Functional characterization of the YUR1, KTR1, and KTR2 genes as members of the yeast KRE2/MNT1 mannosyltransferase gene family.

Eukaryotic glycan structures are progressively elaborated in the secretory pathway. Following the addition of a core N-linked carbohydrate in the endoplasmic reticulum, glycoproteins move to the Golgi complex where the elongation of O-linked sugar chains and processing of complex N-linked oligosaccharide structures take place. In order to better define how such post-translational modifications occur, we have been studying a yeast gene family in which at least one member, KRE2/MNT1, is involved in protein glycosylation. The family currently contains five other members: YUR1, KTR1, KTR2, KTR3 and KTR4 (Mallet, L., Bussereau, F., and Jacquet, M. (1994) Yeast 10, 819-831). All encode putative type II membrane proteins with a short cytoplasmic N terminus, a membrane-spanning region, and a highly conserved catalytic lumenal domain. Kre2p/Mnt1p is a alpha 1,2-mannosyltransferase involved in O- and N-linked glycosylation (Häusler, A., Ballou, L., Ballou, C.E., and Robbins, P.W. (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 6846-6850); however, the role of the other proteins has not yet been established. We have carried out a functional analysis of Ktr1p, Ktr2p, and Yur1p. By in vitro assays, Ktr1p, Ktr2p, and Yur1p have been shown to be mannosyltransferase but, in vivo, do not appear to be involved in O-glycosylation. Examination of the electrophoretic mobility of the N-linked modified protein invertase in null mutant strains indicates that Ktr1p, Ktr2p, and Yur1p are involved in N-linked glycosylation, possibly as redundant enzymes. As found with Kre2p (Hill, K., Boone, C., Goebl, M., Puccia, R., Sdicu, A.-M., and Bussey, H. (1992) Genetics 130, 273-283), Ktr1p, Ktr2p, and Yur1p also seem to be implicated in the glycosylation of cell wall mannoproteins, since yeast cells containing different gene disruptions become K1 killer toxin-resistant. Immunofluorescence microscopy reveals that like Kre2p; Ktr1p, Ktr2p and Yur1p are localized in the Golgi complex.