We study the first steps in the process of infection: how viruses gain entry into cells.
Link to Research Reports to read about our latest work.
Link to our next generation sequencing blog post.
In order for a coronavirus to infect a host, it must perform two crucial functions (1) bind the host receptor and (2) initiate fusion of the viral envelope with the host cell membrane. Receptor binding requires a precise interaction between the virus and the host cell. This interaction is very specific, like a key in a lock, explaining why most viruses don’t cross species barriers. Once a virus binds to a host cell receptor, it still needs to penetrate the cell’s outer membrane. To do this, the virus must change confirmation – exposing a hydrophobic region called the fusion peptide that can be inserted into the lipid bilayer allowing fusion of the host cell membrane and viral envelope. Once fusion is complete, viral proteins and genetic material are released into the host cell and the process of replication can begin. The timing of fusion is critical – it must occur after receptor binding or the fusion apparatus misfires. In order to ensure that receptor binding precedes membrane fusion, coronaviruses use host enzymes called proteases to cut specific amino acid motifs at the surface, triggering conformational changes that expose previously hidden receptor binding sites or fusion peptides. Since individual host proteases are generally specific to a target host tissue, or even to a compartment within the cell, such as an endosome, cleavage events can be timed so viral fusion occurs at the right time and place for successful entry.
The importance the coronavirus spike – one protein to rule them all.
One viral protein is responsible for both receptor binding and membrane fusion: the spike (S) protein. Spike dominates the viral surface giving coronaviruses the crown or corōna-like apparance for which they are named. There are approximately 200 spike proteins arranged in trimers on the surface of each viral particle. The host’s ability to generate neutralizing antibodies against spike is an important part of the immune response. This evolutionary pressure, along with the specific nature of receptor binding, has resulted in divergent spike proteins across viral strains. Further complicating the study of spike function and evolution is the fact that this protein is known to recombine within co-infected cells. This tendency may provide some explanation of why we have seen three novel cornaviruses (SARS-CoV, MERS-CoV, SARS-CoV-2) emerge within the past twenty years. Understanding the spike protein is vital to our understanding of coronavirus infection.
Proteolytic cleavage of the spike protein is necessary for viral fusion: some coronaviruses are cleaved more than once.
The Spike protein consists of two functional subunits named S1 and S2. S1 is responsible for receptor binding while S2 mediates membrane fusion. Receptor binding triggers a conformational change in the spike protein that exposes a proteolytic cleavage site within the S2 subunit known as S2’. Specific host proteases are enlisted to cleave this S2’ site which exposes a hydrophobic fusion peptide. The cleaved viral fusion peptide can be thought of as a harpoon, inserting itself into the host membrane and triggering fusion of the viral envelope with the host cell. Infection is complete and replication can begin.
A subset of coronaviruses possess a second activation site, known as S1/S2 which is located near the boundary of the S1 and S2 subunits. Cleavage at S1/S2 primes the virus for infection. Depending on the strain, cleavage at this additional site results in a conformational change that enhances binding of the S1 subunit to the host cell receptor and/or exposes the critical S2′ site so that fusion can occur. While ALL coronavirus possess a S2’ cleavage site, the S1/S2 site is only found in a subset of coronaviruses including SARS-2 and FCoV serotype I.
Understanding the function of the viral spike protein is key to understanding the process of coronavirus infection.
Basic research, like that performed in our laboratory, is important to drive applied research in diagnostic testing, treatment, and vaccine development. Although no two coronavirus strains are the same, we view research through the lens of One World, One Health – the knowledge that is gained through the study of animal coronaviruses like FIP can inform our understanding of human diseases like COVID-19 and vice versa.
To read more about potential applications of our work, check out this article in the Cornell Chronicle:
https://news.cornell.edu/stories/2020/04/researchers-seek-universal-treatments-impede-coronavirus