Bilayer in Small Bicelles Revealed by Lipid-Protein Interactions Using NMR Spectroscopy

摘要

Micelles formed by a variety of detergents are successfully used to study membrane proteins by solution NMR. However, structures determined under these conditions have been questioned by the argument that micelles cannot mimic a natural membrane environment because of the strong curvature at their surface and the different lateral pressure compared to lipid bilayers. Furthermore, membrane proteins embedded in the micelles often loose their activities. Small bicelles composed of a mixture of short-chain and long-chain phospholipids, usually dihexanoyl phosphatidylcholine (DHPC) and dimyristoyl phosphatidylcholine (DMPC), are presumed to be superior in these regards. From deuterium spectroscopy, it was derived that DMPC in bicelles makes up the flat surface of a bilayered disk, and DHPC covers its rim irrespective of whether more discoid or "swiss cheese"-like bicelles are formed.Because bicelles are more similar in structure to an actual membrane, yet still relatively small in size, they have the potential to serve as replacements for micelles in NMR studies of membrane proteins. The properties of any system for solubilizing membrane proteins will depend primarily on its interactions with the protein. For bicelles, previous NMR studies on the interactions between lipids and protein were performed using peptides. Here, we present a study of the interaction between the intact integral outer membrane protein OmpX from Escherichia coli and small bicelles formed by a DMPC/DHPC mixture.