Moving toward generalizable NZ-1 labeling for 3D structure determination with optimized epitope-tag insertion.

Area of Science:

• Structural Biology
• Biochemistry
• Molecular Biology

Background:

• Antibody labeling is crucial for structural determination via X-ray crystallography and electron microscopy (EM).
• Current methods require target-specific antibodies, limiting broader application.
• Previous epitope insertion methods (e.g., PA12 tag) caused undesirable structural alterations in target proteins.

Purpose of the Study:

• To develop an improved method for antibody complex preparation by inserting exogenous epitopes.
• To overcome the limitations of previous tag-insertion strategies that induced structural changes.
• To enable robust antibody-assisted structural analysis of diverse protein targets.

Main Methods:

• Insertion of an alternative epitope tag (PA14) into various target protein loops.
• Crystallographic analysis to determine the conformation of the PA14 tag within the antibody binding site.
• Molecular-dynamics simulations to assess the impact of tag insertion on target protein structure.
• Negative-stain EM of a bacterial protease using the PA14-labeled antibody complex.

Main Results:

• The PA14 tag adopts a loop-like conformation within the NZ-1 Fab binding pocket, minimizing structural impact on the target.
• PA14 insertion did not induce significant structural changes, even in sterically hindered regions.
• Molecular-dynamics simulations confirmed target protein rigidity post-insertion and complex formation.
• The method was successfully applied to label a bacterial protease, allowing domain arrangement approximation via EM.

Conclusions:

• The PA14 tag offers a superior alternative for antibody-assisted protein labeling and structural studies.
• This improved technique expands the utility of antibody labeling for determining protein structures.
• The method facilitates structural analysis of proteins previously challenging to study with antibody labeling.