Abstract
All ER protein glycosylation reactions occur in the lumen and often involve lumenal mannosylation and glucosylation steps in which mannose and glucose residues are sourced from the glycolipids mannosyl- and glucosyl-phosphoryl dolichol. Paradoxically, these two lipids are synthesized on the cytoplasmic face of the ER and must therefore be flipped across the ER membrane to provide a source of lumenal mannose and glucose. The molecular identities of the specific transporters that are needed to facilitate the transbilayer movement of MPD and GPD across the ER membrane at a physiological rate remain unknown. Taking advantage of the fact that not all N-glycosylation-competent organisms have glucose in their N-glycan precursor, we implemented a bioinformatics approach for assignment of protein function, namely phylogenetic profiling. Using this procedure, we identified a number of polytopic ER membrane proteins as GPD scramblase candidates in yeast.