Breathless: The Scientific Race to Defeat a Deadly Virus

by David Quammen

A furin cleavage site is a sort of trigger within the spike protein of a coronavirus (or an equivalent protein in other viruses) that enhances the capacity of the virus to latch on to and enter cells. First the spike catches hold of a receptor protein on the exterior of the cell, such as ACE2. Then the cleavage site comes into play. It’s located at the junction between the two major lobes of the spike, and when it’s triggered, the spike changes shape—like a Transformer robot metamorphosing suddenly into a truck—so that the spike is “cleaved,” split open, in a way that allows it to fuse with the membrane of the cell. This enables the viral genome to squirt into the cell and begin replicating itself. What triggers the furin cleavage site is the touch of a furin molecule. Furin is an enzyme with important functions, never mind what, and our bodies are full of it. So the furin cleavage site is well suited for triggering, by bodily furin, and helping the virus to penetrate cells. Some viruses possess furin cleavage sites as part of their cell-entry machinery, and those sites seem to help make them highly virulent in humans. SARS-CoV had no such site, and that virus, fortunately, was not very efficiently contagious. So the inclusion of a furin cleavage site in the new coronavirus caused Bob Garry some concern, on the grounds that it might make the thing better able to spread.

Ngl i love that he doesn't try on some of these