Ab initio structure solution of a proteolytic fragment using ARCIMBOLDO.

Area of Science:

• Structural Biology
• X-ray Crystallography
• Protein Science

Background:

• Solving the phase problem is crucial for crystal structure determination.
• Traditional methods like molecular replacement and experimental phasing have limitations, especially for unknown or low-resolution structures.
• Ab initio direct methods are effective for small molecules and high-resolution macromolecules (>1.2 Å).

Purpose of the Study:

• To present a case study demonstrating the utility of ab initio structure solution for an unknown protein fragment.
• To showcase the application of the ARCIMBOLDO program in overcoming phase problem challenges.
• To provide a user-friendly test case for ab initio structure determination.

Main Methods:

• Crystallization of a 30.8 kDa protein sample yielding a 1.6 Å resolution dataset.
• Failed attempts using molecular replacement (homologous and sequence-independent) and iodide ion soaking.
• Successful ab initio structure solution using the ARCIMBOLDO program.

Main Results:

• The crystallized entity was identified as the C-terminal dimerization domain of a target protein from Mycobacterium smegmatis (approximately 8 kDa).
• ARCIMBOLDO successfully identified two α-helical elements, leading to interpretable electron density maps.
• The ab initio approach provided a solution where conventional methods failed.

Conclusions:

• Ab initio structure solution using ARCIMBOLDO is a viable strategy for determining the structures of unknown protein fragments.
• This case highlights the power of ab initio methods when molecular replacement and experimental phasing are unsuccessful.
• The study provides a valuable test case for researchers facing similar challenges in protein structure determination.