Ligand binding and activation properties of the purified bacterial cyclic nucleotide–gated channel SthK

Cyclic nucleotide–modulated ion channels play several essential physiological roles. They are involved in signal transduction in photoreceptors and olfactory sensory neurons as well as pacemaking activity in the heart and brain. Investigations of the molecular mechanism of their actions, including structural and electrophysiological characterization, are restricted by the availability of stable, purified protein obtained from accessible systems. Here, we establish that SthK, a cyclic nucleotide–gated (CNG) channel from Spirochaeta thermophila, is an excellent model for investigating the gating of eukaryotic CNG channels at the molecular level. The channel has high sequence similarity with its eukaryotic counterparts and was previously reported to be activated by cyclic nucleotides in patch-clamp experiments with Xenopus laevis oocytes. We optimized protein expression and purification to obtain large quantities of pure, homogeneous, and active recombinant SthK protein from Escherichia coli. A negative-stain electron microscopy (EM) single-particle analysis indicated that this channel is a promising candidate for structural studies with cryo-EM. Using radioactivity and fluorescence flux assays, as well as single-channel recordings in lipid bilayers, we show that the protein is partially activated by micromolar concentrations of cyclic adenosine monophosphate (cAMP) and that channel activity is increased by depolarization. Unlike previous studies, we find that cyclic guanosine monophosphate (cGMP) is also able to activate SthK, but with much lower efficiency than cAMP. The distinct sensitivities to different ligands resemble eukaryotic CNG and hyperpolarization-activated and cyclic nucleotide–modulated channels. Using a fluorescence binding assay, we show that cGMP and cAMP bind to SthK with similar apparent affinities, suggesting that the large difference in channel activation by cAMP or cGMP is caused by the efficacy with which each ligand promotes the conformational changes toward the open state. We conclude that the functional characteristics of SthK reported here will permit future studies to analyze ligand gating and discrimination in CNG channels.