Conformational States of ABC Transporter MsbA in a Lipid Environment Investigated by Small-Angle Scattering Using Stealth Carrier Nanodiscs

Structural studies of integral membrane proteins (IMPs) are challenging, as many of them are inactive or insoluble in the absence of a lipid environment. Here, we describe an approach making use of fractionally deuterium labeled "stealth carrier" nanodiscs that are effectively invisible to low-resolution neutron diffraction and enable structural studies of IMPs in a lipidic native-like solution environment. We illustrate the potential of the method in a joint small-angle neutron scattering (SANS) and X-ray scattering (SAXS) study of the ATP-binding cassette (ABC) transporter protein MsbA solubilized in the stealth nanodiscs. The data allow for a direct observation of the signal from the solubilized protein without contribution from the surrounding lipid nanodisc. Not only the overall shape but also differences between conformational states of MsbA can be reliably detected from the scattering data, demonstrating the sensitivity of the approach and its general applicability to structural studies of IMPs.