Aspergillus fumigatus Mnn9 is responsible for mannan synthesis and required for covalent linkage of mannoprotein to the cell wall

Owing to the essential role in protection of the Aspergillus fumigatus cell against human defense reactions, its cell wall has long been taken as a promising antifungal target. The cell wall of A. fumigatus composed of chitin, glucan and galactomannan and mannoproteins. Although galactomannan has been used as a diagnostic target for a long time, its biosynthesis remains unknown in A. fumigatus. In this study, a putative alpha 1,6-mannosyltransferase gene mnn9 was identified in A. fumigatus. Deletion of the mrtn9 gene resulted in an increased sensitivity to calcofluor white, Congo red, or hygromycin B as well as in reduced cell wall components and abnormal polarity. Although there was no major effect on N-glycan synthesis, covalently-linked cell wall mannoprotein Mp1 was significantly reduced in the mutant. Based on our results, we propose that Mnn9p is a mannosyltransferase responsible for the formation of the a-mannan in cell wall mannoproteins, potentially via elongation of O-linked mannose chains.