The Molecular Evolution of the SARS-COV-2 Spike Protein: Study of Amino Acid Substitutions and Types

The molecular evolution of SARS-COV-2 has been challenging to predict. Emergence of the Omicron Variant of Concern (VOC) and its sublineages indicated that SARS-COV-2 could evolve more rapidly than previously thought. We analyzed the mutation and amino acid substitution patterns in the spike (S) protein of SARS-COV-2 VOCs to assess how they evolved in response to the selective pressure exerted by both natural immunity and vaccination. Our results indicate less evolutionary constraint on the first part of the S protein, allowing more amino acid substitutions, especially in the NTD, RBD, and subdomain 1. Omicron lineages introduced mutations in the FP and HR1 domains for the first time. The NTD, subdomain 1, and FP domains allowed more radical amino acid substitutions, followed by RBD and HR1, possibly due to their function. There were up to nine conservative and one radical substitutions in the amino acids interacting with the Human ACE2 receptor in the RBM of the Omicron sublineages only, a remarkable departure from the previous VOCs. We show that the molecular evolution of SARS-COV-2 S protein was relatively limited up to the Omicron lineage. The selective pressure from previous VOCs and global vaccination potentially accelerated the emergence of the highly transmissible Omicron lineage. This antigenic drift in Omicron is fueled by a high rate of radical amino acid substitutions in the S1 domains, resulting in positive selection with a high potential to change due to adaptive evolution. However, the conservative nature of changes in the RBM may signal a relative stabilization.