Localization and targeting of the Saccharomyces cerevisiae Kre2p/Mnt1p alpha 1,2-mannosyltransferase to a medial-Golgi compartment

In S. cerevisiae, the various Golgi compartments have not been morphologically well characterized. The yeast Golgi complex is not structurally similar to the perinuclear stacked cisternal subcompartments characteristic of the Golgi apparatus of mammalian cells. Immunoelectron microscopy revealed that the yeast Golgi is composed of as many as 30 concave, disklike membranous cisternae scattered in the cytoplasm and most often not organized in parallel stacks (Preuss et al., 1992) . In indirect immunofluorescence using antibodies to different yeast Golgi proteins, the isolated cisternae are visualized as dispersed punctate spots (Franzusoff et al., 1991; Redding et al., 1991; Antebi and Fink, 1992; Cooper and Bussey, 1992; Roberts et al., 1992) . The establishment and maintenance of the polarized organization of the Golgi apparatus relies on the existence of complex sorting and transport mechanisms permitting specific Golgi proteins to be delivered to their precise cisternal destinations. Resident proteins of the secretory pathway of eukaryotic cells require particular targeting signals that specify their final location (Pelham and Munro, 1993; Gleeson et al., 1994; Low and Hong, 1994) . Mammalian membrane glycoproteins not possessing positive sorting sequences are transported to the cell surface through a bulk flow of proteins and lipids (Pelham and Munro, 1993) . In yeast, secretory pathway membrane glycoproteins lacking targeting sequences are brought by default to the vacuole (Cooper and Bussey, 1992; Roberts et al., 1992; Wilcox et al., 1992; Nothwehr et al., 1993; Gaynor et al., 1994; Hill and Stevens, 1994) .