Regulation of the protein glycosylation pathway in yeast: structural control of N-linked oligosaccharide elongation.

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Abstract
The yeast Saccharomyces cerevisiae X2180 strain with the mnn1 mnn2 mnn9 mutations, all of which affect mannoprotein glycosylation, synthesizes Man10GlcNAc2 oligosaccharides, whereas the mnn1 mnn2 mutant extends the .alpha.1 .fwdarw. 6-linked backbone of some of the core oligosaccharides by adding 20-30 mannose units. Membrane fractions from the mnn1 mnn2 and mnn1 mnn2 mnn9 mutants are equally effective in catalyzing transfer from GDP-[3H]mannose to added mannose in both .alpha.1 .fwdarw. 2 and .alpha.1 .fwdarw. 6 linkages to a Man9GlcNAc2 oligosaccharide, but neither membrane preparation can utilize the homologous mnn1 mnn2 mnn9 oligosaccharide as an acceptor. Thus, addition of .alpha.1 .fwdarw. 2 linked mannose side chain to the terminal .alpha.1 .fwdarw. 6 linked mannose in oligosaccharides of the mnn9 mutant inhibits the elongation reaction and may serve as an important structural control of mannoprotein glycosylation. The mnn9 mutation also increases the transit time for invertase secretion, meaning that this mutation could affect the processing machinery in the Golgi apparatus.