For cryo-EM, samples were incubated directly on holey carbon grids (Fig. 3d,e,h) or incubated in a tube and pipetted onto a grid (Fig. 3a,b,f,g). Samples were vitrified by plunge freezing the grid. Imaging was performed on a Tecnai F30 electron microscope (FEI, Eindhoven, Netherlands), equipped with a 4K FEI Eagle camera. The microscope was operated at 300 kV, with a magnification of 20,000 ×, giving a pixel size of 0.59 nm at the specimen level. Data was collected at −4 µm defocus with an electron dose of 15–25 e−Å−2. The lighter areas in Fig. 3a,d–h represent the 2 µm diameter holes in the carbon support.