The Structure of the Toxin and Type Six Secretion System Substrate Tse2 in Complex with Its Immunity Protein

Area of Science:

• Microbiology
• Structural Biology
• Biochemistry

Background:

• Pseudomonas aeruginosa utilizes a type VI secretion system (T6SS) to secrete effector proteins like the cytotoxin Tse2.
• Tse2 toxin exhibits toxicity in eukaryotic cells upon artificial introduction.
• Understanding Tse2's mechanism is crucial for combating P. aeruginosa infections.

Purpose of the Study:

• To elucidate the structural basis of Tse2-Tsi2 interaction.
• To identify the active site of the Tse2 toxin.
• To propose a potential mechanism for Tse2-mediated toxicity.

Main Methods:

• X-ray crystallography was employed to determine the structure of the Tse2-Tsi2 complex.
• Bioinformatic analysis was used to compare Tse2 with known enzymes.
• Structural comparisons identified conserved catalytic residues.

Main Results:

• The Tse2-Tsi2 complex forms a heterotetrameric structure with a significant binding interface.
• Structural homology was observed between Tse2 and NAD-dependent enzymes, particularly ADP-ribosylating toxins.
• The Tse2 active site was identified and found to be occluded when bound to its immunity protein, Tsi2.

Conclusions:

• The structural similarity to NAD-dependent enzymes suggests a conserved catalytic mechanism.
• The occlusion of the active site by Tsi2 likely serves an auto-immunity function.
• Tse2 toxicity is potentially mediated by an NAD-dependent enzymatic activity.