3.88 Å structure of cytoplasmic polyhedrosis virus by cryo-electron microscopy

Cytoplasmic polyhedrosis virus (CPV) is a member of a large family of double-stranded RNA viruses, but it is unique in having a single shell capsid yet being fully capable of cell entry and mRNA transcription. The structure of this virus has now been determined by single-particle cryo-electron microscopy (cryoEM) to a resolution of 3.88 Å. The technique allows the polypeptide backbone to be traced without the need to make a crystal. The high-resolution structure shows how conformational switching is exploited to make railroad-like 'sliding' tracks for RNA packing and transcription and reveals an mRNA releasing hole coupled with distinctive capping machinery. With this and several other recent publications, cryo-electron microscopy underlines its credentials as a system capable of atomic-resolution in structural studies. This paper presents a high resolution structure of the cytoplasmic polyhedroasis virus (CPV) obtained by single particle cryo electron microscopy, and shows that the polypeptide backbone can be traced without the need of making a crystal. Cytoplasmic polyhedrosis virus (CPV) is unique within the Reoviridae family in having a turreted single-layer capsid contained within polyhedrin inclusion bodies, yet being fully capable of cell entry and endogenous RNA transcription1,2,3,4. Biochemical data have shown that the amino-terminal 79 residues of the CPV turret protein (TP) is sufficient to bring CPV or engineered proteins into the polyhedrin matrix for micro-encapsulation5,6. Here we report the three-dimensional structure of CPV at 3.88 Å resolution using single-particle cryo-electron microscopy. Our map clearly shows the turns and deep grooves of α-helices, the strand separation in β-sheets, and densities for loops and many bulky side chains; thus permitting atomic model-building effort from cryo-electron microscopy maps. We observed a helix-to-β-hairpin conformational change between the two conformational states of the capsid shell protein in the region directly interacting with genomic RNA. We have also discovered a messenger RNA release hole coupled with the mRNA capping machinery unique to CPV. Furthermore, we have identified the polyhedrin-binding domain, a structure that has potential in nanobiotechnology applications.