Solution scattering studies on a virus capsid protein as a building block for nanoscale assemblies

Self-assembled protein cages are versatile building blocks in the construction of biomolecular nanostructures. Because of the defined assembly behaviour the cowpea chlorotic mottle virus (CCMV) protein is often used for such applications. Here we report a detailed solution scattering study of the CCMV virus and empty capsid. Contrast variation in small-angle neutron scattering (SANS) reveals a well-defined protein shell, with RNA associated mainly with its inner surface. The empty capsid has a protein shell with a diameter comparable to that of the virus, and has some weak scattering density associated on the inside, presumably the N-terminal part which is involved in RNA binding. Dynamic light scattering (DLS) and SANS show that the virus swells with increasing pH (5.0 to 7.5), whereas the empty capsid disassembles; the aggregation behaviour of the capsid protein becomes more complex at salt concentrations below 0.5 M NaCl. Incorporation of polystyrene sulfonate (PSS) in the capsid gives a particle with a solvent core, a polymer inner shell, and a protein outer shell, with a much smaller capsid outer diameter.