maybe the furin cleavage site is like having a protective cover on the part of the virus that infects the cell, while the protective cover is on it can't attach to anything, so that it doesn't get "neutralized" by attaching at the wrong point

the furin cleavage site is removed ONLY when it gets to the ACE2 receptor (or something else that can provide the furin enzyme i guess)

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The Furin Cleavage Site

The spike found on the top of coronaviruses is the part of the virus that attaches to cells to infect them. The spike of SARS-CoV-2 has a very special feature, a segment of 4 amino acids called a furin cleavage site (FCS). This allows the spike to attach to the ACE2 receptors found throughout the human body.

The FCS allows the virus to use furin in the ACE2 receptors as an enzyme to dissolve (cleave) its coating so it can release its genetic material to infect cells.



This 4 amino acid sequence furin cleavage site is missing from all the coronaviruses in Betacoronavirus b lineage, the section of viruses that SARS-CoV-2 belongs too. It has been inserted in precisely the best place in the spike to give it the ability to become highly infectious. This is the S1/S2 section of the spike protein diagram below. When it is cleaved (split) at this point, it can release its genetic material to infect the cell via the ACE2 receptor.