Application of the phase extension method in virus crystallography

Area of Science:

• Structural biology
• Crystallography
• Biophysics

Background:

• Phase extension (PX) is vital for improving crystallographic data resolution.
• Viral capsids with icosahedral symmetry offer high non-crystallographic symmetry (NCS), aiding phasing power.

Purpose of the Study:

• To detail the phase extension procedure for crystallographic data.
• To highlight factors critical for successful phase extension and resolution enhancement.

Main Methods:

• Gradual extension of initial phases from low to high resolution.
• Utilizing molecular replacement for accurate particle orientation and positioning.
• Defining molecular masks around non-crystallographic symmetry (NCS) asymmetric units for density modification.

Main Results:

• High NCS in viral capsids significantly improves phasing power for resolution extension.
• Complete diffraction data and accurate molecular replacement solutions are key for smooth PX.
• Density modification techniques like averaging and flattening depend on precise molecular masks.
• PX is effective across space groups with accurate masks and initial phases.
• For P1 space groups, starting PX at higher resolution (e.g., 6Å) resolves Babinet phase and phase flipping errors.

Conclusions:

• Phase extension is a robust method for enhancing crystallographic resolution.
• Non-crystallographic symmetry and accurate data processing are critical for successful phase extension.
• Specific strategies are needed for challenging space groups like P1 to overcome phase extension limitations.