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

Proteins associated with glycan modifications constitute a major class of resident Golgi proteins. Enzymes belonging to different mammalian glycosyltransferase families share a similar type II structural organization, but lack amino acid sequence homology or discernible targeting motifs even if situated in the same Golgi compartment (Shaper and Shaper, 1992; Kleene and Berger, 1993) . It has been established that the membrane-spanning domain of animal glycosyltransferases plays a central role in Golgi localization (Munro, 1991; Nilsson et al., 1991; Swift and Machamer, 1991; Aoki et al., 1992; Burke et al., 1992; Colley et al., 1992; Tang et al., 1992; Teasdale et al., 1992; Wong et al., 1992; Gleeson et al., 1994; Low and Hong, i994) . To define more precisely the S. cerevisiae Golgi complex and to better understand how posttranslational modifications occur, we have studied the glycosyltransferase, Kre2p/Mntlp. KRE2/MNT1 was isolated as a gene implicated in cell-wall assembly conferring K1 killer toxin resistance when mutated (Hill et al., 1992) and found to encode an cd,2-mannosyltransferase (H/iusler and . KRE2/MNT1 encodes a 442-amino acid residue predicted type II membrane protein containing a putative transmembrane domain near its NH2 terminus and one potential luminal N-glycosylation site. Kre2p/Mntlp specifically adds a third mannose during the linear elongation of O-linked carbohydrate chains that may contain up to five mannose residues (H~usler et al., 1992) and is also apparently involved in asparagine-linked glycosylation (Hill et al., 1992; Lussier et al., 1995b) .