Glycolipid MPIase, Essential for Membrane Protein Integration in the Cytoplasmic Membrane of <i>Escherichia coli</i> , Protects Membranes from Aggregation Induced by Chemicals

Artificial liposomes tend to aggregate in the presence of certain chemicals, such as divalent cations. In contrast, membrane vesicles derived from biological membranes, such as the inner and outer membrane vesicles of the diderm Gram-negative bacterium Escherichia coli (E. coli), are highly resistant to these chemicals. We hypothesized that a protective factor, referred to as Biomembrane-Protecting Factor (BPF) based on its function, was present in biomembranes. In this study, we purified the factor from both the inner and outer membranes of E. coli and revealed that BPF is identical with the glycolipid known to drive membrane protein integration (MPIase; Membrane Protein Integrase). Immunoelectron microscopy analysis revealed that BPF (MPIase) is asymmetrically distributed across the membranes, i.e., predominantly localizing on the periplasmic leaflet of both membranes, with a smaller amount also detected on the cytoplasmic side of the inner membrane. Using chemically synthesized BPF (MPIase) derivatives, we demonstrated that the length of the sugar chain and the pyrophosphate linker are important for membrane protection, while the 6-O-acetyl substitution on GlcNAc, which enhances the membrane protein integration activity of MPIase, negatively affected BPF activity. Since BPF’s ability to protect membranes from chemical-induced aggregation is comparable to that of polyethylene glycol (PEG)-modified lipids, BPF is expected to be adopted in drug delivery systems and nucleic acid vaccines.