Structural and mechanistic insights into protein glycosylation and glycosylphosphatidylinositol anchoring in the endoplasmic reticulum membrane

The endoplasmic reticulum (ER) hosts several integral membrane enzymes responsible for post-translational modifications of proteins entering the secretory pathway. These include protein glycosylation and the attachment of glycosylphosphatidylinositol (GPI) anchors. At the ER membrane, protein glycosylation is catalyzed by glycosyltransferases from the C-superfamily (GT-C), which use lipid donor substrates to attach a complex oligosaccharide to asparagine residues (N-glycosylation), or a single mannose unit to threonine, serine (O-mannosylation), or tryptophan (C-mannosylation) residues. In contrast, the attachment of GPI anchors to acceptor proteins is catalyzed by the multimeric enzyme transamidase, which cleaves a C-terminal GPI signal peptide of the acceptor protein and replaces it with a GPI anchor. In the present review, we will discuss recent mechanistic studies that shed light on the architecture of these membrane protein machineries and on how they recognize their substrates and catalyze protein glycan modifications at the ER membrane.