Phylogenetic profiling identifies glucosyl-phosphoryl dolichol scramblase candidates

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.