Toward Bridging a Gap Between Model and Natural Membranes for Biophysical Studies

Abstract

To study their lipid interactions or obtain valuable structural information, membrane peptides or proteins need to be maintained in a lipid system as close as possible to their natural environment. Model membranes are employed in a variety of biophysical techniques such as nuclear magnetic resonance (NMR). However, developing a method that could allow peptide extraction and reconstitution in a native or near-native membrane would be an asset. In this work, we propose an approach inspired by bicelles, which can orient in the magnetic field. We prepared magnetically-orientable phosphatidylcholine (PC) liposomes using the membrane softening detergent Tween80 (polyoxyethylene sorbitan monooleate) which is also used to extract soluble and membrane-bound proteins. Our 31P and 2H solid-state NMR results show that TWeen80 mixed to DMPC and DPPC bilayers leads to elongated oriented structures which align over a wide range of lipid-to-detergent molar ratios (2<q<13) and temperatures (25-80oC). This can be ascribed to the miscibility of Tween80 and PC molecules, as shown by the thermotropic behavior of these mixture in infrared spectroscopy. We show that the orientation with respect to the magnetic field direction can be flipped from parallel to perpendicular by addition of lanthanides ions. Using a 15N-labeled model KALP transmembrane peptide, 15N solid-state NMR spectra showed that Tween80-based PC membranes can reveal a peptide orientation, thus providing valuable data for structural determination. Altogether, our work shows that Tween80 could be exploited for membrane protein extraction, reconstitution and structural studies without requiring to a detergent removal step. This approach has the potentiel to be used with more complex membrane bilayers including native bological membranes.