A minimalist's approach to the phase problem - phasing selenomethionyl protein structures using Cu Kalpha data

Area of Science:

• Structural Biology
• Biophysics
• X-ray Crystallography

Background:

• Protein structure determination is crucial for understanding biological function.
• Access to synchrotron X-ray sources can be a limitation for many researchers.
• Selenomethionyl (Se-Met) derivatives offer potential for phasing using anomalous scattering.

Purpose of the Study:

• To investigate the feasibility of protein structure phasing using Se-Met derivatives with a standard laboratory X-ray source.
• To assess the quality of electron-density maps obtained through this method.
• To evaluate a combined phasing procedure for proteins lacking synchrotron access.

Main Methods:

• Collected X-ray diffraction data from a Se-Met derivative of avian sarcoma virus integrase using a laboratory Cu Kalpha source.
• Employed single isomorphous replacement/single anomalous scattering (SIR/SAS) phasing techniques.
• Applied solvent flattening to enhance phase accuracy.
• Tested a combined phasing procedure on two different proteins.

Main Results:

• Obtained interpretable electron-density maps for the core domain of avian sarcoma virus integrase (156 amino acids, 4 Met residues).
• Achieved an r.m.s. difference of 55 degrees between experimental and final phases at 3.1 A resolution.
• Demonstrated the effectiveness of the combined SIR/SAS and solvent flattening procedure.

Conclusions:

• Protein structure phasing is feasible using Se-Met derivatives and standard laboratory X-ray sources.
• The developed phasing procedure is a promising alternative when synchrotron facilities are unavailable.
• This approach can facilitate broader access to protein structure determination.