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

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 angstrom) 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-angstrom diameter at 3.3-angstrom resolution and of RNA-dependent RNA polymerase (RdRp) complexes within the capsid at 3.9-angstrom 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-angstrom resolution structural determinations for biological assemblies of relatively small sizes using cryo-EM at 200 kV was discussed. (C) 2016 Elsevier Ltd. All rights reserved.