Scai scanner

To vitrify particles for cryo-EM observation from defined conditions of elevated temperature and humidity, we used a custom-made environmental chamber that mounts over a cryo-station (see Fig. S1 in the supplemental material). In brief, 3-μl drops of specimen were applied to EM grids mounted within the chamber, incubated for 10 min at 65°C or 70°C, and then blotted to thin films and vitrified in an otherwise conventional manner. Finally, grids were transferred into the electron microscope and low-dose micrographs were recorded on a CM200-FEG electron microscope (FEI, Hillsboro, OR) equipped with a Gatan 626 cryo-holder using procedures previously described (43 (link)). The Head I* sample was imaged at a magnification of ×38,000 and defocus values in the range of 0.78 µm to 2.15 µm. Films were digitized with a Zeiss SCAI scanner with a 7-µm sampling rate to yield 1.84 Å/pixel at the sample. The Head I sample was imaged at a magnification of ×50,000, and films were scanned on a Nikon Super CoolScan 9000 scanner with a 6.35-µm sampling rate, yielding 1.27 Å/pixel. Capsid diameters were measured by hand as averages of three measurements at positions 60° apart for each capsid. To estimate the incidence of damaged capsids, ~400 capsids from three representative micrographs were scored independently for each sample by two observers.