that YAP1 and MAPK pathways respond to phenolic compounds. Following this and other examples, we explore here how oxidative-stress sensing networks of fungi might have evolved to detect chemical stressors. Furthermore, we draw functional parallels between fungal YAP1 and mammalian Keap1-Nrf2 signaling systems.
Highlights
Fungi have extraordinary capacity to adapt to a changing environment, responding to environmental cues via signaling pathways poised for activation by specific stimuli
Signal transduction is mediated by transcription factors that reprogram gene expression to cope with the stress
Specificity of fAP1 to the activating stimulus could be determined at different levels: at the promoters of target genes, or by integration of the outputs of several regulatory hierarchies
Summary
Fungi have extraordinary capacity to adapt to a changing environment, responding to environmental cues via signaling pathways poised for activation by specific stimuli. Fungal cells are exposed to a multitude of stimuli, many of which are interpreted as stressors (Brown et al, 2017; Rangel et al, 2018) These stimuli include xenobiotics or reactive oxygen species of extra- or intra-cellular origin. Yeast commonly serve as a model to help understand general cell biological principles, with respect to the question posed here, the mammalian Keap1-Nrf pathway has been extensively studied due to its central importance in stress response at the cellular level, and, more broadly, human disease. This pathway is activated by both xenobiotics and reactive oxygen species and mediates detoxification of both. In a review aptly titled “an evolutionary journey through stressful space and time” (Fuse and Kobayashi, 2017) chart the evolution of the Keap1-Nrf signaling cascade
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