Preparation of SapA picodiscs

SapA picodiscs were prepared by a variation of a previously described method^23,42. Small unilamellar vesicles (SUVs) were prepared by mixing BC-cholesterol, bis(monoacylglycero)phosphate (BMP) and egg phosphatidylcholine (DOPC, Avanti Polar Lipids) at a ratio of 10:10:80% moles in chloroform and evaporating the solvent under N2. The dry lipids were dispersed by vortexing in acidic buffer (50 mM sodium acetate pH 4.8, 150 mM NaCl). The suspension was subjected to 10 cycles of freezing (with liquid nitrogen) and thawing followed by sonication in a bath sonicator. SapA was added to a final concentration of 1 mg/mL to solutions of 1 mM liposomes in 50 mM sodium acetate pH 4.8, 150 mM NaCl. The mixture was incubated at room temperature for 20 min and centrifuged at 21,000 × g to pellet unbound liposomes. The supernatant containing the SapA picodiscs was stored at 4 °C.