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

None of the studied animal glycosylation enzymes appear to need an intact cytoplasmic domain to achieve Golgi retention (Munro, 1991; Aoki et al., 1992; Colley et al., 1992; Shaper and Shaper, 1992; Teasdale et al., 1992; Dahdal et al., 1993; Burke et al., 1994; Gleeson et al., 1994; Low and Hong, 1994) . Kre2p, thus, constitutes the first demonstration of an eukaryotic glycosyltransferase requiring a short cytoplasmic amino-terminal domain for correct intracellular localization. The 11-amino acid residue-targeting domain of Kre2p seemingly parallels that of the late yeast Golgi enzymes Kexlp, Kex2p and DPAP A (Cooper and Bussey, 1992; Wilcox et al., 1992; Nothwehr et al., 1993) . It was shown for DPAP A that a 10-amino acid region within the cytoplasmic domain was both required and sufficient for proper retention (Nothwehr et al., 1993) , and for Kex2p, a 27-amino acid region was found to be essential (Wilcox et al., 1992) . The accurate Golgi targeting of Kex2p and DPAP A is clathrin dependent and aromatic residues are thought necessary for this retention process (Wilcox et al., 1992; Nothwehr et al., 1993; Wilsbach and Payne, 1993) . While unrelated at a sequence level with Kex2p and DPAP A, the amino-terminal domain of Kre2p contains a single aromatic phenylalanine residue (MALFLS-KRLLR) which was mutated to an alanine, an alteration showing no effect on targeting and, therefore, providing no evidence for clathrin-based retention of Kre2p (Lussier, M., A.-M. Sdicu, T. Ketela, and H. Bussey, unpublished results) .