Functional reconstitution of an ABC transporter in nanodiscs for use in electron paramagnetic resonance spectroscopy.

Electron paramagnetic resonance (EPR) spectroscopy is a powerful biophysical technique for study of the structural dynamics of membrane proteins. Many of these proteins interact with ligands or proteins on one or both sides of the membrane. Membrane proteins are typically reconstituted in proteoliposomes to observe their function in a physiologically relevant environment. However, membrane proteins can insert into liposomes in two different orientations, and surfaces facing the lumen of the vesicle can be inaccessible to ligands. This heterogeneity can lead to subpopulations that do not respond to ligand binding, complicating EPR spectral analysis, particularly for distance measurements. Using the well-characterized maltose transporter, an ATP binding cassette (ABC) transporter that interacts with ligands on both sides of the membrane, we provide evidence that reconstitution into nanodiscs, which are soluble disk-shaped phospholipid bilayers, is an ideal solution to these problems. We describe the functional reconstitution of the maltose transporter into nanodiscs and demonstrate that this system is ideally suited to study conformational changes and intramolecular distances by EPR.