Sub-3 Å resolution protein structure determination by single-particle cryo-EM at 100 keV

Abstract

Cryo-electron microscopy (cryo-EM) has revolutionized structural biology by providing high-resolution insights into biological macromolecules. Here, we present sub-3 Å resolution structures determined using the 100 keV Tundra cryogenic transmission electron microscope (cryo-TEM), equipped with the newly developed Falcon C direct electron detector (DED). Our results demonstrate that this lower voltage microscope, when combined with advanced electron optics and detectors, can achieve high-resolution reconstructions that were previously only attainable with higher voltage systems. The implementation of an extreme-brightness field emission gun (XFEG) and an SP-TWIN objective lens significantly enhanced the spatial and temporal coherence of the system. Furthermore, the semi-automated sample loader minimized contamination and drift, allowing extended data collection sessions without manual intervention. The high detective quantum efficiency (DQE) of Falcon C further improved the signal-to-noise ratio, which is critical for achieving high-resolution structures. We validated the performance of this microscope by determining the structures of various biological samples, including apoferritin, T20S proteasome, GABAA receptor, haemoglobin, and human transthyretin ranging in size from 440 kDa to 50 kDa. The highest resolutions achieved were 2.1 Å for apoferritin, 2.7 Å for the 20S proteasome, 2.8 Å for the GABAA receptor, 5.0 Å for haemoglobin, and 3.5 Å for transthyretin. We also explored a larger specimen, a 3.9 MDa Adeno-associated virus (AAV9) capsid and resolved it a 2.8 Å. This work highlights the potential of 100 keV TEMs to make high-resolution cryo-EM more accessible to the structural biology community. Furthermore, it sets a precedent for the use of lower voltage TEMs in routine cryo-EM studies, not only for screening grids for single particle analysis but also for achieving high-resolution structures of protein samples.