Membrane protein integration is a vital event in cells. We identified a novel factor involved in this process in Escherichia coli, which we named MPIase after its function. A combination of spectroscopic analyses and synthetic work has revealed that MPIase is a glycolipid despite its enzyme-like activity. MPIase has a long glycan chain composed of repeating trisaccharide units and an anchor composed of a pyrophosphate and a diacylglycerol. To determine the mechanism of activity, we synthesized a trisaccharyl pyrophospholipid termed mini-MPIase-3, a minimal unit of MPIase, and its derivatives. Structure-activity relationship studies demonstrated that the glycan part of MPIase prevents the aggregation of substrate proteins. Moreover, MPIase embedded in the membrane alters the physicochemical properties of membranes to facilitate proteins to interact with the inner part of the membrane.
