Coronaviruses are responsible for almost a third of mild, cold-like symptoms and atypical pneumonia in the world. The deadliest forms are SARS-CoV in 2002 and of MERS-CoV in 2012 with fatality rates between 10 percent to 37 percent.

These outbreaks demonstrated that coronaviruses can transmit from different animals to people. Until now, only six coronaviruses are known to infect people. However, a variety of coronaviruses naturally infect animals. The latest outbreaks resulted from coronaviruses breaking the species barrier. It is possible that other novel coronavirus with pandemic potential may occur. But now we have no approved vaccines or antiviral drugs against SARS-CoV or MERS-CoV.

A transmembrane spike glycoprotein allows coronaviruses to bind to and enter host cells. The glycoprotein forms trimers on the surface of the virus. Trimers are assembled from the same three protein units.

In the study, the researchers investigated a spike responsible for binding to and fusing with the membrane of a living cell. The spike determines which animals and which cells a coronavirus can infect.

Combining the latest technique, known as single particle cryo-electron microscopy and supercomputing analysis, the research team identified the atom structure of a mouse coronavirus spike glycoprotein trimer in great detail. The structure is maintained in its pre-fusion state, and then undergoes major rearrangements to induce fusion of the viral and host membranes and start infection.

Another family of viruses called paramyxoviruses, including respiratory syncytial virus and those cause measles and mumps, enter host cells in the similar way with coronaviruses. The researchers suggested the two classes of two virus families share a common evolutionary origin.

The team also compared crystal architectures of the spike protein in mouse and human coronaviruses. The results could help to understand how the molecular structure of these protein domains might affect which animal species the virus can infect.