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.
Highlights
Icosahedral capsids come in two varieties: spherical, whereby all pentamers are equidistant from the center of the capsid, or prolate, that retains quasi-icosahedral symmetry with one direction of the capsid elongated by additional hexameric rings
We focus on macromolecular assemblies that interrupt true icosahedral symmetry at a single 5-fold vertex, generating a symmetry mismatch within the coat shell
The lack of identical binding sites at the interfaces exposed by portal protomers is possibly the reason why head completion proteins that bind at the end of DNA packaging such as gp4 in P22 [76], gp15 in SPP1 [77] and gpFII in phage λ [78] are monomeric in solution but oligomerize upon binding to the portal vertex
Summary
The principles governing assembly of icosahedral capsids are well understood and have been reviewed in detail [1,2,3,4,5,6]. All dsDNA-containing viruses use a scaffolding protein-mediated assembly and utilize a canonical HK97-like coat fold, which was first identified in the crystal structure of the bacteriophage HK97 mature capsid [13]. The portal protein ( known as “head-to-tail connector” in bacteriophages) is one of the best examples of non-icosahedral structures breaking the global symmetry of virus capsids [30]. Cryo-EM single particle analysis of frozen hydrated virions reconstructed without applying icosahedral symmetry (methodology referred to as “asymmetric” reconstructions) enabled direct visualization of the portal vertex in situ [51] (Tables 2 and 3).
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
