It’s this computing power, along with refinements to the image-processing software, that allowed the researchers to generate such a high-resolution model. Because they had the resources to handle a massive input of data, they could combine many more images to create the composite.
Earlier uses of electron cryomicroscopy to model the structures of viruses have relied on shortcuts, such as assuming that the virus’s structure will be highly symmetrical. Thanks to the Condor pool, Jiang’s group was able to avoid such simplifications in determining how the virus’s surface proteins fit together.
“They did the pure experiment, which was to solve the structure without assuming symmetry,” says Matsudaira. That, he says, is the project’s most significant innovation–even more so than the 4.5-angstrom resolution.
From approximately 36,000 single-particle images, the researchers pieced together a model of epsilon 15’s protein shell, known as a capsid. Earlier work suggested that the capsid only incorporated one major protein. But in addition to tracing that protein’s backbone, the new model revealed a mysterious second protein–much smaller than the first–that no previous structural or biochemical study had predicted.
When the group reanalyzed the virus’s constituent proteins using a more sensitive biochemical screening method, they indeed found evidence of the smaller protein.
Jiang says that this outcome turns conventional structural analysis on its head. Usually, a particle’s biochemical makeup is called upon to help derive its structure. Here, the virus’s structure, as revealed by this powerful new analysis, helped uncover a previously overlooked biochemical feature.
“Usually the structure relies on the biochemistry,” says Matsudaira, “but this was exactly the opposite.”
In the future, Jiang hopes to further improve the resolution of images produced by single-particle electron cryomicroscopy. By further refining the software and perhaps invoking even more computing power, he anticipates that it may be possible to reach three-angstrom resolution within the next few years. That level of detail would uncover atomic-level features.
Beyond epsilon 15, the technique could be used to create structural models of other, more clinically relevant viruses. Jiang’s lab is currently applying the new approach to West Nile virus and dengue virus. Elaborate protein structures other than viral capsids would also be ideal targets.
“This is just scratching the surface of this technique,” says Jiang. “The potential of the technique is so much more than what we have achieved so far.”