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, Electron Microscopy Sciences) were plasma cleaned for 30 s using a Gatan Solarus 950 with a 4:1 O 2 :H 2 ratio. A 0.57 mg/mL solution of L-P complex in 50 mM Tris-HCl pH 8.0, 500 mM NaCl, 1 mM TCEP and 10% glycerol was diluted with an equal volume of 20 mM Tris-HCl pH 8.0, 200 mM NaCl immediately before 3 mL was deposited onto grids and plunge-frozen in liquid ethane using a Vitrobot Mark IV (Thermo Scientific) set to 100% humidity and 4 C, with a wait time of 0.5 s, a blot time of 4 s and a blot force of 1. Data were collected at 22,500x magnification, corresponding to a calibrated pixel size of 1.075 Å . A total of 30 frames were collected for each micrograph

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