High-resolution cryo-EM using a common LaB6 120-keV electron microscope equipped with a sub-200-keV direct electron detector

High-resolution cryo-electron microscopy (cryo-EM) requires costly 200- to 300-keV cryo-transmission electron microscopes (cryo-TEMs) with field emission gun (FEG) sources, stable columns, constant-powered lenses, autoloader, and direct electron detectors (DED). Recent advances in 100-keV imaging with the emergence of sub-200-keV optimized DED technology promises the development of more affordable cryo-TEMs. So far, 100-keV imaging has required microscopes with FEG sources. We here explored whether a standard 120-keV TEMs with thermionic lanthanum hexaboride (LaB6) source can be upgraded with a sub-200-keV DED for high-resolution cryo-EM. Using this imaging configuration, we successfully obtained a 2.65 & Aring; reconstruction for apoferritin, 4.33 & Aring; for 64-kDa hemoglobin, and 4.4 & Aring; for an asymmetric 153kDa membrane protein GPCR. All results were achieved using standard automated data collection with SerialEM, demonstrating the feasibility to collect large cryo-EM datasets with a side-entry cryo-holder. These results showcase a widely accessible solution to obtaining interpretable cryo-EM structures at low cost and contribute to the "democratization" of cryo-EM.