Direct Measurements of the Assembly of Individual Viral Capsids Around Their RNA Genome

Viral capsids are often constructed from many copies of a single protein subunit arranged with high symmetry into a closed shell. The architecture of these structures has been determined in exquisite detail using high-resolution structural techniques such as x-ray crystallography and cryo-electron microscopy. However, we do not yet understand how the capsids form. Kinetic measurements on bulk solutions of assembling capsids obscure the full pathways, because they measure the signal from viruses at different stages of growth. I will describe experiments in which we use interferometric scattering microscopy to monitor the assembly of single viral capsids from start to finish. In the experiments, we flow capsid proteins from bacteriophage MS2 into a sample chamber and monitor their binding to individual copies of the RNA genome that are tethered to the chamber surface. We find that the assembly kinetics are characterized by long delays preceding comparatively fast growth, suggesting a nucleation-and-growth assembly pathway. These findings should inform the construction of quantitative models of the assembly process.