Effect of coat-protein concentration on the self-assembly of bacteriophage MS2 capsids around RNA
Self-assembly is a vital part of the life cycle of certain icosahedral RNA viruses. Furthermore, the assembly process can be harnessed to make icosahedral virus-like particles (VLPs) from coat protein and RNA in vitro. Although much previous work has explored the effects of RNA-protein interactions on the assembly products, relatively little research has explored the effects of coat-protein concentration. We mix coat protein and RNA from bacteriophage MS2, and we use a combination of gel electrophoresis, dynamic light scattering, and transmission electron microscopy to investigate the assembly products. We show that with increasing coat-protein concentration, the products transition from well-formed MS2 VLPs to "monster" structures consisting of multiple partial capsids to RNA-protein condensates consisting of large networks of RNA and protein. We argue that the variation in structure arises because the assembly follows a nucleation-and-growth pathway in which the nucleation rate depends sensitively on the coat-protein concentration. At high coat-protein concentration, multiple nuclei can form on each RNA strand, leading to malformed structures. Monte Carlo simulations with coarse-grained models of capsomers and RNA validate this physical picture. Our results provide insight into an important biophysical process and could inform design rules for making VLPs for various applications.