Non-cytotoxic variants of the Kid protein that retain their auto-regulatory activity

Area of Science:

• Bacteriology
• Molecular Biology
• Protein Structure and Function

Background:

• The Kid toxin and Kis antitoxin are part of the parD operon on plasmid R1.
• The Kid toxin shares structural similarities with the CcdB toxin from plasmid F.
• Previous studies identified toxic residues in CcdB but not in Kid.

Purpose of the Study:

• To identify the specific residues responsible for Kid toxin's toxicity.
• To investigate how mutations affect Kid's toxicity and co-regulatory activity.
• To understand the structure-function relationship of Kid and related bacterial toxins.

Main Methods:

• Characterization of non-toxic Kid mutants.
• Circular dichroism (CD) spectroscopy to assess protein structure.
• Sedimentation equilibrium analysis to determine protein association state.
• Thermal denaturation and tryptophan fluorescence to probe local structural changes.

Main Results:

• Mutations affecting toxicity were localized to the amino- and carboxy-terminal regions of Kid.
• Specific residues (E18 and R85) were identified as critical for toxicity.
• These mutations did not alter the overall protein structure or association state.
• Subtle local structural changes were observed at the N-terminal end.

Conclusions:

• The Kid toxin's toxicity is mediated by residues at both its N- and C-termini.
• Conserved residues E18 and R85 play a crucial role in Kid toxicity.
• Structural integrity is maintained despite mutations affecting toxicity.
• These findings refine the understanding of bacterial toxin-structure-function relationships.