Importance of cell wall mannoproteins for septum formation in Saccharomyces cerevisiae.

The mannosyltransferase mutants mnn9 and mnn10 were isolated in a genetic screen for septation defects in Saccharomyces cerevisiae. Ultrastructural examination of mutant cell walls revealed markedly thin septal structures and occasional failure to construct trilaminar septa, which then led to the formation of bulky default septa at the bud neck. In the absence of a functional septation apparatus, mnn10 mutants are unable to complete cytokinesis and die as cell chains with incompletely separated cytoplasms, indicating that mannosylation defects impair the ability to form remedial septa. We could not detect N-linked glycosylation of the beta(1,3)glucan synthase Fks1p and mnn10 defects do not change the molecular weight or abundance of the protein. We discuss a model explaining the pleiotropic effects of impaired N-linked protein glycosylation on septation in S. cerevisiae.