The Ktr1p, Ktr3p, and Kre2p/Mnt1p Mannosyltransferases Participate in the Elaboration of Yeast O- andN-linked Carbohydrate Chains

Abstract We have determined a role for Ktr1p and Ktr3p as mannosyltransferases in the synthesis of the carbohydrate chains attached to Saccharomyces cerevisiae O- andN-modified proteins. KTR1 and KTR3encode related proteins that are highly similar to the Kre2p/Mnt1p Golgi α1,2-mannosyltransferase (Lussier, M., Camirand, A., Sdicu, A.-M., and Bussey, H. (1993) Yeast 9, 1057–1063; Mallet, L., Bussereau, F., and Jacquet, M. (1994) Yeast 10, 819–831). Examination of the electrophoretic mobility of a specifically O-linked protein from mutants and an analysis of their total O-linked mannosyl chains demonstrates that Ktr1p, Ktr3p, and Kre2p/Mnt1p have overlapping roles and collectively add most of the second and the third α1,2-linked mannose residues onO-linked oligosaccharides. Determination of the mobility of the specifically N-linked glycoprotein invertase in different null strains indicates that Ktr1p, Ktr3p, and Kre2p are also jointly involved in N-linked glycosylation, possibly in establishing some of the outer chain α1,2-linkages.