O-60 - Polybasic furin cleavage site (PRRA insertion) at the S1-S2 junction, absent in related lineage B betacoronaviruses
SARS-CoV-2 has a polybasic (furin) cleavage site formed by a 12-nucleotide insertion adding the amino acids PRRA (yielding an RRAR motif) at the S1/S2 boundary of the spike; such sites are absent in the related lineage B betacoronaviruses.
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Why this is evidence
The polybasic furin site — the one human-adapting feature absent from close lineage-B relatives — is the crux the members split on: H-43 predicts an inserted/engineered site, H-41/H-42 a naturally acquired one. Its flanking O-glycans (O-61) and non-relative status (O-72/O-77) are read oppositely by the two sides; direction contested, but it discriminates H-43 from the natural members.