0.04% sodium cholate. For the measurements, MsbA-loaded nanodiscs were enriched based on the affinity of the His-tagged MsbA for Ni 2+. The supernatant from the solubilization of proteoliposomes was mixed with Ni-NTA beads (Thermo Fisher Scientific) at a ratio of 100 µL resin/mL solubilized protein, and after incubation at 4 °C overnight with gentle rotation the samples were transferred to a gravity flow column. The resin was washed with 10 column volumes of 100 mM NaCl and 20 mM Tris/HCl, with 0.1 mM TCEP and 20 mM imidazole, pH 7.4, and elution was achieved by increasing imidazole to 200 mM. Eluted fractions were analyzed on gels (16% SDS-PAGE) stained with Instant Blue (Expedeon) and used for the ATPase measurements. Estimation of nanodiscs size by dynamic light scattering (DLS). Measurements were performed at 22 °C on a Zetasizer Nano ZSP (Malvern Instruments, Westborough, MA) using 40-µL disposable microcuvettes. Size-number distributions were generated using the Zetasizer software version 7.11 and were analyzed using the protein analysis distribution. Statistics. Statistical comparisons were performed by the Student's t test for paired or unpaired data, or one-way analysis of variance, as appropriate. P < 0.05 in a two-tailed analysis was considered significant. The number of experiments given in the main text and figure legends corresponds to independent measurements.