In order to penetrate human cells, the Sars-CoV-2 coronavirus with one of its surface proteins has to dock onto the cell surface. Researchers have now succeeded in decoding the crucial part of this so-called spike protein. Using cryo-electron microscopy, they depicted the three-dimensional, atom-precise structure of this binding protein. Additional experiments also revealed that this protein looks similar to that of the Sars and Mers pathogens, but reacts significantly differently. Knowledge of the structure and behavior of this key protein now offers new starting points for future vaccines and drugs against the pathogen.
Even though the new infections in China seem to be slowly declining, hundreds of people are still contracting the SARS-CoV-2 coronavirus every day. The number of cases has now increased to more than 74,000, and more than 2,000 people have already died from the Covid-19 viral disease. The corona virus, which first appeared in the Chinese city of Wuhan, has spread to more than 24 countries, and there are 16 cases in Germany as well. According to current knowledge, Sars-CoV-2 is passed on via droplet infection and is therefore similarly contagious as the flu. While most of the patients infected with the virus appear to develop mild symptoms, the disease can also lead to severe pneumonia and death. So far there is no cure or vaccine against Sars-CoV-2, researchers worldwide are working flat out to find active substances against the virus.
Docking protein as the key to antivirals
The new findings by Daniel Wrapp from the University of Texas at Austin and his colleagues could be of great help. Because they have succeeded in deciphering the exact structure of a crucial surface protein from Sars-CoV – the spike protein. “The coronavirus spike glycoprotein is a key target for the much needed vaccines, therapeutic antibodies and diagnostics,” the researchers explain. With this three-part protein, the virus docks onto the so-called ACE2 receptor of human cells and can then penetrate the cell through the cell membrane. As the scientists report, the spike protein undergoes a conformational change – it opens to a certain extent and thus takes on a shape that is suitable for the cell receptor. The problem, however, is that this conformation is unstable and therefore cannot be easily mapped or structurally analyzed.
To work around this problem, Wrapp and his colleagues used a strategy that they had previously used for the closely related Mers-CoV and Sars-CoV viruses: They extracted the genetic building instructions for the spike from the genome of the virus, which was supplied by Chinese researchers Protein and inserted a mutation in it at two points, which led to the exchange of an amino acid. This change stabilized the resulting spike protein. “We knew exactly which mutations we had to use because we had tried this with a number of other corona viruses,” explains senior author Jason McLellan from the University of Texas. Using a laboratory system, the researchers then used the building instructions to generate the viral spike protein and prepared samples for cryo-electron microscopy – the virus proteins were quasi freeze-dried.
Particularly loyal
The images made it possible for the first time to depict the three-dimensional structure of the part of the spike protein that is important for binding to the cell with atomic resolution. “We were also able to observe how the spike subunits performed the opening movement,” report Wrapp and his colleagues. “Observation of this phenomenon suggests that it has the same trigger mechanism as other coronaviruses.” The structure of the spike protein of Sars-CoV-2 is very similar to that of the known types of these pathogens – but behaves differently, however Additional experiments revealed: “Surprisingly, the ACE2 receptor binds to the spike binding site of the new coronavirus with 10 to 20 times higher affinity than to Sars-CoV,” said the researchers. “That could explain why this virus spreads so easily from person to person.”
Another result of the team is important for the development of new vaccines and antidotes: Supplementary experiments revealed that the spike protein of Sars-CoV-2, despite superficial similarities with those of the SARS-CoV and MERS-CoV, apparently not against antibodies against these two related types of viruses responded. “In tests with three monoclonal antibodies against Sars, no binding to the spike protein of Sars-CoV-2 could be detected,” the researchers report. In their view, this suggests that it is probably not very expedient to try to use existing means against the related corona viruses against the new Sars-CoV. Instead, it makes more sense to start from the structure of the pathogen itself. “Knowing the atomic structure of the spike protein can now make the design of vaccines and antivirals more precise and facilitate the development of medical countermeasures,” concludes Wrapp and his colleagues.
Source: Daniel Wrapp (University of Texas, Austin) et al., Science, in press