Measurements of the Assembly of Individual HIV-1 Gag Protein Capsids Around Spherical Templates*

Despite the abundance of structural information about virus capsids, relatively little is known about how they assemble. The small sizes of the capsids and the wide range of timescales involved in the assembly process make it difficult to directly measure the assembly kinetics. We use a microscopy method based on interferometric scattering that addresses both of these challenges, allowing us to measure the the size of individual capsids as a function of time. We apply this method to study the self-assembly of the Gag protein of HIV-1 around a spherical template that is coated in nucleic acid. We find that a simple Langmuir adsorption model can describe the growth kinetics of each capsid. In addition, the assembly rate has a strong concentration dependence, and the binding of capsid proteins is reversible.