Global view of domain-specific O-linked mannose glycosylation in glycoengineered cells

Abstract

Protein O-linked mannose (O-Man) glycosylation is an evolutionary conserved post-translational modification (PTM) that fulfills important biological roles during embryonic development. Three non-redundant enzyme families, POMT1/POMT2, TMTC1-4 and TMEM260, selectively coordinate the initiation of protein O-Man glycosylation on distinct classes of transmembrane proteins, including -dystroglycan, cadherins and plexin receptors. However, a systematic investigation of their substrate specificities is lacking, in part due to the ubiquitous expression of O-Man glycosyltransferases in cells, which precludes analysis of pathway-specific O-Man glycosylation on a proteome-wide scale. Here, we apply a targeted workflow for membrane glycoproteomics across five human cell lines to extensively map O-Man substrates and genetically deconstruct O-Man initiation by individual and combinatorial knock-out (KO) of O-Man glycosyltransferase genes. We established a human cell library for analysis of substrate specificities of individual O-Man initiation pathways by quantitative glycoproteomics. Our results identify 180 O-Man glycoproteins, demonstrate new protein targets for the POMT1/POMT2 pathway and show that TMTC1-4 and TMEM260 pathways widely target distinct Ig-like protein domains of plasma membrane proteins involved in cell-cell and cell-extracellular matrix interactions. The identification of O-Man on Ig-like folds adds further knowledge on the emerging concept of domain-specific O-Man glycosylation which opens for functional studies of O-Man glycosylated adhesion molecules and receptors.