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

cesses have been studied from genetic and biochemical perspectives (Kukuruzinska et al., 1987; Pryer et al., 1992) resulting in a functionally defined Golgi apparatus that has been subdivided into several subcompartments on the basis of asparagine-linked (N-linked) 1 oligosaccharide modifications (Franzusoff and Schekman, 1989; Graham and Emr, 1991; Wilsbach and Payne, 1993; Gaynor et al., 1994) . In yeast, secretory pathway glycoproteins can acquire two types of N-linked oligosaccharides, either a simple core carbohydrate or one extended by an outer-chain glycan structure. The N-linked core oligosaccharide elaborated in the ER is mainly constituted of MansGlcNAc2 and may undergo Golgi maturation resulting in Man9.13_ GlcNAc2. In other cases, glycoproteins traversing the Golgi apparatus have their core oligosaccharide extended by outer chains (see Fig. 2 ; Ballou, 1990; Herscovics and Orlean, 1993) . The initial step in outer-chain synthesis takes place and defines an early Golgi compartment in which a backbone of ed,6-1inked mannose residues is attached to the core oligosaccharide. This Golgi compartment has been likened to the mammalian cis-Golgi network (Wilsbach and Payne, 1993) . Outer chain elaboration is then brought to completion: the al,6-1inked mannose structure is enlarged in a sequential fashion by al,2-and al,3-mannosyltransferases in a subsequent distinct com-partment defining the yeast medial-Golgi compartment.