Mnt2p and Mnt3p of Saccharomyces cerevisiae are members of the Mnn1p family of α -1,3-mannosyltransferases responsible for adding the terminal mannose residues of O-linked oligosaccharides

The genome of Saccharomyces cerevisiae contains five genes that encode type II transmembrane proteins with significant amino acid similarity to the α-1,3-mannosyltransferase Mnn1p. The roles of the three genes most closely related to MNN1 were examined in mutants carrying single and multiple combinations of the disrupted genes. Paper chromatographic analysis of [2-3H]mannose-labeled O-linked oligosaccharides released by β-elimination showed that the MNT2 (YGL257c) and MNT3 (YIL014w) genes in combination with MNN1 have overlapping roles in the addition of the fourth and fifth α-1,3-linked mannose residues to form Man4 and Man5 oligosaccharides whereas MNT4 (YNR059w) does not appear to be required for O-glycan synthesis.