Structure and Mechanism of an Active Lipid-linked Oligosaccharide Flippase

In the process of N-linked protein glycosylation, the translocation of lipid-linked oligosaccharides that serves as a donor, is the result of an unknown machinery requiring the action of “flippases”. In Campylobacter jejuni, this process is catalyzed by the ABC transporter PglK. Using newly designed in vitro flipping assay and in vivo studies given rise to distinct stages of the mechanisms could be obtained. 3D crystalline structures were established, providing a structural dissection of the different stages involved in the flipping mechanism. While the pyrophosphate-oligosaccharide head group of lipid-linked oligosaccharides enters the translocation cavity and interacts with positively charged side chains, the lipidic polyprenyl tail binds and activates the transporter but remains exposed to the lipid bilayer during the reaction.