Hypoxic conditions and iron restriction affect the cell-wall proteome of Candida albicans grown under vagina-simulative conditions

Abstract

Proteins that are covalently linked to the skeletal polysaccharides of the cell wall of Candida albicans play a major role in the colonization of the vaginal mucosal surface, which may result in vaginitis. Here we report on the variability of the cell-wall proteome of C. albicans as a function of the ambient O2 concentration and iron availability. For these studies, cells were cultured at 37 °C in vagina-simulative medium and aerated with a gas mixture consisting of 6 % (v/v) CO2, 0.01–7 % (v/v) O2 and N2, reflecting the gas composition in the vaginal environment. Under these conditions, the cells grew exclusively in the non-hyphal form, with the relative growth rate being halved at ∼0.02 % (v/v) O2. Using tandem MS and immunoblot analysis, we identified 15 covalently linked glycosylphosphatidylinositol (GPI) proteins in isolated walls (Als1, Als3, Cht2, Crh11, Ecm33, Hwp1, Pga4, Pga10, Phr2, Rbt5, Rhd3, Sod4, Ssr1, Ywp1, Utr2) and 4 covalently linked non-GPI proteins (MP65, Pir1, Sim1/Sun42, Tos1). Five of them (Als3, Hwp1, Sim1, Tos1, Utr2) were absent in cells grown in rich medium. Immunoblot analysis revealed that restricted O2 availability resulted in higher levels of the non-GPI protein Pir1, a putative β-1,3-glucan cross-linking protein, and of the GPI-proteins Hwp1, an adhesion protein, and Pga10 and Rbt5, which are involved in iron acquisition. Addition of the iron chelator ferrozine at saturating levels of O2 resulted in higher cell wall levels of Hwp1 and Rbt5, suggesting that the responses to hypoxic conditions and iron restriction are related.