An aspartyl protease-mediated cleavage regulates structure and function of a flavodoxin-like protein and aids oxidative stress survival

The yapsin family of aspartyl proteases regulate glucose homeostasis in Candida glabrata

Invasive candidiasis poses a major healthcare threat. The human opportunistic fungal pathogen Candida glabrata, which causes mucosal and deep-seated infections, is armed with distinct virulence attributes, including a family of 11 glycosylphosphatidylinositol-linked aspartyl proteases, CgYapsins. Here, we have profiled total membrane proteomes of the C. glabrata wildtype and 11 proteases-deficient strain, Cgyps1-11Δ, by mass spectrometry analysis and uncovered a novel role for fungal yapsins in glucose sensing and homeostasis. Furthermore, through label-free quantitative membrane proteome analysis, we showed differential abundance of 42% of identified membrane proteins, with electron transport chain and glycolysis proteins displaying lower and higher abundance in Cgyps1-11Δ cells, compared with wildtype cells, respectively. We also demonstrated elevated glucose uptake and upregulation of genes that code for the low-glucose sensor CgSnf3, transcriptional regulators CgMig1 and CgRgt1, and hexose transporter CgHxt2/10 in the Cgyps1-11Δ mutant. We further elucidated a potential underlying mechanism through genetic and transcript measurement analysis under low- and high-glucose conditions and found CgSNF3 deletion to rescue high glucose uptake and attenuated growth of the Cgyps1-11Δ mutant in YPD medium, thereby linking CgYapsins with regulation of the CgSnf3-dependent low-glucose sensing pathway. Last, high ethanol production, diminished mitochondrial membrane potential, and elevated susceptibility to oxidative phosphorylation inhibitors point toward increased fermentative and decreased respiratory metabolism in the Cgyps1-11Δ mutant. Altogether, our findings revealed new possible glucose metabolism-regulatory roles for putative cell surface-associated CgYapsins and advanced our understanding of fungal carbohydrate homeostasis mechanisms.

An Assay to Determine NAD(P)H: Quinone Oxidoreductase Activity in Cell Extracts from Candida glabrata

Flavodoxin-like proteins (Fld-LPs) are an important constituent of the oxidative stress defense system in several organisms and highly conserved from bacteria to humans. These proteins possess NAD(P)H:quinone oxidoreductase activity and convert quinones to hydroquinones through two-electron reduction, using NAD(P)H and quinone as electron donor and acceptor, respectively. Purified yeast and bacterial Fld-LPs exhibit NAD(P)H:quinone oxidoreductase activity in vitro. Here, we describe a protocol to measure oxidoreductase activity of Fld-LPs that are present in extracts of whole cells. We have recently shown that the assembly and activity of a Fld-LP, CgPst2, is regulated by an aspartyl protease-mediated cleavage of its C-terminus in the pathogenic yeast Candida glabrata. Mutant yeast where the CgPST2 gene was deleted lacked cellular NAD(P)H:quinone oxidoreductase activity and displayed elevated susceptibility to menadione stress. The protocol described herein is based on the measurement of NADH oxidation (conversion of NADH to NAD⁺) by endogenous Fld-LPs in the presence of quinone menadione. This assay can be performed with whole cell lysates prepared by the mechanical lysis of C. glabrata cells and does not require expression and purification of Fld-LPs from a heterogeneous system, thereby allowing researchers to study the effect of different posttranslational modifications and varied structural states of Fld-LPs on their enzymatic activities. Since many FLP-LPs are known to exist in dimeric and tetrameric states possessing differential activities, our efficient and easy-to-use assay can reliably detect and validate their quinone reductase activities. Although we have used menadione with CgPst2 enzyme in our study, the protocol can easily be modified to examine the presence of Fld-LPs with specificity for other quinones. As this assay does not require many expensive chemicals, it can readily be scaled up and adapted for other medically important fungi and potentially be a useful tool to characterize fungal oxidative stress response systems and screen inhibitors specific for fungal Fld-LPs, thereby contributing to our understanding of fungal pathogenesis mechanisms.