Aspergillus fumigatusRasA Regulates Asexual Development and Cell Wall Integrity

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Abstract
The Ras family of proteins is a large group of monomeric GTPases. Members of the fungal Ras family act as molecular switches that transduce signals from the outside of the cell to signaling cascades inside the cell.A. fumigatusRasA is 94% identical to the essential RasA gene ofAspergillus nidulansand is the Ras family member sharing the highest identity to Ras homologs studied in many other fungi. In this study, we report thatrasAis not essential inA. fumigatus, but its absence is associated with slowed germination and a severe defect in radial growth. The ΔrasAhyphae were more than two times the diameter of wild-type hyphae, and they displayed repeated changes in the axis of polarity during hyphal growth. The deformed hyphae accumulated numerous nuclei within each hyphal compartment. The ΔrasAmutant conidiated poorly, but this phenotype could be ameliorated by growth on osmotically stabilized media. The ΔrasAmutant also showed increased susceptibility to cell wall stressors, stained more intensely with calcofluor white, and was refractory to lysing enzymes used to make protoplasts, suggesting an alteration of the cell wall. All phenotypes associated with deletion ofrasAcould be corrected by reinsertion of the wild-type gene. These data demonstrate a crucial role for RasA in both hyphal growth and asexual development inA. fumigatusand provide evidence that RasA function is linked to cell wall integrity.