Virus particles can be as simple as a molecule of RNA or DNA inside a spherical shell – the “capsid” – made up of multiple copies of a single protein. Further, because viruses only “become alive” when they are inside their hosts, it is possible to study them as physical objects, i.e., to do the same controlled experiments (and theory) on them that one routinely does with more familiar polymer and colloidal systems.
This approach is best illustrated with the example of one of the simplest viruses we work on – cowpea chlorotic mottle virus (CCMV) – a plant virus with a single-stranded RNA genome. Each particle of CCMV is a one-molecule-thick spherical shell consisting of (exactly!) 180 copies of a single protein, surrounding and protecting single copies of RNA genes of the virus. The 180 capsid proteins are organized into groups of 12 pentamers and 20 hexamers, involving 3 sets of inequivalent positions, labeled schematically in red, green and blue in the figure above.
Most remarkably, this icosahedrally-symmetric (soccer-ball-like/”Bucky-ball”-like) structure can form spontaneously: one simply has to mix the purified RNA molecules and purified capsid proteins under the right conditions of pH and ionic strength and a virus forms that is infectious and indistinguishable from those found in infected plants. By changing these solution conditions, and replacing the viral RNA by non-viral RNA (and even by synthetic anionic polymers) and replacing the wildtype capsid protein by genetically- and/or chemically- modified forms of it, we are able to learn a lot about the fundamental physical chemistry of this biological process – the formation of an infectious virus.