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

The Mnnlp al,3-mannosyltransferase is the only other yeast glycosyltransferase where targeting has been examined. Mnnlp targeting appears different from that of Kre2p since a mutant Mnnlp lacking its NH2-terminal cytoplasmic tail is properly localized to the Golgi complex . Correct retention of Mnnlp is clathrin dependent but, contrary to Kex2p and DPAP A, does not seem to be mediated by a direct interaction through its cytoplasmic tail. The reason for the Kre2p requirement for its cytoplasmic tail remains unclear. It could be clathrin dependent but not involving an aromatic residue, or through some other process. The Gdalp type II Golgi membrane protein (Abeijon et al., 1989; Abeijon el al., 1993) has been shown to be properly localized in a strain lacking clathrin heavy chains (Seeger and Payne, 1992; Wilsbach and Payne, 1993) . The Mnnlp, Kre2p, and Gdalp enzymes appear to be in close proximity or even in the same Golgi compartment (this work; Abeijon et al., 1989; Graham et al., 1994) . This raises the possibility that specific Golgi membrane proteins showing a similar compartmental distribution may be retained by more than one mechanism.