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The fungal cell possesses an essential carbohydrate cell wall. The outer layer, mannan, is formed by mannoproteins carrying highly mannosylated  O - and  N -linked glycans. Yeast mannan biosynthesis is initiated by a Golgi-located complex (M-Pol I) of two GT-62 mannosyltransferases, Mnn9p and Van1p, that are conserved in fungal pathogens.  Saccharomyces cerevisiae  and  Candida albicans mnn9  knockouts show an aberrant cell wall and increased antibiotic sensitivity, suggesting the enzyme is a potential drug target. Here, we present the structure of  Sc Mnn9 in complex with GDP and Mn 2+ , defining the fold and catalytic machinery of the GT-62 family. Compared with distantly related GT-78/GT-15 enzymes,  Sc Mnn9 carries an unusual extension. Using a novel enzyme assay and site-directed mutagenesis, we identify conserved amino acids essential for  Sc Mnn9 ‘priming’ α-1,6-mannosyltransferase activity. Strikingly, both the presence of the  Sc Mnn9 protein and its product, but not  Sc Mnn9 catalytic activity, are required to activate subsequent  Sc Van1 processive α-1,6-mannosyltransferase activity in the M-Pol I complex. These results reveal the molecular basis of mannan synthesis and will aid development of inhibitors targeting this process.

Yeast Mnn9 is both a priming glycosyltransferase and an allosteric activator of mannan biosynthesis