Putative Stress Sensors WscA and WscB Are Involved in Hypo-Osmotic and Acidic pH Stress Tolerance in Aspergillus nidulans

Wsc proteins have been identified in fungi and are believed to be stress sensors in the cell wall integrity (CWI) signaling pathway. In this study, we characterized the sensor orthologs WscA and WscB inAspergillus nidulans . Using hemagglutinin-tagged WscA and WscB, we showed both Wsc proteins to beN - andO -glycosylated and localized in the cell wall and membrane, implying that they are potential cell surface sensors. ThewscA disruptant (ΔwscA ) strain was characterized by reduced colony and conidia formation and a high frequency of swollen hyphae under hypo-osmotic conditions. The deficient phenotype of the ΔwscA strain was facilitated by acidification, but not by alkalization or antifungal agents. In contrast, osmotic stabilization restored the normal phenotype in the ΔwscA strain. A similar inhibition was observed in thewscB disruptant strain, but to a lesser extent. In addition, a doublewscA andwscB disruptant (ΔwscA ΔwscB ) strain was viable, but its growth was inhibited to a greater degree, indicating that the functions of the products of these genes are redundant. Transcription ofα -1,3-glucan synthase genes (agsA andagsB ) was significantly altered in thewscA disruptant strain, resulting in an increase in the amount of alkali-soluble cell wall glucan compared to that in the wild-type (wt) strain. An increase in mitogen-activated protein kinase (MpkA) phosphorylation was observed as a result ofwsc disruption. Moreover, the transient transcriptional upregulation of theagsB gene via MpkA signaling was observed in the ΔwscA ΔwscB strain to the same degree as in the wt strain. These results indicate thatA. nidulans Wsc proteins have a different sensing spectrum and downstream signaling pathway than those in the yeastSaccharomyces cerevisiae and that they play an important role in CWI under hypo-osmotic and acidic pH conditions.