SARS-related coronavirus S-protein structures reveal synergistic RBM interactions underpinning high-affinity human...

Area of Science:

• Structural biology
• Virology
• Molecular mechanisms of viral entry

Background:

• The binding of severe acute respiratory syndrome coronavirus (SARS)-related coronaviruses (SARSr-CoVs) to the human angiotensin-converting enzyme 2 (hACE2) receptor is crucial for viral entry but not fully understood.
• SARSr-CoVs utilize their spike (S)-proteins to mediate this interaction, with variations in binding affinity and specificity observed across different viral strains.

Purpose of the Study:

• To elucidate the structural basis of hACE2 receptor recognition by diverse SARSr-CoVs.
• To understand the molecular determinants that enable SARSr-CoV S-proteins to achieve high-affinity binding to hACE2.

Main Methods:

• Cryo-electron microscopy (cryo-EM) was employed to determine the structures of eight different SARSr-CoV S-proteins.
• Structure-guided mutagenesis was performed on specific S-protein regions, particularly the receptor-binding motif (RBM), to assess the impact on ACE2 binding.

Main Results:

• Cryo-EM structures revealed that SARSr-CoV S-proteins adopt locked, prefusion conformations, allowing classification into three types based on RBM structure and ACE2 binding.
• Type-2 S-proteins showed preferential binding to bat ACE2 (bACE2) over hACE2. A structure of a type-2 BtKY72-RBD complexed with bACE2 was determined.
• Mutagenesis studies demonstrated that multiple synergistic mutations within four distinct RBM regions are necessary for high-affinity hACE2 binding, and these changes can confer hACE2 binding to previously non-binding S-proteins.

Conclusions:

• The study provides detailed structural insights into SARSr-CoV S-protein interactions with ACE2 receptors.
• Understanding the acquisition of high-affinity hACE2 binding through specific RBM mutations is critical for predicting viral tropism and developing therapeutic strategies against SARSr-CoVs.