London, Mar 6 : Researchers at Purdue University have achieved a major breakthrough in examining microscopic structures by using a technique called cryo-EM that has successfully given them images of a virus in detail two times greater than achieved in the past.
The research team led by Wen Jiang, an assistant professor of biological sciences at Purdue, used the emerging technique of single-particle electron cryomicroscopy for capturing a three-dimensional image of a virus at a resolution as small as that of 4.5 angstroms (1 million angstroms would equal the diameter of a human hair).
"This is one of the first projects to refine the technique to the point of near atomic-level resolution. This breaks a threshold and allows us to now see a whole new level of detail in the structure. This is the highest resolution ever achieved for a living organism of this size," Nature quoted Jiang, as saying.
Jiang pointed out that the knowledge of detailed structure of a virus can offer important information for development of disease treatments, saying: "If we understand the system - how the virus particles assemble and how they infect a host cell - it will greatly improve our ability to design a treatment. Structural biologists perform the basic science and provide information to help those working on the clinical aspects."
Another researcher Roger Hendrix, said their knowledge about the viruses can be put to use in many other biological systems.
"Understanding the proteins that create the structure of a virus gives us insight into the tiny biological machines found throughout our bodies. Getting to 4.5 angstrom using this technique is a watershed of sorts because it is the first time we can actually trace the polypeptide chain - the backbone of proteins. Now we can see the tiny gears and levers that allow the proteins to move and interact as they carry out their intricate biological roles," he said.
The imaging technique, cryo-EM, is additionally useful as it can enable a sample to be studied in a state very similar to its natural environment, unlike other commonly used imaging techniques, such as X-ray crystallography, that require manipulation of the sample.
"This method offers a new approach for modeling the structure of proteins in other macromolecular assemblies, such as DNA, at near-native states. The sample is purified in a solution that is very similar to the environment that would be found in a host cell. It is as if the virus is frozen in glass and it is alive and infectious while we examine it," said Jiang.
He also said that the team could get a three-dimensional map of the capsid, or protein shell, of the epsilon15 bacteriophage, a virus that infects bacteria and is a member of a family of viruses that are the most abundant life forms on Earth.
"This demonstration shows that cryo-EM is doable and is a major step in reaching the full potential of this technique. The goal is to have it reach a 3 to 4 angstrom resolution, which would allow us to clearly see the amino acids that make up a protein," he said. "."
It is possible to cool the specimens to temperatures much below the freezing point of water through Cryo-EM. This lessens any kind of damage from the electron beam and enables the specimens to be examined for a longer period of time. It is possible to take more detailed images through longer exposure time.
By using cryo-EM researchers could obtain images at a resolution of 6-9 angstroms but failed to differentiate between smaller elements of the structure spaced only 4.5 angstroms apart.
"There are different elements that make up the protein building blocks of the virus. It is like examining a striped blanket. From a distance, the stripes blur together and the blanket appears to be one solid color. As you get closer you can see the different stripes, and if you use a magnifying glass you can see the strands of string that make up the material. The resolution needs to be smaller than the distance between the strands of thread in order to see two separate strands. By being able to zoom in, researchers were able to see components that blurred together at the earlier achieved resolution," said Jiang.
Cryo-EM requires high-end electron microscopes and powerful computing resources.
A detailed paper of the work was published in a recent issue of Nature.