  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.

Biological Physics