SARS-CoV-2 spike and its adaptable furin cleavage site

Much attention has been drawn to the origin of SARS-CoV-2, the causative agent of COVID-19. One notable feature of SARS-CoV-2 is a four-amino acid insert starting with proline (SPRRAR|S) at the junction of the receptor-binding (S1) and fusion (S2) domains of the spike protein. Following the release of the SARS-CoV-2 genome, several groups identified this insert as a potential cleavage site for the protease furin—the insert has also been referred to as a polybasic site and proposed to be part of the proximal origin of the ongoing pandemic.