it looks like that polybasic site occurred at the 680th amino acid locus. in corona viruses, specifically in the spike proteins, there is a stretch that contains a lot of arginine residues and residues that can are close enough to arginine that function isn't destroyed. this makes it more likely for mutations to occur there, as the virus would still be able to function as a virus. this area (670-700) also happens to be the location of the S1/S2 cleavage site in corona viruses and mutations in that regions have been seen in other human and non-human coronaviruses. In this particular case, the insertion mutation simply resulted in an amino acid shift to RXXR or RXK, which makes it cleavable by furins. thus far, all evidence suggests natural mutation, not targeted genome splicing.
it looks like that polybasic site occurred at the 680th amino acid locus. in corona viruses, specifically in the spike proteins, there is a stretch that contains a lot of arginine residues and residues that can are close enough to arginine that function isn't destroyed. this makes it more likely for mutations to occur there, as the virus would still be able to function as a virus. this area (670-700) also happens to be the location of the S1/S2 cleavage site in corona viruses and mutations in that regions have been seen in other human and non-human coronaviruses. In this particular case, the insertion mutation simply resulted in an amino acid shift to RXXR or RXK, which makes it cleavable by furins. thus far, all evidence suggests natural mutation, not targeted genome splicing.