Re: More about the Covid furin cleavage site....
the furin cleavage site is like the vampire mouth of the virus...it opens up the host cell so that the virus can disgorge it's 'contents' into the cell. The spike protein has an S1 region and an S2 region...S1 is like the key that fits into the angiotensin lock. S1 binds to the receptor in other words. S2 is like velcro that sticks the virus to the surface of the cell.
But without the furin cleavage site between S1 & S2 that would be the end of things. The furin cleavage site splits the surface of the host cell so that the viral innards can invade.
NO OTHER SARS RELATED CORONAVIRUS HAS A FURIN CLEAVAGE SITE. That doesn't mean it 'came from a lab' though. But it does mean this is a very unusual thing to find & we should figure out how it came to pass.
The furin cleavage site is four amino acids. So that's a lot of little bits that happen to be in the right place in the right order. Plus, the placement of the cleavage site is just right with respect to the S1 & S2 regions of the spike. Doesn't mean that can't happen naturally but it does mean it's a real, real long shot that it's just a random mutation.
So how the heck did it get there? Were there intermediate hosts involved? Which one? We should find out...