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The formation of a viral capsid -- the highly-ordered protein shell thatsurrounds the genome of a virus -- is the canonical example of self-assembly.The capsids of many positive-sense RNA viruses spontaneously assemble from invitro mixtures of the coat protein and RNA. The high yield of proper capsidsthat assemble is remarkable, given their structural complexity: 180 identicalproteins must arrange into three distinct local configurations to form anicosahedral capsid with a triangulation number of 3 (T = 3). Despite a wealthof data from structural studies and simulations, even the most fundamentalquestions about how these structures assemble remain unresolved. Experimentshave not determined whether the assembly pathway involves aggregation ornucleation, or how the RNA controls the process. Here we use interferometricscattering microscopy to directly observe the in vitro assembly kinetics ofindividual, unlabeled capsids of bacteriophage MS2. By measuring how many coatproteins bind to individual MS2 RNA strands over time scales from 1 ms to 900s, we find that the start of assembly is broadly distributed in time and isfollowed by a rapid increase in the number of bound proteins. Thesemeasurements provide strong evidence for a nucleation-and-growth pathway. Wealso find that malformed structures assemble when multiple nuclei appear on thesame RNA before the first nucleus has finished growing. Our measurements revealthe complex assembly pathways for viral capsids around RNA in quantitativedetail, including the nucleation threshold, nucleation time, growth time, andconstraints on the critical nucleus size. These results may inform strategiesfor engineering synthetic capsids or for derailing the assembly of pathogenicviruses.

Measurements of the self-assembly kinetics of individual viral capsids around their RNA genome