Capturing a flavivirus pre-fusion intermediate.

Area of Science:

• Virology
• Structural Biology
• Molecular Biology

Background:

• Flaviviruses, including West Nile virus, initiate cell entry via low endosomal pH.
• This pH drop triggers viral surface glycoprotein rearrangement to a fusion-active state.
• This fusion process releases the viral RNA into the host cell cytoplasm.

Purpose of the Study:

• To capture and structurally characterize a flavivirus pre-fusion intermediate state.
• To gain insights into the molecular mechanisms of flavivirus cell entry.
• To utilize antibody fragments for trapping dynamic viral structural transitions.

Main Methods:

• Complexation of West Nile virus with Fab fragments of the neutralizing monoclonal antibody E16.
• Exposure of the complex to low pH to induce a pre-fusion state.
• Cryo-electron microscopy to determine the structure of the trapped intermediate.

Main Results:

• The cryo-electron microscopy structure revealed a radial expansion of the virion's outer protein layer at low pH compared to neutral pH.
• A low-density shell, approximately 60 Å wide, was observed between the lipid bilayer and the outer protein layer.
• This region is proposed to accommodate the stem region of the E glycoprotein during fusion.

Conclusions:

• Antibody fragments can successfully trap virus structural intermediates occurring during cell entry.
• The study provides the first structural view of a flavivirus fusion intermediate under near-physiological conditions.
• This methodology is applicable to studying other flavivirus life cycle stages and cellular events involving conformational changes.