Revolution in an RNA-packaging capsid
Artificial nucleocapsid proteins, which could be analogous to those used by viruses to package their genomes, are a promising way to protect and deliver RNAs. Using an escalating challenge by nucleases, Tetter et al. evolved a protein that forms multimeric, spherical cages into a highly efficient capsid that selectively packages its own encoding RNA. Cryo–electron microscopy of the final design and intermediates revealed a stepwise expansion in size, enabled by destabilizing amino acid substitutions and a domain swap that results in a change of oligomerization interfaces for the base units of the cage. In addition to altering the protein, directed evolution resulted in changes to the encoding RNA structure that enabled efficient uptake versus other cellular RNAs.
Science, abg2822, this issue p. 1220
Viruses are ubiquitous pathogens of global impact. Prompted by the hypothesis that their earliest progenitors recruited host proteins for virion formation, we have used stringent laboratory evolution to convert a bacterial enzyme that lacks affinity for nucleic acids into an artificial nucleocapsid that efficiently packages and protects multiple copies of its own encoding messenger RNA. Revealing remarkable convergence on the molecular hallmarks of natural viruses, the accompanying changes reorganized the protein building blocks into an interlaced 240-subunit icosahedral capsid that is impermeable to nucleases, and emergence of a robust RNA stem-loop packaging cassette ensured high encapsidation yields and specificity. In addition to evincing a plausible evolutionary pathway for primordial