The putative stress sensor protein MtlA is required for conidia formation, cell wall stress tolerance, and cell wall integrity in Aspergillus nidulans

The Mid2-like protein MtlA is a putative sensor of the cell wall integrity (CWI) signaling pathway in Aspergillus nidulans. An MtlA-EGFP fusion protein was localized at the cell surface and septa. The mtlA disruptant (ΔmtlA) showed radial colony growth similar to the wild-type (wt) strain, but showed reduced conidia formation. The ΔmtlA mutant showed growth deficiency in the presence of inhibitors of cell wall synthesis. Moreover, mtlA disruption resulted in a reduction in the glucan and chitin content in the cell wall. These results suggest that MtlA plays a significant role in asexual sporulation, cell wall stress tolerance, and the maintenance of CWI in A. nidulans, but transcriptional upregulation of α-1,3-glucan synthase gene agsB induced by micafungin was observed in the ΔmtlA strain as well as the wt strain. Thus, MtlA is not essential for activation of the downstream CWI signaling pathway components identified in previous studies of Saccharomyces cerevisiae.