CLONING AND ANALYSIS OF THE SACCHAROMYCES-CEREVISIAE MNN9 AND MNN1 GENES REQUIRED FOR COMPLEX GLYCOSYLATION OF SECRETED PROTEINS

Proteins secreted by the yeast Saccharomyces cerevisiae are usually modified by the addition at asparagine-linked glycosylation sites of large heterogeneous mannan units that are highly immunogenic. Secreted proteins from mnn1 mnn9 mutant strains, in contrast, have homogeneous Man10GlcNAC2 oligosaccharides that lack the immunogenic alpha1,3-mannose linkages. We have cloned and sequenced the MNN9 and MNN1 genes, both of which encode proteins with the characteristics of type II membrane proteins. Mnn9p is a membrane-associated protein with unknown function that is required for the addition of the long alpha 1,6-mannose backbone of the complex mannan, whereas Mnn1p is most likely the alpha1,3-mannosyltransferase located in the Golgi apparatus.