Distinctive Features of the XBB.1.5 and XBB.1.16 Spike Protein Receptor-Binding Domains and Their Roles in Conformational Changes and Angiotensin-Converting Enzyme 2 Binding.

The emergence and the high transmissibility of the XBB.1.5 and XBB.1.16 subvariants of the SARS-CoV-2 omicron has reignited concerns over the potential impact on vaccine efficacy for these and future variants. We investigated the roles of the XBB.1.5 and XBB.1.16 mutations on the structure of the spike protein's receptor-binding domain (RBD) and its interactions with the host cell receptor ACE2. To bind to ACE2, the RBD must transition from the closed-form to the open-form configuration. We found that the XBB variants have less stable closed-form structures that may make the transition to the open-form easier. We found that the mutations enhance the RBD-ACE2 interactions in XBB.1.16 compared to XBB.1.5. We observed significant structural changes in the loop and motif regions of the RBD, altering well-known antibody-binding sites and potentially rendering primary RBD-specific antibodies ineffective. Our findings elucidate how subtle structural changes and interactions contribute to the subvariants' fitness over their predecessors.