Med15B Regulates Acid Stress Response and Tolerance in Candida glabrata by Altering Membrane Lipid Composition

Abstract

<i>Candida glabrata</i> is a promising producer of organic acids. To elucidate the physiological function of the Mediator tail subunit Med15B in the response to low-pH stress, we constructed a deletion strain, <i>C. glabrata</i><i>med15B</i>Δ, and an overexpression strain, <i>C. glabrata HTU</i>Δ/<i>CgMED15B</i> Deletion of <i>MED15B</i> caused biomass production, glucose consumption rate, and cell viability to decrease by 28.3%, 31.7%, and 26.5%, respectively, compared with those of the parent (<i>HTU</i>Δ) strain at pH 2.0. Expression of lipid metabolism-related genes was significantly downregulated in the <i>med15B</i>Δ strain, whereas key genes of ergosterol biosynthesis showed abnormal upregulation. This caused the proportion of C_18:1 fatty acids, the ratio of unsaturated to saturated fatty acids (UFA/SFA), and the total phospholipid content to decrease by 11.6%, 27.4%, and 37.6%, respectively. Cells failed to synthesize fecosterol and ergosterol, leading to the accumulation and a 60.3-fold increase in the concentration of zymosterol. Additionally, cells showed reductions of 69.2%, 11.6%, and 21.8% in membrane integrity, fluidity, and H⁺-ATPase activity, respectively. In contrast, overexpression of Med15B increased the C_18:1 levels, total phospholipids, ergosterol content, and UFA/SFA by 18.6%, 143.5%, 94.5%, and 18.7%, respectively. Membrane integrity, fluidity, and H⁺-ATPase activity also increased by 30.2%, 6.9%, and 51.8%, respectively. Furthermore, in the absence of pH buffering, dry weight of cells and pyruvate concentrations were 29.3% and 61.2% higher, respectively, than those of the parent strain. These results indicated that in <i>C. glabrata</i>, Med15B regulates tolerance toward low pH via transcriptional regulation of acid stress response genes and alteration in lipid composition.<b>IMPORTANCE</b> This study explored the role of the Mediator tail subunit Med15B in the metabolism of <i>Candida glabrata</i> under acidic conditions. Overexpression of <i>MED15B</i> enhanced yeast tolerance to low pH and improved biomass production, cell viability, and pyruvate yield. Membrane lipid composition data indicated that Med15B might play a critical role in membrane integrity, fluidity, and H⁺-ATPase activity homeostasis at low pH. Thus, controlling membrane composition may serve to increase <i>C. glabrata</i> productivity at low pH.