Morphological changes of bacterial model membrane vesicles

Unilamellar vesicles frequently serve as model membrane systems to study membrane-peptide and membrane-protein interactions. They are formed either from natural lipid extracts that contain complex lipid mixtures, or from synthetic one-, two-, or multi-component lipid mixtures mimicking the lipid composition of biological membranes. Unilamellar vesicles can be prepared by extrusion of aqueous multilamellar phospholipid suspensions and they can have limited stability due to aggregation and fusion processes. We used cryo-transmission electron microscopy (cryoTEM) to study the morphology of vesicles of phosphatidylethanol-amines (PEs), phosphatidylglycerols (PGs), and cardiolipins (CLs) that represent the major lipid components of cytoplasmic membranes of Gram-negative bacteria. Our results show that single-component vesicles of PE and PG stored at RT are stable in shape and size for at least 1 wk. In contrast, two-component vesicles of PE/PG and PE/CL transform into lamellar sheets already several hours after preparation, which is probably due to a compensation of different spontaneous curvatures of PG lipids compared to PE and CL. With this investigation we intend to alert researchers working with model membranes to scrutinize the shape and stability of vesicles before using them in protein-vesicle interaction studies.