Many viruses have an outer protein coat with the structure of a truncated
icosahedron, and can expand following changes to the environment around the
virus. The protein coat consists of chemically identical protein subunits that
form pentagonal or hexagonal capsomeres; these move apart during expansion,
opening interstices and allowing access to the interior of the virus coat.
A virus whose expansion has been well characterised is the cowpea chlorotic mottle virus (CCMV). A simple physical model of the CCMV has been constructed which consists of rigid pentagonal and hexagonal plates modelling the capsomeres, connected by spherically jointed bars that preserves the double link nature of the connection between capsomeres.
Analysis has shown that this model is guaranteed to have a totally symmetric swelling mode - this expansion mode of the physical model reproduces the key characteristics of the expansion of the CCMV.This mechanical model helps to explain the expansion of the virion in terms of classical principles of structural mechanics.
