Crystallization of single-chain Fv proteins

Area of Science:

• Protein crystallography
• Structural biology
• Immunology

Background:

• Single-chain variable fragment (sFv) proteins are antibody fragments comprising variable heavy (VH) and variable light (VL) domains linked by a polypeptide.
• sFv aggregation during concentration hinders crystallization, impeding structural analysis.
• Aggregation is influenced by linker integrity, protein identity, ligand presence, and linker characteristics.

Purpose of the Study:

• To develop methods for crystallizing single-chain Fv proteins.
• To investigate the causes of sFv aggregation and find solutions.
• To enable structural determination of sFv proteins for therapeutic and diagnostic applications.

Main Methods:

• Crystallization of sFv proteins specific for fluorescein (4-4-20 sFv) and TAG-72 antigen (CC49 sFv).
• Investigated monomer-multimer equilibrium and aggregation during protein concentration.
• Developed two methods to overcome aggregation: proteolytic linker cleavage and purification of sFv dimers.

Main Results:

• Two strategies successfully yielded X-ray diffraction-quality crystals of sFv proteins.
• Proteolytic cleavage of the linker produced crystallizable Fv fragments.
• Purification of sFv dimers, with intact linkers, provided an alternative crystallization approach.
• A novel dimer interface model involving intermolecular VL/VH interactions was proposed.

Conclusions:

• Methods to overcome sFv aggregation are essential for structural studies.
• Proteolytic cleavage and dimer purification are effective strategies for sFv crystallization.
• Further structural determination of sFv dimers will validate proposed molecular association models.
• Optimizing linker length and antigen binding are potential strategies for difficult-to-crystallize sFv proteins.