A group of NIH has reported the effects of O-glycan modification around the furin cleavage site of the new coronavirus (SARS-CoV-2) to the increased infectivity of the UK variant (B.1.1.7).
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7872346/
At the S1/S2 boundary of SARS-CoV-2 S-protein, an unique sequence of 4 amino acids (PRRA) is inserted forming the furin cleavage site as shown below.
S673, T676 are the positions where O-glycans are modified, as shown above. The presence of proline at 681 was deeply related to O-glycan modification at those positions. The authors also found that GALNT1 is the most active one glycosylating the furin proximal region among glycosyltransferase GALNT family. Furthermore, addition of O-glycans was shown to significantly suppress the cleavage at the furin cleavage site into S1 and S2. The UK variant (B.1.1.7) has a mutation called P681H, which replaces proline with histidine. Based on the above mentioned results, it is inferred that the non-presence of proline at 681 in UK variant suppresses O-glycan modification, resulting in increased cleavage efficiency and increased infectability.