Mapping ligand interactions with the hyperpolarization activated cyclic nucleotide modulated (HCN) ion channel binding domain using a soluble construct.

Hyperpolarization activated cyclic nucleotide modulated (HCN) ion channel currents are activated by hyperpolarization and modulated in response to changes in cytosolic adenosine 3',5'-cyclic monophosphate (cAMP) concentrations. A cDNA chimera combining the rat HCN2 cyclic nucleotide binding domain and the DNA binding domain of the cAMP receptor protein (CRP) from E. coli and the histidine tag (HCN2/CRP) was expressed and purified. The construct is capable of forming only non-ligand dependent dimers because the C-linker region of the channel is not present in this construct. The construct binds 8-[[2-[(fluoresceinylthioureido) amino] ethyl] thio] adenosine-3',5'-cyclic monophosphate (8-fluo cAMP) with a Kd of 0.299 microM as determined with a monomer binding model. The Ki values of 20 ligands related to cAMP were measured in order to determine the properties necessary for a ligand to bind to the HCN2 binding domain. This is the first report of cAMP and gunaosine 3',5'-cyclic monophosphate (cGMP) affinities to the HCN2 binding domain being equivalent, even though they modulate the channel with a 10-fold difference in K0.5. Furthermore, the array of ligands measured allows the preference rank order for each purine ring position to be determined: position 1, H > NH2 > O; position 2, NH2 > Cl > H > O; position 6, NH2 > Cl > H > O; and position 8, NH2 > Cl > H > O. Finally, the ability of HCN2/CRP to bind cyclic nucleotide pyrimidine rings at concentrations approximately 1.33 times greater than cAMP suggests that ribofuranose is key for binding.