Negative-stain TEM sample preparation

The above purified sample (5 μl) was adsorbed on a commercially supplied formvar-stabilized carbon type-B, 400-mesh copper grids (Ted Pella, part number 01814-F) that was glow-discharged for 1 min at 15 mA using a Pelco easiGlow glow-discharge system (Ted Pella, Redding, CA, USA) and stained using 5 μl of a freshly prepared 2% aqueous uranyl formate solution containing 25 mM NaOH. Samples were incubated for 15 to 300 s depending on the concentration of the sample. Excess liquid was wicked away with Whatmann filter paper, and grids were left to dry for 30 to 60 min before imaging.

Images were acquired on an FEI Tecnai TF20 TEM microscope operated at 120 kV of accelerating voltage using a charge-coupled device camera at ×50,000 magnification. Particles (see details in table S3) were manually picked and class-averaged using Relion3.0 without CTF correction.