Functional compartments of the yeast Golgi apparatus are defined by the sec7 mutation

The role of the SEC7 gene product in yeast intercompartmental protein transport was examined. A spectrum of N-linked oligosaccharide structures, ranging from core to nearly complete outer chain carbohydrate, was observed on glycoproteins accumulated in secretion-defective sec7 mutant cells. Terminal alpha 1-3-linked outer chain mannose residues failed to be added to N-linked glycoproteins in sec7 cells at the restrictive temperature. These results suggest that outer chain glycosyl modifications do not occur within a single compartment. Additional evidence consistent with subdivision of the yeast Golgi apparatus came from a cell-free glycoprotein transport reaction in which wild-type membranes sustained outer chain carbohydrate growth up to, but not including, addition of alpha 1-3 mannose residues. Golgi apparatus compartments may specialize in addition of distinct outer chain determinants. The SEC7 gene product was suggested to regulate protein transport between and from functional compartments of the yeast Golgi apparatus.