Subunit interactions in the activation of cyclic nucleotide-gated ion channels

Stoichiometry and arrangement of heteromeric olfactory cyclic nucleotide-gated ion channels

Native cylic nucleotide-gated (CNG) channels are composed of alpha and beta subunits. Olfactory CNG channels were expressed from rat cDNA clones in Xenopus oocytes and studied in inside-out patches. Using tandem dimers composed of linked subunits, we investigated the stoichiometry and arrangement of the alpha and beta subunits. Dimers contained three subunit types: alphawt, betawt, and alpham. The alpham subunit lacks an amino-terminal domain that greatly influences gating, decreasing the apparent affinity of the channel for ligand by 9-fold, making it a reporter for inclusion in the tetramer. Homomeric channels from injection of alphawtalphawt dimers and from alphawt monomers were indistinguishable. Channels from injection of alphawtalpham dimers had apparent affinities 3-fold lower than alphawt homomultimers, suggesting a channel with two alphawt and two alpham subunits. Channels from coinjection of alphawtalphawt and betabeta dimers were indistinguishable from those composed of alpha and beta monomers and shared all of the characteristics of the alpha+beta phenotype of heteromeric channels. Coinjection of alphawtalpham and beta beta dimers yielded channels also of the alpha+beta phenotype but with an apparent affinity 3-fold lower, indicating the presence of alpham in the tetramer and that alpha+beta channels have adjacent alpha-subunits. To distinguish between an alpha-alpha-alpha-beta and an alpha-alpha-beta-beta arrangement, we compared apparent affinities for channels from coinjection of alphawtalphawt and betaalphawt or alphawtalphawt and betaalpham dimers. These channels were indistinguishable. To further argue against an alpha-alpha-alpha-beta arrangement, we quantitatively compared dose-response data for channels from coinjection of alphawtalpham and beta beta dimers to those from alpha and beta monomers. Taken together, our results are most consistent with an alpha-alpha-beta-beta arrangement for the heteromeric olfactory CNG channel.

Molecular and pharmacological analysis of cyclic nucleotide-gated channel function in the central nervous system

Most functional studies of cyclic nucleotide-gated (CNG) channels have been confined to photoreceptors and olfactory epithelium, in which CNG channels are abundant and easy to study. The widespread distribution of CNG channels in tissues throughout the body has only recently been recognized and the functions of this channel family in many of these tissues remain largely unknown. The molecular biological and pharmacological properties of the CNG channel family are summarized in order to put in context studies aimed at probing CNG channel functions in these tissues using pharmacological and genetic methods. Compounds have now been identified that are useful in distinguishing CNG channel activated pathways from cAMP/cGMP dependent-protein kinases or other pathways. The ways in which these interact with CNG channels are understood and this knowledge is leading to the identification of more potent and more specific CNG channel subtype-specific agonists or antagonists. Recent molecular and genetic analyses have identified novel roles of CNG channels in neuronal development and plasticity in both invertebrates and vertebrates. Targeting CNG channels via specific drugs and genetic manipulation (such as knockout mice) will permit better understanding of the role of CNG channels in both basic and higher orders of brain function.

Constraining ligand-binding site stoichiometry suggests that a cyclic nucleotide-gated channel is composed of two functional dimers

Cyclic nucleotide-gated ion channels are composed of four pore-forming subunits. Binding of cyclic nucleotide to a site in the intracellular carboxyl terminus of each subunit leads to channel activation. Since there are four subunits, four binding events are possible. In this study, we investigate the effects of individual binding events on activation by studying channels containing one, two, three, or four functional binding sites. The binding of a single ligand significantly increases opening, although four ligands are required for full activation. The data are inconsistent with models in which the four subunits activate in a single concerted step (Monod-Wyman-Changeux model) or in four independent steps (Hodgkin-Huxley model). Instead, the four subunits may associate and activate as two independent dimers.

Three amino acids in the C-linker are major determinants of gating in cyclic nucleotide-gated channels

The activation of cyclic nucleotide-gated (CNG) channels is a complex process comprising the initial ligand binding and a consecutive allosteric transition from a closed to an open configuration. The cone and olfactory CNG channels differ considerably in cyclic nucleotide affinity and efficacy. In each channel, the cyclic nucleotide-binding site is connected to the last transmembrane segment of the channel by a linker peptide (C-linker) of approximately 90 amino acids. Here we report that replacement of three amino acids in the cone C-linker by the corresponding amino acids of the olfactory channel (I439V, D481A and D494S) profoundly enhanced the cAMP efficacy and increased the affinities for cAMP and cGMP. Unlike the wild-type cone channel, the mutated channel exhibited similar single-channel kinetics for both cGMP and cAMP, explaining the increase in cAMP efficacy. We thus conclude that the identified amino acids are major determinants of channel gating.

Interdomain interactions underlying activation of cyclic nucleotide-gated channels

Cyclic nucleotide-gated (CNG) ion channels are multimeric proteins that activate in response to the binding of cyclic nucleotide to intracellular domains. Here, an intramolecular protein-protein interaction between the amino-terminal domain and the carboxyl-terminal ligand-binding domain of the rat olfactory CNG channel was shown to exert an autoexcitatory effect on channel activation. Calcium-calmodulin, which modulates CNG channel activity during odorant adaptation, blocked this interaction. A specific deletion within the amino-terminal domain disrupted the interdomain interaction in vitro and altered the gating properties and calmodulin sensitivity of expressed channels. Thus, the amino-terminal domain may promote channel opening by directly interacting with the carboxyl-terminal gating machinery; calmodulin regulates channel activity by targeting this interaction.

Single cyclic nucleotide-gated channels locked in different ligand-bound states

Cyclic nucleotide-gated (CNG) channels are directly activated by the binding of several ligands. For these channels as well as for other allosteric proteins, the functional effects of each ligand-binding event have been difficult to assess because ligands continuously bind and unbind at each site. Furthermore, in retinal rod photoreceptors the low cytoplasmic concentration of cyclic GMP means that channels exist primarily in partially liganded states, so it is important to determine how such channels behave. Previous studies of single channels have suggested that they occasionally open to subconducting states at low cGMP, but the significance of these states and how they arise is poorly understood. Here we combine the high resolution of single-channel recording with the use of a photoaffinity analogue of cGMP that tethers cGMP moieties covalently to their binding sites to show single retinal CNG channels can be effectively locked in four distinct ligand-bound states. Our results indicate that channels open more than they would spontaneously when two ligands are bound (approximately 1% of the maximum current), significantly more with three ligands bound (approximately 33%), and open maximally with four ligands bound. In each ligand-bound state, channels opened to two or three different conductance states. These findings place strong constraints on the activation mechanism of CNG channels.

Allosteric gating of a large conductance Ca-activated K+ channel

Large-conductance Ca-activated potassium channels (BK channels) are uniquely sensitive to both membrane potential and intracellular Ca2+. Recent work has demonstrated that in the gating of these channels there are voltage-sensitive steps that are separate from Ca2+ binding steps. Based on this result and the macroscopic steady state and kinetic properties of the cloned BK channel mslo, we have recently proposed a general kinetic scheme to describe the interaction between voltage and Ca2+ in the gating of the mslo channel (Cui, J., D.H. Cox, and R.W. Aldrich. 1997. J. Gen. Physiol. In press.). This scheme supposes that the channel exists in two main conformations, closed and open. The conformational change between closed and open is voltage dependent. Ca2+ binds to both the closed and open conformations, but on average binds more tightly to the open conformation and thereby promotes channel opening. Here we describe the basic properties of models of this form and test their ability to mimic mslo macroscopic steady state and kinetic behavior. The simplest form of this scheme corresponds to a voltage-dependent version of the Monod-Wyman-Changeux (MWC) model of allosteric proteins. The success of voltage-dependent MWC models in describing many aspects of mslo gating suggests that these channels may share a common molecular mechanism with other allosteric proteins whose behaviors have been modeled using the MWC formalism. We also demonstrate how this scheme can arise as a simplification of a more complex scheme that is based on the premise that the channel is a homotetramer with a single Ca2+ binding site and a single voltage sensor in each subunit. Aspects of the mslo data not well fitted by the simplified scheme will likely be better accounted for by this more general scheme. The kinetic schemes discussed in this paper may be useful in interpreting the effects of BK channel modifications or mutations.

Direct interaction between amino- and carboxyl-terminal domains of cyclic nucleotide-gated channels

We have examined domain interactions in the rod cyclic nucleotide-gated ion channel using both physiological and biochemical interaction assays. We have found an interaction between two regions of the channel distant in primary structure, the amino-terminal region and the carboxyl-terminal region containing the cyclic nucleotide-binding (CNB) domain. The interaction in functional channels was detected by the formation of a disulfide bond between cysteine residues at position 35 in the amino-terminal region and 481 in the carboxyl-terminal region. The disulfide bond resulted in channel potentiation, which was due, in part, to an increase in availability of C481 to modification when the channels were open. This state dependence is likely to underlie previously reported potentiation of cyclic nucleotide-gated channels by sulfhydryl-reactive compounds. Polypeptides derived from the amino-terminal and carboxyl-terminal regions were shown to interact, even under conditions which precluded disulfide bond formation. These data argue for a previously unknown, direct interaction between disparate regions of channel sequence.

Single-channel properties of ionic channels gated by cyclic nucleotides

This paper presents an extensive analysis of single-channel properties of cyclic nucleotide gated (CNG) channels, obtained by injecting into Xenopus laevis oocytes the mRNA encoding for the alpha and beta subunits from bovine rods. When the alpha and beta subunits of the CNG channel are coexpressed, at least three types of channels with different properties are observed. One type of channel has well-resolved, multiple conductive levels at negative voltages, but not at positive voltages. The other two types of channel are characterized by flickering openings, but are distinguished because they have a low and a high conductance. The alpha subunit of CNG channels has a well-defined conductance of about 28 pS, but multiple conductive levels are observed in mutant channels E363D and T364M. The conductance of these open states is modulated by protons and the membrane voltage, and has an activation energy around 44 kJ/mol. The relative probability of occupying any of these open states is independent of the cGMP concentration, but depends on extracellular protons. The open probability in the presence of saturating cGMP was 0.78, 0.47, 0.5, and 0.007 in the w.t. and mutants E363D, T364M, and E363G, and its dependence on temperature indicates that the thermodynamics of the transition between the closed and open state is also affected by mutations in the pore region. These results suggest that CNG channels have different conductive levels, leading to the existence of multiple open states in homomeric channels and to the flickering behavior in heteromeric channels, and that the pore is an essential part of the gating of CNG channels.

Mechanism of tetracaine block of cyclic nucleotide-gated channels

Local anesthetics are a diverse group of ion channel blockers that can be used to probe conformational changes in the pore. We examined the effects of the local anesthetic tetracaine on rod and olfactory cyclic nucleotide-gated channels expressed from subunit 1 in Xenopus oocytes. We found that 40 microM tetracaine effectively blocked the bovine rod channel but not the rat olfactory channel at saturating concentrations of cGMP. By testing chimeric channels containing regions of sequence from both rod and olfactory channels, we found that determinants of apparent affinity for tetracaine at saturating cGMP did not map to any one region of the channel sequence. Rather, the differences in apparent affinity could be explained by differences between the chimeras in the free energy of the opening allosteric transition. If a channel construct (such as the rod channel) spent appreciable time in the closed state at saturating cGMP, then it had a high apparent affinity for tetracaine. If, on the other hand, a channel construct (such as the olfactory channel) spent little time in the closed state at saturating cGMP, then it had a low apparent affinity for tetracaine. Furthermore, tetracaine became more effective at low concentrations of cGMP and at saturating concentrations of cAMP, conditions which permit the channels to spend more time in the closed configuration. These results were well fit by a model in which tetracaine binds more tightly to the closed channel than to the open channel. Dose-response curves for tetracaine in the presence of saturating cGMP are well fit with a Michaelis-Menten binding scheme indicating that a single tetracaine molecule is sufficient to produce block. In addition, tetracaine block is voltage dependent with an effective z delta of +0.56. These data are consistent with a pore-block hypothesis. The finding that tetracaine is a state-dependent pore blocker suggests that the inner mouth of the pore of cyclic nucleotide-gated channels undergoes a conformational change during channel opening.

Identification of competitive antagonists of the rod photoreceptor cGMP-gated cation channel: beta-phenyl-1,N2-etheno-substituted cGMP analogues as probes of the cGMP-binding site

cGMP is the natural activator of the cyclic nucleotide-gated channel originally isolated from rod photoreceptors but now known to be expressed in a wide variety of neural and non-neural cells. To identify antagonists of cGMP action and to better understand the interaction between cGMP and the channel protein, experimental studies were undertaken using four synthetic cGMP analogues, PET-cGMP, 8-Br-PET-cGMP, Rp-8-Br-PET-cGMPS, and Sp-8-Br-PET-cGMPS. With excised patches from either Xenopus oocytes expressing a cloned rat rod channel alpha-subunit or from native Xenopus rod photoreceptors, Rp-8-Br-PET-cGMPS competitively suppressed the cGMP-induced current with an IC50 of 25 microM and Sp-8-Br-PET-cGMPS inhibited this current with an IC50 of 105 microM. On the expressed rat rod channel, 8-Br-PET-cGMP behaved as a very weak partial agonist at high concentrations and an antagonist (IC50 = 64 microM) at lower concentrations when coapplied with cGMP. PET-cGMP did not activate channel currents alone but showed a synergism when coapplied with subsaturating concentrations of cGMP. Because Sp-8-Br-PET-cGMPS is a potent activator of type I cGMP-dependent protein kinase, but a competitive antagonist of channel activation, it will be a useful reagent for discriminating between those effects of cGMP that are mediated by a protein kinase and those mediated by channel activation. Because the PET derivatives all contain a phenyl-substituted 5-membered ring system fused to the amino group in position 2 and the nitrogen in position 1 of the guanine ring, the results support the idea that N1 and N2 are important for channel activation. They also suggest a minor role for the cyclic phosphate group in binding or activation.