Cyclic GMP contact points within the 63-kDa subunit and a 240-kDa associated protein of retinal rod cGMP-activated channels

The glutamic acid-rich protein is a gating inhibitor of cyclic nucleotide-gated channels

The cyclic nucleotide-gated (CNG) cation channel of rod photoreceptors is a heterotetramer consisting of homologous CNGA1 and CNGB1a subunits. While CNGA1 is indispensable for channel activation, the specific role of CNGB1a in this process has remained elusive. Here, we show that the N-terminal glutamic acid-rich protein (GARP) domain of CNGB1a and soluble GARP2, which corresponds to the proximal portion of the GARP domain, act as autoinhibitory domains that decrease the opening probability of the CNG channel. In the presence of mutations that structurally impair the cyclic nucleotide-binding domain (CNBD) of CNGB1a, the GARP domain completely abolishes channel activity. In agreement with an inhibitory function of GARP, the activity of mutant CNG channels could be fully restored by deletion of the GARP domain. We identified two sequences within the GARP domain that confer most of the inhibitory effect and demonstrate that the profound inhibition imposed by the GARP domain is caused by direct and autonomous protein-protein interaction with the CNG channel complex. In wild-type rod CNG channels, this inhibitory effect can be relieved by binding of cGMP to the CNBD of CNGB1a. In conclusion, we propose that the N terminus of CNGB1a and soluble GARPs act as molecular gate keepers that control the activation of heteromeric rod CNG channels. Our results suggest that the GARP domain has evolved in rod photoreceptors to reduce current noise resulting from openings of CNG channels in the absence of cGMP.

The pharmacology of cyclic nucleotide-gated channels: emerging from the darkness

Cyclic nucleotide-gated (CNG) ion channels play a central role in vision and olfaction, generating the electrical responses to light in photoreceptors and to odorants in olfactory receptors. These channels have been detected in many other tissues where their functions are largely unclear. The use of gene knockouts and other methods have yielded some information, but there is a pressing need for potent and specific pharmacological agents directed at CNG channels. To date there has been very little systematic effort in this direction - most of what can be termed CNG channel pharmacology arose from testing reagents known to target protein kinases or other ion channels, or by accident when researchers were investigating other intracellular pathways that may regulate the activity of CNG channels. Predictably, these studies have not produced selective agents. However, taking advantage of emerging structural information and the increasing knowledge of the biophysical properties of these channels, some promising compounds and strategies have begun to emerge. In this review we discuss progress on two fronts, cyclic nucleotide analogs as both activators and competitive inhibitors, and inhibitors that target the pore or gating machinery of the channel. We also discuss the potential of these compounds for treating certain forms of retinal degeneration.

Stoichiometry and assembly of olfactory cyclic nucleotide-gated channels

Native ion channels are precisely tuned to their physiological role in neuronal signaling. This tuning frequently involves the controlled assembly of heteromeric channels comprising multiple types of subunits. Cyclic nucleotide-gated (CNG) channels of olfactory neurons are tetramers and require three types of subunits, CNGA2, CNGA4, and CNGB1b, to exhibit properties necessary for olfactory transduction. Using fluorescently tagged subunits and fluorescence resonance energy transfer (FRET), we find the subunit composition of heteromeric olfactory channels in the surface membrane is fixed, with 2:1:1 CNGA2:CNGA4:CNGB1b. Furthermore, when expressed individually with CNGA2, CNGA4 and CNGB1b subunits were still present in only a single copy and, when expressed alone, did not self-assemble. These results suggest that the precise assembly of heteromeric olfactory channels results from a mechanism where CNGA4 and CNGB1b subunits have a high affinity for CNGA2 but not for self-assembly, precluding more than one CNGA4 or CNGB1b subunit in the channel complex.

Efficient coupling of ligand binding to channel opening by the binding domain of a modulatory (beta) subunit of the olfactory cyclic nucleotide-gated channel

CNG channels in vivo are heteromers of homologous alpha and beta subunits that each contain a six-transmembrane segment domain and a COOH-terminal cytoplasmic cyclic nucleotide binding domain (BD). In heterologous expression systems, heteromeric alphabeta channels activate with greater sensitivity to ligand than do homomeric alpha channels; however, ligand-gating of channels containing only beta subunit BDs has never been studied because beta subunits cannot form functional homomeric CNG channels. To characterize directly the contribution of the beta subunit BD to ligand-gating, we constructed a chimeric subunit, X-beta, whose BD sequence was that of the beta subunit CNG5 from rat, but whose sequence outside the BD was derived from alpha subunits. For comparison, we constructed another chimera, X-alpha, whose sequence outside the BD was identical to that of X-beta, but whose BD sequence was that of the alpha subunit CNG2 from catfish. When expressed in Xenopus oocytes, X-beta and X-alpha each formed functional homomeric channels activated by both cAMP and cGMP. This is the first demonstration that the beta subunit BD can couple ligand binding to activation in the absence of alpha subunit BD residues. Notably, both agonists activate X-beta more effectively than X-alpha (higher opening efficacy and lower K(1/2)). The BD is believed to comprise two functionally distinct subdomains: (1) the roll subdomain (beta-roll and flanking A- and B-helices) and (2) the C-helix subdomain. Opening efficacy was previously believed to be controlled primarily by the C-helix, but when we made additional chimeras by exchanging the subdomains between X-beta and X-alpha, we found that both subdomains contain significant determinants of efficacy and agonist selectivity. In particular, only channels containing the roll subdomain of the beta subunit had high efficacy. Thermodynamic linkage analysis shows that interaction between the two subdomains accounts for a significant portion of their contribution to activation energetics.

Constraining the subunit order of rod cyclic nucleotide-gated channels reveals a diagonal arrangement of like subunits

Retinal rod cyclic nucleotide-gated channels are composed of alpha and beta subunits. We have explored possible subunit arrangements by expressing tandemly linked dimers of both subunits and examining their responses to three different modulating agents. Channels formed from either alpha-beta or beta-alpha heterodimers had functional properties similar to those formed from coexpressed alpha and beta monomers and to native channels. These results point to an alpha-beta-alpha-beta arrangement. To ensure that heterodimers had not flipped around, we coexpressed alpha-alpha dimers with an excess of either beta monomers or beta-beta dimers. Our data indicate that heteromultimers do not form efficiently in an alpha-alpha-beta-beta arrangement. Thus, we propose that native rod cyclic nucleotide-gated channels are arranged with like subunits diagonally opposed: alpha-beta-alpha-beta.

Opening mechanism of a cyclic nucleotide-gated channel based on analysis of single channels locked in each liganded state

Cyclic nucleotide-gated channels contain four subunits, each with a binding site for cGMP or cAMP in the cytoplasmic COOH-terminal domain. Previous studies of the kinetic mechanism of activation have been hampered by the complication that ligands are continuously binding and unbinding at each of these sites. Thus, even at the single channel level, it has been difficult to distinguish changes in behavior that arise from a channel with a fixed number of ligands bound from those that occur upon the binding and unbinding of ligands. For example, it is often assumed that complex behaviors like multiple conductance levels and bursting occur only as a consequence of changes in the number of bound ligands. We have overcome these ambiguities by covalently tethering one ligand at a time to single rod cyclic nucleotide-gated channels (Ruiz, ML., and J.W. Karpen. 1997. Nature. 389:389-392). We find that with a fixed number of ligands locked in place the channel freely moves between three conductance states and undergoes bursting behavior. Furthermore, a thorough kinetic analysis of channels locked in doubly, triply, and fully liganded states reveals more than one kinetically distinguishable state at each conductance level. Thus, even when the channel contains a fixed number of bound ligands, it can assume at least nine distinct states. Such complex behavior is inconsistent with simple concerted or sequential allosteric models. The data at each level of liganding can be successfully described by the same connected state model (with different rate constants), suggesting that the channel undergoes the same set of conformational changes regardless of the number of bound ligands. A general allosteric model, which postulates one conformational change per subunit in both the absence and presence of ligand, comes close to providing enough kinetically distinct states. We propose an extension of this model, in which more than one conformational change per subunit can occur during the process of channel activation.

Pseudechetoxin: a peptide blocker of cyclic nucleotide-gated ion channels

Ion channels activated by the binding of cyclic nucleotides first were discovered in retinal rods where they generate the cell's response to light. In other systems, however, it has been difficult to unambiguously determine whether cyclic nucleotide-dependent processes are mediated by protein kinases, their classical effector enzymes, or cyclic nucleotide-gated (CNG) ion channels. Part of this difficulty has been caused by the lack of specific pharmacological tools. Here we report the purification from the venom of the Australian King Brown snake of a peptide toxin that inhibits current through CNG channels. This toxin, which we have named Pseudechetoxin (PsTx), was purified by cation exchange and RP-HPLC and has a molecular mass of about 24 kDa. When applied to the extracellular face of membrane patches containing the alpha-subunit of the rat olfactory CNG channel, PsTx blocked the cGMP-dependent current with a Ki of 5 nM. Block was independent of voltage and required only a single molecule of toxin. PsTx also blocked CNG channels containing the bovine rod alpha-subunit with high affinity (100 nM), but it was less effective on the heteromeric version of the rod channel (Ki approximately 3 microM). We have obtained N-terminal and partial internal sequence data and the amino acid composition of PsTx. These data indicate that PsTx is a basic protein that exhibits some homology with helothermine, a toxin isolated from the venom of the Mexican beaded lizard. PsTx promises to be a valuable pharmacological tool for studies on the structure and physiology of CNG channels.

Photoaffinity labeling as an approach to study supramolecular nucleoprotein complexes

The modern approaches for studying the detailed structure of nucleoprotein complexes involved in replication and transcription, based on the use of nucleic acids with photoreactive groups incorporated into definite positions of polynucleotide chain, are considered. Methods of preparation of photoreactive nucleic acids of this type are presented. Their use for positioning of RNA polymerase III and transcription factors as well as of the main participants of the replication machinery at the respective templates is described. A survey of the data concerning the amino acid residues modified in the course of photoaffinity labeling of proteins is also presented and some complications are discussed.

Movement of gating machinery during the activation of rod cyclic nucleotide-gated channels

In the visual and olfactory systems, cyclic nucleotide-gated (CNG) ion channels convert stimulus-induced changes in the internal concentrations of cGMP and cAMP into changes in membrane potential. Although it is known that significant activation of these channels requires the binding of three or more molecules of ligand, the detailed molecular mechanism remains obscure. We have probed the structural changes that occur during channel activation by using sulfhydryl-reactive methanethiosulfonate (MTS) reagents and N-ethylmaleimide (NEM). When expressed in Xenopus oocytes, the alpha-subunit of the bovine retinal channel forms homomultimeric channels that are activated by cGMP with a K1/2 of approximately 100 microM. Cyclic AMP, on the other hand, is a very poor activator; a saturating concentration elicits only 1% of the maximum current produced by cGMP. Treatment of excised patches with MTS-ethyltrimethylamine (MTSET) or NEM dramatically potentiated the channel's response to both cyclic nucleotides. After MTSET treatment, the dose-response relation for cGMP was shifted by over two orders of magnitude to lower concentrations. The effect on channel activation by cAMP was even more striking. After modification, the channels were fully activated by cAMP with a K1/2 of approximately 60 microM. This potentiation was abolished by conversion of Cys481 to a nonreactive alanine residue. Potentiation occurred more rapidly in the presence of saturating cGMP, indicating that this region of the channel is more accessible when the channel is open. Cys481 is located in a linker region between the transmembrane and cGMP-binding domains of the channel. These results suggest that this region of the channel undergoes significant movement during the activation process and is critical for coupling ligand binding to pore opening. Potentiation, however, is not mediated by the recently reported interaction between the amino- and carboxy-terminal regions of the alpha-subunit. Deletion of the entire amino-terminal domain had little effect on potentiation by MTSET.

Total colourblindness is caused by mutations in the gene encoding the alpha-subunit of the cone photoreceptor cGMP-gated cation channel

Total colourblindness (OMIM 216900), also referred to as rod monochromacy (RM) or complete achromatopsia, is a rare, autosomal recessive inherited and congenital disorder characterized by photophobia, reduced visual acuity, nystagmus and the complete inability to discriminate between colours. Electroretinographic recordings show that in RM, rod photoreceptor function is normal, whereas cone photoreceptor responses are absent. The locus for RM has been mapped to chromosome 2q11 (ref. 2), however the gene underlying RM has not yet been identified. Recently, a suitable candidate gene, CNGA3, encoding the alpha-subunit of the cone photoreceptor cGMP-gated cation channel, a key component of the phototransduction pathway, has been cloned and assigned to human chromosome 2q11 (refs 3,4). We report the identification of missense mutations in CNGA3 in five families with RM. Homozygous mutations are present in two families, whereas the remaining families show compound heterozygous mutations. In all cases, the segregation pattern of the mutations is consistent with the autosomal recessive inheritance of the disease and all mutations affect amino acids that are highly conserved among cyclic nucleotide gated channels (CNG) in various species. This is the first report of a colour vision disorder caused by defects other than mutations in the cone pigment genes, and implies at least in this instance a common genetic basis for phototransduction in the three different cone photoreceptors of the human retina.

Cloning, chromosomal localization and functional expression of the gene encoding the alpha-subunit of the cGMP-gated channel in human cone photoreceptors

Cyclic nucleotide-gated (CNG) ion channels serve as final targets of signal transduction in vertebrate photoreceptors. While the basic mechanisms of phototransduction are similar in rod and cone photoreceptors, both cell types express distinct sets of components of the transduction pathway. We report here the cloning of the cDNA encoding the alpha-subunit of the cGMP-gated channel of human cone photoreceptors. The open reading frame predicts a polypeptide of 694 amino acid residues with conserved functional parts and amino acid positions typical for the alpha-subunit of CNG-channels. Heterologous expression of the cDNA in Xenopus oocytes gave rise to cGMP-gated channel activity. Antiserum directed against the C-terminus of the bovine cone CNG channel alpha-subunit crossreacted specifically with the heterologously expressed polypeptide and stained cone photoreceptors and weakly also the outer plexiform layer in human retinal sections. Northern blot analysis detected a prominent mRNA species of approximately 3.8 kb in human retina. The entire gene spans approximately 30 kb of genomic sequence and is located on the pericentric band q11.2 of human chromosome 2. The gene is composed of seven exons, with introns located at positions which are preserved with respect to the human rod gene, indicating a common ancestral gene structure. RT-PCR analysis gave no evidence for alternatively spliced transcripts.

Calmodulin regulation of cyclic-nucleotide-gated channels

Cyclic-nucleotide-gated (CNG) channels play key roles in photoreceptor and olfactory signal-transduction pathways. Recent studies have focused on the molecular characterization of CNG channel subunits and on the identification of the structural domains that contribute to ligand selectivity and affinity, ion gating and permeation, and regulation of channel activity. Calmodulin has been shown to bind directly to the rod and olfactory channels and to modulate their sensitivity to cyclic nucleotides. This Ca2+-dependent regulation of channel activity appears to play a role in the termination of the signal-transduction pathway in olfactory neurons and rod photoreceptor cells. It remains to be determined whether calmodulin also regulates the activity of related channels in other cells.

Covalent activation of retinal rod cGMP-gated channels reveals a functional heterogeneity in the ligand binding sites

Ion channels gated by the binding of multiple ligands play a critical role in synaptic transmission and sensory transduction. It has been difficult to resolve the contribution of individual binding events to channel gating because ligands are continuously binding and unbinding at each site. In examining the allosteric mechanism of retinal rod cGMP-gated channels, we have circumvented this problem by making use of a cGMP derivative, 8-p-azidophenacylthio-cGMP (APT-cGMP), that can be covalently tethered to the binding sites in the presence of long-wavelength UV light. In excised membrane patches, a population of channels was isolated that contained covalently-attached ligands at all but one site. Activation of these channels by cGMP revealed a previously unknown heterogeneity in the ligand-binding sites. The dose-response relations were much shallower than predicted by single-site activation models, but were well described by models in which there are two populations of sites, in roughly equal proportion, that bind cGMP with apparent affinities that differ by a factor of 25. The two apparent affinities, incorporated into a four-site model of the channel, provided an accurate description of the patch's original dose-response relation. A comparison of results on native and expressed channels suggests that the heterogeneity in the native channel arises at least in part from the presence of two different cGMP-binding subunits.

Subunit interactions in coordination of Ni2+ in cyclic nucleotide-gated channels

Cyclic nucleotide-gated (CNG) channels present a unique model for studying the molecular mechanisms of channel gating. We have studied the mechanism of potentiation of expressed rod CNG channels by Ni2+ as a first step toward understanding the channel gating process. Here we report that coordination of Ni2+ between histidine residues (H420) on adjacent channel subunits occurs when the channels are open. Mutation of H420 to lysine completely eliminated the potentiation by Ni2+ but did not markedly alter the apparent cGMP affinity of the channel, indicating that the introduction of positive charge at the Ni(2+)-binding site was not sufficient to produce potentiation. Deletion or mutation of most of the other histidines present in the channel did not diminish potentiation by Ni2+. We studied the role of subunit interactions in Ni2+ potentiation by generating heteromultimeric channels using tandem dimers of the rod CNG channel sequence. Injection of single heterodimers in which one subunit contained H420 and the other did not (wt/H420Q or H420Q/wt) resulted in channels that were not potentiated by Ni2+. However, coinjection of both heterodimers into Xenopus oocytes resulted in channels that exhibited potentiation. The H420 residues probably occurred predominantly in nonadjacent subunits when each heterodimer was injected individually, but, when the two heterodimers were coinjected, the H420 residues could occur in adjacent subunits as well. These results suggest that the mechanism of Ni2+ potentiation involves intersubunit coordination of Ni2+ by H420. Based on the preferential binding of Ni2+ to open channels, we suggest that alignment of H420 residues of neighboring subunits into the Ni(2+)-coordinating position may be associated with channel opening.