MAD phasing using the (Ta6Br12)2+ cluster: a retrospective study.

Area of Science:

• Structural Biology
• Crystallography
• Biochemistry

Background:

• Cytokinin-specific binding protein (CSBP) plays a crucial role in plant hormone signaling.
• Determining the high-resolution structure of CSBP is essential for understanding its function.
• Traditional structure determination methods faced challenges, necessitating alternative approaches.

Purpose of the Study:

• To solve the crystal structure of CSBP using a novel heavy atom derivatization method.
• To investigate the utility of the dodecabromohexatantalum cation ((Ta6Br12)2+) for phasing.
• To analyze the optimal conditions for heavy atom derivatization and crystal soaking.

Main Methods:

• Three-wavelength MAD (Multi-wavelength Anomalous Dispersion) phasing was employed.
• Crystals were derivatized with the (Ta6Br12)2+ cluster, and 1.8 angstrom resolution data were collected.
• Refinement of the derivative structure was performed to R = 0.158.

Main Results:

• The crystal structure of CSBP, containing four independent molecules, was successfully determined.
• The (Ta6Br12)2+ cluster provided a strong anomalous signal, enabling phasing despite low occupancy.
• Insights into the (Ta6Br12)2+ cluster geometry, protein interactions, and ligand backsoaking were obtained.

Conclusions:

• The (Ta6Br12)2+ cluster is a viable phasing tool in crystallography, provided specific binding and adequate crystal solvent channels.
• Resolution limits and crystal properties significantly impact the effectiveness of heavy atom phasing.
• Understanding these parameters is crucial for successful structure determination of proteins like CSBP.