Lesson from a Fab-enabled co-crystallization study of TDRD2 and PIWIL1

Area of Science:

• Molecular Biology
• Structural Biology
• Genetics

Background:

• Tudor domain-containing proteins (TDRDs) and PIWI proteins are crucial for transposon silencing and spermatogenesis.
• Most TDRDs bind methylated PIWI proteins, but TDRD2 uniquely recognizes unmethylated PIWI proteins.

Purpose of the Study:

• To elucidate the structural basis of TDRD2's recognition of unmethylated PIWI proteins.
• To overcome challenges in co-crystallizing the TDRD2 eTudor domain with PIWIL1.

Main Methods:

• Extensive co-crystallization attempts of TDRD2 eTudor and PIWIL1 peptides were performed.
• Recombinant antigen-binding fragments (Fabs) were generated against the TDRD2 eTudor domain.
• Fab-assisted co-crystallization was employed to obtain the complex structure.

Main Results:

• A Fab fragment successfully facilitated the co-crystallization of TDRD2 and PIWIL1.
• Structural analysis revealed that PIWIL1 residues G3-R8 bind between the Tudor core and SN domain of TDRD2.
• The C-terminal PIWIL1 peptide residues were not resolved due to steric hindrance from the Fab.

Conclusions:

• Fab-assisted crystallization is a viable method for studying protein-ligand interactions when native co-crystallization fails.
• The study provides structural insights into TDRD2's unique binding of unmethylated PIWI proteins.
• This technique can aid in the structural analysis of challenging protein complexes.