Transmembrane motions of PglB induced by LLO are coupled with EL5 loop conformational changes necessary for OST...

N-linked glycosylation is an enzymatic reaction in which an oligosaccharide is transferred en bloc onto an asparagine residue of an acceptor polypeptide, catalyzed by oligosaccharyltransferase (OST). Despite the available crystal structures, the role of the external loop EL5, which is critical for the catalytic cycle, is enigmatic as EL5 in the crystal structures is partially absent or blocks a pathway of lipid-linked oligosaccharide to the active site. Here we report the molecular origin of EL5 conformational changes through a series of molecular dynamics simulations of a bacterial OST, Campylobacter lari PglB. The simulations reveal that the isoprenoid moiety of lipid-linked oligosaccharide favorably binds to a hydrophobic groove of the PglB transmembrane domain. This binding triggers the conformational changes of the transmembrane domain and subsequently impairs the structural stability of EL5, leading to disordered EL5 with open conformations that are required for correct placement of the oligosaccharide in the active site.