SARS-CoV heptad repeat 2 is a trimer of parallel helices

Area of Science:

• Structural biology
• Virology
• Biophysics

Background:

• The severe acute respiratory syndrome coronavirus (SARS-CoV) envelope heptad repeat 2 (HR2) is crucial for viral entry.
• HR2 is a target for antiviral therapies and a potential therapeutic agent itself.
• Previous structural studies yielded conflicting results: NMR showed a trimer of parallel helices, while X-ray crystallography indicated a tetramer of anti-parallel helices.

Purpose of the Study:

• To resolve the structural discrepancies of the SARS-CoV HR2 peptide.
• To determine the helical orientation and oligomerization state of HR2 under various solution conditions.
• To investigate the influence of N-terminal residues on HR2 structure.

Main Methods:

• Site-directed spin labeling of the HR2 N-terminus with a nitroxide radical.
• Utilizing Electron Paramagnetic Resonance (EPR) spectroscopy, specifically paramagnetic relaxation enhancement (PRE).
• Assessing helical orientation and oligomerization in the presence and absence of trifluoroethanol (TFE).

Main Results:

• Paramagnetic relaxation enhancement data consistently support a trimer of parallel helices for HR2.
• This parallel helical trimer conformation was observed both with and without the co-solvent TFE.
• The findings suggest that the N-terminal extension in the NMR construct accounts for the observed structural differences.

Conclusions:

• The SARS-CoV HR2 peptide adopts a trimer of parallel helices in solution.
• The structural variability observed in prior NMR and X-ray studies is likely due to differences in the N-terminal sequence.
• This clarification is vital for developing effective HR2-targeted antiviral strategies.