Membrane tubulation induced by a bacterial glycolipid

Membrane protein integrase (MPIase) is a glycolipid found in Escherichia coli cell membranes. It consists of diacylglycerol and a sugar chain comprising approximately 10 repeating trisaccharide units, made up of three types of N-acetylated amino sugars linked by pyrophosphate. In cooperation with proteinaceous translocons, MPIase regulates membrane protein integration. In this study, using various microscopic techniques, we demonstrated that externally added MPIase induces the formation of a single tubule protruding outward from giant unilamellar vesicles (GUVs). These tubules resembled those reported in studies involving membrane-perturbing proteins but differed in that MPIase formed aggregates at the base of the tubules. We also showed that hydrophobic interactions between longer sugar chains of MPIase play a key role in forming multiple aggregates on the membrane, which in turn induce membrane budding and triggers membrane protrusion. Once a single tubule begins to form from one of the aggregates, elongating a longer and narrower tubule can reduce line tension and elastic energy for a given area differences between the internal and external leaflets. These findings provide insights into the mechanism underlying glycolipid-induced membrane tubulation and suggest that the unique long sugar chain of MPIase can offer functions beyond its essential role in membrane protein integration.