Crystal Structure of the Coat Protein of the Flexible Filamentous Papaya Mosaic Virus

摘要

Papaya Mosaic Virus (PapMV) is a filamentous plant virus that belongs to the Alphaflexiviridae family. Flexible filamentous viruses have defied more than two decades of effort in fiber diffraction, and no high-resolution structure is available for any member of the Alphaflexiviridae family. Here we report our structural characterization of PapMV by X-ray crystallography and cryo-EM 3D reconstruction. We found that PapMV is 135 Å in diameter with a helical symmetry of ~ 10 subunits per turn. Crystal structure of the C-terminal truncated PMV coat protein reveals a novel all helix fold with seven α-helices. Thus the PMV CP structure is different from the four-helix bundle fold of Tobacco Mosaic Virus (TMV) in which helix bundling dominates the subunit interface in TMV and conveys rigidity to the rod virus. PapMV coat protein was crystallized as an asymmetrical dimer in which one protein lassoes the other by the N-terminal peptide. Mutation of residues critical to the inter-subunit lasso interaction abolishes coat protein polymerization. The crystal structure suggests that PMV may polymerize via the consecutive N-terminal loop lassoing mechanism. The structure of PapMV will be useful for rational design and engineering of the PapMV nanoparticles into innovative vaccines.