Protein nanocompartments are good at carrying protein and bad at carrying RNA. We are trying to change that by evolution rather than by design — and the hard part is not the shell, it is building a selection that cannot be satisfied by anything except the behaviour we want.
These four pages follow that argument in order: what encapsulins already do and where they fall short, which shell we chose and how we gave it an RNA handle, how survival was wired to encapsulation, and what the experiments have shown so far.