Prokaryotic homologs of the major vault protein form engineerable vault nanocapsules

Vaults, the largest ribonucleoprotein particles found in eukaryotic cells, are composed of 78 major vault protein (MVP) copies, but their purpose has been a mystery for four decades since their discovery. MVP homologs have been identified in prokaryotes and bacteriophages, yet it was uncertain whether they form vaults. Researchers have now discovered that prokaryotic and bacteriophage MVP homologs can assemble into vault particles.

By examining around 15 billion proteins, the researchers identified MVP homologs in 29 bacterial and 4 archaeal phyla. They used size-exclusion chromatography to rapidly screen and isolate vault particles of eight homologs that were expressed in Escherichia coli. The cryo-electron microscopy structures of the phage homolog L05, obtained at high resolution, revealed a shell composed of 96 MVP chains enclosing a volume approximately 25% larger than human vaults, making it one of the largest protein shells of a defined size outside virus capsids.

Prokaryotic MVPs generally contain seven wall repeats, while eukaryotic MVPs consist of nine. The addition of wall repeats to L05 results in the formation of larger vaults. These prokaryotic vaults not only expand the known diversity of vault architecture but also provide a more accessible system for studying vault assembly and a size-tunable scaffold for cargo encapsulation.