Mapping ligand interactions with the hyperpolarization activated cyclic nucleotide modulated (HCN) ion channel...

Area of Science:

• Molecular biology
• Biochemistry
• Ion channel function

Background:

• Hyperpolarization-activated cyclic nucleotide-modulated (HCN) channels are crucial for regulating cardiac rhythm and neuronal excitability.
• Cytosolic cyclic adenosine 3',5'-monophosphate (cAMP) levels directly modulate HCN channel activity.
• Understanding the specific interactions between cyclic nucleotides and the HCN channel's binding domain is essential for drug development and disease research.

Purpose of the Study:

• To characterize the ligand-binding properties of the HCN2 cyclic nucleotide binding domain (CNBD).
• To determine the structural requirements for cyclic nucleotide binding to HCN2.
• To investigate the binding affinities of various cAMP analogs to the HCN2 CNBD.

Main Methods:

• Expression and purification of a novel HCN2/CRP chimera construct.
• Fluorescence polarization assays to measure binding affinity (Kd) of 8-fluo cAMP.
• Ki measurements for 20 cAMP-related ligands to determine binding preferences.

Main Results:

• The HCN2/CRP construct exhibited a Kd of 0.299 microM for 8-fluo cAMP.
• Identified equivalent binding affinities for cAMP and cyclic guanosine 3',5'-monophosphate (cGMP) to the HCN2 CNBD, despite differing channel modulation.
• Established a rank order of ligand preference based on purine ring substitutions, highlighting the importance of specific positions (1, 2, 6, and 8).
• Demonstrated a higher binding affinity for cyclic nucleotide pyrimidine rings compared to cAMP, suggesting ribofuranose is critical for binding.

Conclusions:

• The study provides the first detailed analysis of cAMP and cGMP binding affinities to the HCN2 CNBD.
• Specific structural features of cyclic nucleotides dictate binding preference to the HCN2 CNBD.
• The findings offer insights into the molecular mechanisms of HCN channel modulation by cyclic nucleotides.