Mapping function to structure in a channel-blocking peptide: electrostatic mutants of charybdotoxin

Area of Science:

• Molecular biology
• Biophysics
• Neuroscience

Background:

• Charybdotoxin (CTX) is a specific peptide that blocks potassium (K+) channels.
• Calcium-activated potassium channels are crucial for neuronal excitability and function.
• Understanding the electrostatic interactions between CTX and K+ channels is key to elucidating channel gating mechanisms.

Purpose of the Study:

• To investigate the electrostatic interactions between charybdotoxin (CTX) and calcium-activated potassium channels.
• To identify specific charged residues on CTX critical for high-affinity binding and channel interaction.
• To map the molecular surface of CTX involved in direct contact with the K+ channel pore.

Main Methods:

• Utilized a genetically manipulable recombinant charybdotoxin (CTX) for site-directed mutagenesis.
• Assessed the impact of point mutations on CTX binding affinity to the target K+ channel.
• Analyzed toxin association and dissociation rates to understand the kinetics of interaction.

Main Results:

• Mutations at Lys11, Glu12, Arg19, His21, Lys31, and Lys32 had minimal effects on CTX binding affinity.
• Replacing Arg25, Lys27, or Lys34 with glutamine drastically reduced CTX affinity, primarily by increasing dissociation rates.
• Neutralizing Lys27 abolished the voltage-dependent dissociation of CTX, indicating its role in voltage-sensing interactions.

Conclusions:

• Specific charged residues (Arg25, Lys27, Lys34) are critical for high-affinity CTX binding to Ca(2+)-activated K+ channels.
• These residues likely mediate close-range interactions at the extracellular mouth of the K+ channel pore.
• The study reveals a detailed molecular map of CTX-channel interactions, highlighting the importance of electrostatic forces and voltage dependence.