Simon Guest: Virus Structures

Introduction

Many viruses consist of an outer protein coat (the virion) containing a DNA or RNA 'payload', where the virion undergoes reversible structural changes that allow switchable access to the interior by the opening of interstices through expansion. These changes may be driven, for example, by variations in pH of the biological medium. We have looked at mechanical models that helps to explain the expansion of the virion in terms of classical principles of structural mechanics.

CCMV

An expanding virus that has been well characterised is the cowpea chlorotic mottle virus (CCMV); a cryoelectron microscopy and image reconstruction of CCMV is shown below (a) in an unswollen condition compatible with low pH; (b) in a swollen condition induced by high pH. This image of CCMV was generated by Tim Baker's lab at Purdue University

CCMV has a structure based on the truncated icosahedron (T=3 in the standard notation for triangulated icosahedral structures). In the native form, stable around pH 5, 180 chemically identical protein subunits form a shell of diameter 28.6 nm. The protein subunits form into either pentagonal or hexagonal capsomeres. At pH 7, the virus particles undergo a 10% increase in radius, thought to occur as a result of deprotonation of carboxyl moieties at the inter-capsomere contacts, leading to electrostatic repulsion that opens out the structure, but the process falls short of complete disassembly through preservation of interwoven carboxyl/protein links between capsomeres. Discrete swollen states have also been observed in many plant viruses, and similar phenomena have been inferred for animal viruses such as poliovirus.

Expandohedra

I have worked on mechanical models of the CCMV with Tibor Tarnai and Florian Kovacs of the Budapest University of Technology and Economics, and Patrick Fowler of the University of Exeter. We have christened our models the "expanodhedra"; two systems have been described. Below is a model I built of the expandohedra that most closely models the CCMV.

The model consists of rigid plates representing the capsomeres, together with double-links — spherically jointed bars that represent a protein link between capsomeres. One important fact that we have elucidated is that a system of this type has a totally symmetric expansion mode whatever the detailed geometry of the links. The ability of the virus to expand only depends on evolution getting the topology, and not the geometry, correct.

Relevant Publications

Guest, S.D., Kovács, K., Tarnai, T. and Fowler, P.W. (2004) “Construction of a Mechanical Model for the Expansion of a Virus”, in Proceedings, IASS-2004, Montpellier, France, September 20–24, 2004. (pdf copy)

Kovács, F., Tarnai, T., Fowler, P.W. and Guest, S.D. (2004), "Double-link expandohedra: a mechanical model for expansion of a virus." Submitted to the Proceedings of the Royal Society: Mathematical, Physical & Engineering Sciences. (pdf preprint)

Kovács, F., Tarnai, T., Fowler, P.W. and Guest, S.D. (2004), "A class of expandable polyhedral structures." International Journal of Solids and Structures. 41(3-4), 1119-1137. doi:10.1016/j.ijsolstr.2003.09.046

Last updated on the 7th of June, 2004

S.D. Guest - sdg@eng.cam.ac.uk