Expression and purification of the membrane proteins
All three membrane proteins were cloned from genomic DNA into standard expression vectors. Expression was performed in prokaryotic cells using standard protocols. The cells were harvested at 3000g and resuspended in lysis buffer containing protease inhibitor tablets (Roche). After lysis using a cell disruptor (Constant Cell Disruption Systems), unbroken cells were removed by centrifugation at 8000g for 15 min. Membranes were harvested from the resulting supernatant by ultracentrifugation for 1 h at 100,000g. For each of the target proteins, the membranes were solubilized in 1% DDM. Insoluble material was then removed by ultracentrifugation at 100,000g for 1 h. All steps following cell harvest were carried out at 4°C; all purification buffers contained 300 mM NaCl. The solubilized proteins were individually purified by one-step Ni2+-NTA (Qiagen) affinity chromatography. The fractions containing the purified target proteins, as assessed by SDS-PAGE, were pooled prior to UDS. The concentration of the protein was confirmed using the BCA assay (Pierce).