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Research background: Glycosyltransferases represent a large and diverse family of enzymes that catalyze the transfer of sugar residues to proteins and lipids, thereby regulating essential cellular processes such as protein quality control and cell wall biosynthesis. In yeast, protein O-mannosyltransferases and other glycosyltransferases are crucial for maintaining cell wall integrity. While the functions of many of these enzymes are well characterized, the role of some of them, such as Gmh5p, remains unknown. This study aims to elucidate the function of Gmh5p, a previously uncharacterized member of the GT34 glycosyltransferase family, in the context of protein and cell wall biosynthesis in Experimental approach: To identify proteins and pathways compensating for reduced O-mannosylation, we performed a genetic screening for multicopy suppressors in a conditional lethal nmt81- Results and conclusions: Gmh5p was identified as a suppressor of O-mannosylation defects. Contrary to its predicted function, Gmh5p did not exhibit α-1,2-galactosyltransferase activity but instead showed mannosyltransferase activity. Expression of Novelty and scientific contribution: This study provides the first functional characterization of Gmh5p as a mannosyltransferase of the GT34 family and demonstrates its role in N-glycan biosynthesis. Our findings expand the current understanding of the diversity and specificity of glycosyltransferases in eukaryotes and highlight their importance in cell wall biology.