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Geometric Defects and Icosahedral Viruses
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Viruses 2018, 10(2), 67; https://doi.org/10.3390/v10020067

Breaking Symmetry in Viral Icosahedral Capsids as Seen through the Lenses of X-ray Crystallography and Cryo-Electron Microscopy

1
Department of Biochemistry and Molecular Biology, Michigan State University, East Lansing, MI 48824, USA
2
Department of Biochemistry and Molecular Biology, Thomas Jefferson University, Philadelphia, PA 19107, USA
3
Institute of Biomembranes and Bioenergetics, National Research Council, Via Amendola 165/A, 70126 Bari, Italy
*
Authors to whom correspondence should be addressed.
Received: 6 January 2018 / Revised: 26 January 2018 / Accepted: 31 January 2018 / Published: 7 February 2018
(This article belongs to the Special Issue Advances in Structural Virology via Cryo-EM)
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Abstract

The majority of viruses on Earth form capsids built by multiple copies of one or more types of a coat protein arranged with 532 symmetry, generating an icosahedral shell. This highly repetitive structure is ideal to closely pack identical protein subunits and to enclose the nucleic acid genomes. However, the icosahedral capsid is not merely a passive cage but undergoes dynamic events to promote packaging, maturation and the transfer of the viral genome into the host. These essential processes are often mediated by proteinaceous complexes that interrupt the shell’s icosahedral symmetry, providing a gateway through the capsid. In this review, we take an inventory of molecular structures observed either internally, or at the 5-fold vertices of icosahedral DNA viruses that infect bacteria, archea and eukaryotes. Taking advantage of the recent revolution in cryo-electron microscopy (cryo-EM) and building upon a wealth of crystallographic structures of individual components, we review the design principles of non-icosahedral structural components that interrupt icosahedral symmetry and discuss how these macromolecules play vital roles in genome packaging, ejection and host receptor-binding. View Full-Text
Keywords: icosahedral symmetry; symmetry mismatch; cryo-EM; cryo-ET; X-ray crystallography; portal protein; bacteriophages; giant viruses; archeal viruses icosahedral symmetry; symmetry mismatch; cryo-EM; cryo-ET; X-ray crystallography; portal protein; bacteriophages; giant viruses; archeal viruses
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).
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Parent, K.N.; Schrad, J.R.; Cingolani, G. Breaking Symmetry in Viral Icosahedral Capsids as Seen through the Lenses of X-ray Crystallography and Cryo-Electron Microscopy. Viruses 2018, 10, 67.

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