Budding pathway in the templated assembly of viruslike particles.

A new pathway for the assembly of viral capsid protein around inorganic nanoparticle cores was observed by time-course light scattering and cryo-electron tomography. Gold nanoparticles with an average diameter of 11.3 nm have been used as a template for the assembly of Brome mosaic virus (BMV) capsid protein at different concentrations. At least at low protein concentrations the kinetic features of the scattering and extinction measurements are consistent with the initial rapid formation of large nanoparticle-protein clusters, which subsequently separate into individual viruslike particles (VLPs). The occurrence of multiparticle clusters at short times after mixing nanoparticles and proteins was confirmed by cryo-EM. Cryo-electron tomography of the multiparticle clusters yielded an average surface-to-surface interparticle distance of ∼7.5 nm, equivalent to ∼1.5 times the thickness of a protein shell. We propose a scenario in which VLP generation may take place through monomer exchange between aggregated particles with defect-ridden or incomplete shells, leading to the formation of stable icosahedral shells, which eventually bud off the aggregate. Together with results from previous works, the findings highlight the astonishing versatility of plant virus capsid protein assembly. This previously unknown mechanism for VLP formation has features that may have relevance for the crowded environment characterizing virus factories in the cell.