All protein purification steps were carried out at 4 °C unless otherwise stated. Cell pellets were resuspended in 40 mM TRIS buffer at pH 7.6, 1 mM EDTA supplemented with Complete EDTA-free protease inhibitor cocktail tablets (Roche) and disrupted at ~15 000 psi using a microfluidizer (Processor M-110L Pneumatic, Microfluidics). Membranes were pelleted by ultra-centrifugation at 200 000 g for 50 minutes, and then subjected to a high salt wash in a buffer containing 40