Cryo-EM reconstruction of the Cafeteria roenbergensis virus capsid suggests novel assembly pathway for giant viruses

Chuan Xiao,Matthias G Fischer,Gustavo A Avila,Curtis A Suttle,Duer M Bolotaulo,Nancy Ulloa-Rondeau

doi:10.1038/s41598-017-05824-w

Abstract

Whereas the protein composition and overall shape of several giant virus capsids have been described, the mechanism by which these large capsids assemble remains enigmatic. Here, we present a reconstruction of the capsid of Cafeteria roenbergensis virus (CroV), one of the largest viruses analyzed by cryo-electron microscopy (cryo-EM) to date. The CroV capsid has a diameter of 3,000 Å and a Triangulation number of 499. Unlike related mimiviruses, the CroV capsid is not decorated with glycosylated surface fibers, but features 30 Å-long surface protrusions that are formed by loops of the major capsid protein. Based on the orientation of capsomers in the cryo-EM reconstruction, we propose that the capsids of CroV and related giant viruses are assembled by a newly conceived assembly pathway that initiates at a five-fold vertex and continuously proceeds outwards in a spiraling fashion.

Highlights

Viruses with long dsDNA genomes (>200 kilobases) and large particles (>0.2 μm) are a common occurrence in nature and several new families of giant viruses have been reported in recent years[1,2,3,4,5,6], which inspired various discussions regarding their evolutionary origin[7,8,9,10,11]
We push the limits of cryo-electron microscopy (cryo-EM) application to large virus particles by reconstructing the capsid of Cafeteria roenbergensis virus (CroV)[38]
The major capsid protein (MCP) that forms the trimeric capsomers in most icosahedral giant viruses consists of a double “jelly-roll” fold[12]

Summary

Introduction

Viruses with long dsDNA genomes (>200 kilobases) and large particles (>0.2 μm) are a common occurrence in nature and several new families of giant viruses have been reported in recent years[1,2,3,4,5,6], which inspired various discussions regarding their evolutionary origin[7,8,9,10,11]. Whereas rapidly advancing DNA sequencing methods facilitate genomic analysis of giant viruses, structural studies of large viral capsids are lagging behind. In contrast to small and medium-sized capsids, the >200 nm isometric capsids of giant DNA viruses still pose a significant technical challenge for high-resolution methods such as X-ray crystallography and cryo-EM12. We push the limits of cryo-EM application to large virus particles by reconstructing the capsid of Cafeteria roenbergensis virus (CroV)[38]. We observed that the CroV particles were homogenous and could be averaged to high resolution Such detailed structural information of an intact giant virus capsid may help to shed light on their assembly mechanism. Based on our cryo-EM reconstruction of the CroV capsid and by comparison with other giant viruses, we propose a new spiral assembly pathway for the formation of large icosahedral virus capsids

Methods

Results

Conclusion