N-terminal domain mutations of the spike protein are structurally implicated in epitope recognition in emerging SARS-CoV-2 strains

During the past two years, the world has been ravaged by a global pandemic caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Acquired mutations in the SARS-CoV-2 genome affecting virus infectivity and/or immunogenicity have led to a number of novel strains with higher transmissibility compared to the original Wuhan strain. Mutations in the receptor binding domain (RBD) of the SARS-CoV-2 spike protein have been extensively studied in this context. However, mutations and deletions within the N-terminal domain (NTD) located adjacent to the RBD are less studied. Many of these are found within certain β sheet-linking loops, which are surprisingly long in SARS-CoV-2 in comparison to SARS-CoV and other related β coronaviruses. Here, we perform a structural and epidemiological study of novel strains carrying mutations and deletions within these loops. We identify short and long-distance interactions that stabilize the NTD loops and form a critical epitope that is essential for the recognition by a wide variety of neutralizing antibodies from convalescent plasma. Among the different mutations/deletions found in these loops, Ala 67 and Asp 80 mutations as well as His 69/Val 70 and Tyr 144 deletions have been identified in different fast-spreading strains. Similarly, deletions in amino acids 241–243 and 246–252 have been found to affect the network of NTD loops in strains with high transmissibility. Our structural findings provide insight regarding the role of these mutations/deletions in altering the epitope structure and thus affecting the immunoreactivity of the NTD region of spike protein.