Crystallization and preliminary X-ray diffraction analysis of the human XRCC4-XLF complex

Area of Science:

• Molecular Biology
• Structural Biology
• DNA Repair Mechanisms

Background:

• The proteins XRCC4 (X-ray Repair Cross Complementing protein 4) and XLF (XRCC4-Like Factor) are crucial for repairing DNA double-strand breaks via nonhomologous end-joining (NHEJ).
• These proteins form a complex that facilitates the ligation step in DNA repair.
• The filamentous structure of the XRCC4-XLF complex has hindered previous structural studies using X-ray crystallography.

Purpose of the Study:

• To determine the high-resolution structure of the XRCC4-XLF complex.
• To overcome the challenges associated with crystallizing and phasing filamentous protein complexes.
• To provide structural insights into the mechanism of nonhomologous end-joining.

Main Methods:

• Co-crystallization of truncated XRCC4 and XLF protein variants.
• Optimization of crystal growth conditions using microseeding, dehydration, and heavy metal derivatization.
• X-ray diffraction data collection and structure determination using molecular replacement and anomalous scattering from heavy atom clusters.

Main Results:

• Improved diffraction resolution of XRCC4(Δ157)-XLF(Δ224) crystals to 3.9 Å.
• Successful initial phasing using the anomalous signal from tantalum bromide clusters, overcoming limitations of molecular replacement alone.
• Obtained structural data for the XRCC4-XLF complex, advancing understanding of its role in DNA repair.

Conclusions:

• The developed crystallization and phasing strategies are effective for studying challenging filamentous protein complexes.
• The structural data obtained provides a foundation for understanding the molecular mechanisms of DNA double-strand break repair by NHEJ.
• Further structural and functional studies can now be pursued with greater confidence.