Membrane proteins are present in the cell membranes of all organisms and are deeply involved in basic biological phenomena such as signal transduction and metabolite transport between the inside and the outside of the cell. Recently, we discovered a novel essential factor for membrane protein integration, a glycolipid named MPIase (Membrane Protein Integrase), in the inner membrane of Escherichia coli. Structural analysis revealed that MPIase has approximately ten repeating trisaccharide units, featureing 4-acetamido-4-deoxyfucose (Fuc4NAc), 2-acetamido-2-deoxymannuronic acid (ManNAcA), and 2-acetamido-2-deoxyglucose (GlcNAc), and diacylglycerol at the reducing end of the glycan through a pyrophosphate linkage. About 30% of the 6-hydroxy groups of GlcNAc are acetylated. In order to elucidate the mechanism of membrane protein integration, we chemically synthesized trisaccharyl pyrophospholipid (mini-MPIase-3), which is the minimum unit of MPIase, and its derivatives. Structure–activity relationship studies demonstrated that mini-MPIase-3 showed significant activity, indicating that it contains the minimum active structure. The phosphorylated glycan part of MPIase has chaperone-like activity to suppress the aggregation of proteins, and anchoring of MPIase by the lipid moiety in the membrane is essential for the integration activity. Since chemically synthesized mini-MPIase-3 is active, properly designed synthetic molecules will enable the determination of the detailed structure–activity relationship. Clarifying the molecular basis of preprotein translocation and membrane protein integration in E. coli would bring new insights not only for uncovering the biological functions of glycolipids, but also for developing antibacterial agents, protein aggregation inhibitors, and membrane protein reconstruction techniques.
