Cytoplasmic Polyhedrosis Virus Structure at 8 Å by Electron Cryomicroscopy: Structural Basis of Capsid Stability and mRNA Processing Regulation

The single-shelled cytoplasmic polyhedrosis virus (CPV) is a unique member of the Reoviridae. Despite lacking protective outer shells, it exhibits striking capsid stability and is capable of endogenous RNA transcription and processing. The 8 Å three-dimensional structure of CPV by electron cryomicroscopy reveals secondary structure elements present in the capsid proteins CSP, LPP, and TP, which have α+β folds. The extensive nonequivalent interactions between CSP and LPP, the unique CSP protrusion domain, and the perfect inter-CSP surface complementarities may account for the enhanced capsid stability. The slanted disposition of TP functional domains and the stacking of channel constrictions suggest an iris diaphragm-like mechanism for opening/closing capsid pores and turret channels in regulating the highly coordinated steps of mRNA transcription, processing, and release.