Near-Atomic Resolution Structure Determination of a Cypovirus Capsid and Polymerase Complex Using Cryo-EM at 200Â kV

•The cypovirus capsid structure was resolved at 3.3-Å resolution using a 200-kV TEM.•The criterion for particle image selection was proposed.•The structure of RdRp complex within the capsid was resolved at 3.9-Å resolution.•The conformational change of RdRp suggests that the RdRp might also function as an RNA helicase.

Single-particle cryo-electron microscopy (cryo-EM) allows the high-resolution structural determination of biological assemblies in a near-native environment. However, all high-resolution (better than 3.5 Å) cryo-EM structures reported to date were obtained by using 300 kV transmission electron microscopes (TEMs). We report here the structures of a cypovirus capsid of 750-Å diameter at 3.3-Å resolution and of RNA-dependent RNA polymerase (RdRp) complexes within the capsid at 3.9-Å resolution using a 200-kV TEM. The newly resolved structure revealed conformational changes of two subdomains in the RdRp. These conformational changes, which were involved in RdRp's switch from non-transcribing to transcribing mode, suggest that the RdRp may facilitate the unwinding of genomic double-stranded RNA. The possibility of 3-Å resolution structural determinations for biological assemblies of relatively small sizes using cryo-EM at 200 kV was discussed.

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