The spike glycoprotein of the newly emerged SARS-CoV-2 contains a potential cleavage site for furin proteases. This observation has implications for the zoonotic origin of the virus and its epidemic spread in China.
The membrane of coronaviruses harbors a trimeric transmembrane spike (S) glycoprotein (pictured) which is essential for entry of virus particles into the cell. The S protein contains two functional domains: a receptor binding domain, and a second domain which contains sequences that mediate fusion of the viral and cell membranes. The S glycoprotein must be cleaved by cell proteases to enable exposure of the fusion sequences and hence is needed for cell entry.
The nature of the cell protease that cleaves the S glycoprotein varies according to the coronavirus. The MERS-CoV S glycoprotein contains a furin cleavage site and is probably processed by these intracellular proteases during exit from the cell. The virus particles are therefore ready for entry into the next cell. In contrast, the SARS-CoV S glycoprotein is uncleaved upon virus release from cells; it is likely cleaved during virus entry into a cell.
Proteolytic cleavage of the S glycoprotein can determine whether the virus can cross species, e.g. from bats to humans. For example, the S glycoprotein from a MERS-like CoV from Ugandan bats can bind to human cells but cannot mediate virus entry. However, if the protease trypsin is included during infection, the S glycoprotein is cleaved and virus entry takes place. This observation demonstrates that cleavage of the S glycoprotein is a barrier to zoonotic coronavirus transmission.
Examination of the protein sequence of the S glycoprotein of SARS-CoV-2 reveals the presence of a furin cleavage sequence (PRRARS|V). The CoV with the highest nucleotide sequence homology, isolated from a bat in Yunnan in 2013 (RaTG-13), does not have the furin cleavage sequence. Because furin proteases are abundant in the respiratory tract, it is possible that SARS-CoV-2 S glycoprotein is cleaved upon exit from epithelial cells and consequently can efficiently infect other cells. In contrast, the highly related bat CoV RaTG-13 does not have the furin cleavage site.
Whether or not the furin cleavage site within the S glycoprotein of SARS-CoV-2 is actually cleaved remains to be determined. Meanwhile, it is possible that the insertion of a furin cleavage site allowed a bat CoV to gain the ability to infect humans. The furin cleavage site might have been acquired by recombination with another virus possessing that site. This event could have happened thousands of years ago, or weeks ago. Upon introduction into a human – likely in an outdoor meat market – the virus began its epidemic spread.
Furins are also known to control infection by avian influenza A viruses, in which cleavage of the HA glycoprotein is needed for entry into the cell. Low-pathogenic avian influenza viruses contain a single basic amino acid at the cleavage site in the HA protein which is cleaved by proteases that are restricted to the respiratory tract. Insertion of a furin cleavage site in the HA of highly pathogenic avian H5N1 influenza viruses leads to replication in multiple tissues and higher pathogenicity, due to the distribution of furins in multiple tissues.
Acquisition of the furin cleavage site might be viewed as a ‘gain of function’ that enabled a bat CoV to jump into humans and begin its current epidemic spread.