Structural snapshot shows monster protein

A structural snapshot of a protein capsule has revealed details of the largest cellular component ever imaged by x-ray crystallography. 

Big enough to hold an entire ribosome - the protein assembly factory of the cell - the ’vault’ capsule is a barrel-shaped particle found in all mammalian cells. The researchers who produced the draft structure of the protein suggest that the container could find use as a drug delivery vehicle, improving on viral carriers that can potentially trigger the body’s immune response.

The team, led by David Eisenberg and Leonard Rome of the University of California, Los Angeles, used a combination of cryo-electron microscopy and protein crystallography to build up a picture of the particle.¹

Exactly 96 copies of a smaller molecule, known as major vault protein (MVP), stack together to form a hollow shell, 725? (72.5nm) long and 410? (41.0nm) wide. Each half of the central barrel is built of 48 parallel protein ’staves’ which taper together at each end to form a ’cap’. At the waist of the protein, all 96 MVP molecules mesh together like a zipper.

Vaults normally contains an enzyme and some RNA, but since their discovery more than 20 years ago scientists have been unable to pin down exactly what vaults do inside the body. 

Although the structure is unlikely to clear up that puzzle, Rome says that it does suggest some ways to engineer the particle to improve its potential as a drug carrier. The group has already shown that proteins can be stowed inside the hollow shell.²

’We could try to manipulate the zipper-like structure around the waist of the particle,’ he told Chemistry World. ’And there are some areas around the cap . where the chain is predicted to bulge out into the media. We may be able to zoom in on an interesting area and attach a molecule there.’

However, the team admits their picture has ’substantial uncertainties’ because their vault shell crystals diffracted x-rays poorly. The next step, says Rome, will be to get better data: ’We would like to get to higher resolution to place side chains in the model and see the positions of individual amino acids.’

Stephen Fuller of the University of Oxford, UK, studies membrane viruses using similar methods. ’This is a marvellous example of the power of combining cryo-electron microscopy and x-ray crystallography to reveal the structure of macromolecular complexes,’ he told Chemistry World. ’One realises, as so often in recent work on complexes, that this is not the final step, but rather encouragement for future efforts.’

Nanomedicine researcher Chad Mirkin of Northwestern University, US, said that the work holds promise. ’Drug delivery is an excellent avenue to pursue, but they’ll need to learn how to modify the structures to target cells and facilitate uptake.’

Ananyo Bhattacharya