# The Schizosaccharomyces pombe Glycosyltransferase Gmh5 is a Functional Homologue of the α-1,6-Mannosyltransferase Mnn10 Crucial for N-Glycan Processing

## TL;DR

This study identifies a yeast enzyme, Gmh5p, as a key player in sugar transfer during protein modification, similar to a known enzyme in another yeast species.

## Contribution

The first functional characterization of Gmh5p as a mannosyltransferase involved in N-glycan biosynthesis.

## Key findings

- Gmh5p suppresses defects in O-mannosylation and exhibits mannosyltransferase activity.
- Expression of Gmh5p in a mutant yeast strain restores hygromycin tolerance to near wild-type levels.
- N-glycosylation of model substrates is reduced in gmh5Δ mutants.

## Abstract

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 Schizosaccharomyces pombe.

To identify proteins and pathways compensating for reduced O-mannosylation, we performed a genetic screening for multicopy suppressors in a conditional lethal nmt81-oma2+ mutant background. The enzymatic activity of Gmh5p was biochemically characterized, and its functional homology to known mannosyltransferases was assessed through complementation experiments in Saccharomyces cerevisiae. In addition, the N-glycosylation status of model substrates was analyzed in gmh5Δ mutant strains.

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 gmh5+ in S. cerevisiae mnn10Δ mutants restored hygromycin tolerance to near wild-type levels. Furthermore, N-glycosylation of model substrates was reduced in gmh5Δ mutants. These results demonstrate that Gmh5p is a mannosyltransferase involved in the outer chain elongation of N-linked glycans and functions as a homologue of Mnn10p.

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.