Specific transbilayer translocation of dolichol-linked oligosaccharides by an endoplasmic reticulum flippase

Abstract

The oligosaccharide donor for protein N-glycosylation, Glc 3Man9GlcNAc2-PP-dolichol, is synthesized via a multistep pathway that starts on the cytoplasmic face of the endoplasmic reticulum (ER) and ends in the lumen where the glycosylation reaction occurs. This necessitates transbilayer translocation or flipping of the lipid intermediate Man5GlcNAc2-PP-dolichol (M5-DLO) across the ER membrane. The mechanism by which M5-DLO - or any other lipid - is flipped across the ER is unknown, except that specific transport proteins or flippases are required. We recently demonstrated M5-DLO flipping activity in proteoliposomes reconstituted from detergent-solubilized ER membrane proteins and showed that it was ATP-independent and required a trypsin-sensitive protein that sedimented at approximately 4S. By using an activity-enriched fraction devoid of glycerophospholipid flippase activity, we now report that M5-DLO is rapidly flipped in the reconstituted system with a time constant τ <2 min, whereas its triantennary structural isomer is flipped slowly with τ >200 min. DLOs larger than M5-DLO are also poorly translocated, with τ ranging from approximately 10 min to >200 min. We conclude that (i) the number and arrangement of mannoses in the DLO glycan has a profound effect on the ability of the DLO to be translocated by the flippase, (ii) glycan size per se does not dictate whether a DLO will be flipped, and (iii) the flippase is highly specific for M5-DLO. Our results suggest a simple structural model for the interaction between the DLO head group and the flippase. © 2009 by The National Academy of Sciences of the USA.