101
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More answers about cGMP-gated channels pose more questions. Behav Brain Sci 1995. [DOI: 10.1017/s0140525x00039509] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
Abstract
AbstractOur understanding of the molecular properties and cellular role of cGMP-gated channels in outer segments of vertebrate photo-receptors has come from over a decade of studies which have continuously altered and refined ideas about these channels. Further examination of this current view may lead to future surprises and further refine the understanding of cGMP-gated channels.
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102
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Cyclic nucleotides as regulators of light-adaptation in photoreceptors. Behav Brain Sci 1995. [DOI: 10.1017/s0140525x00039510] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
Abstract
AbstractCyclic nucleotides can regulate the sensitivity of retinal rods to light through phosducin. The phosphorylation state of phosducin determines the amount of G available for activation by Rho*. Phosducin phosphorylation is regulated by cyclic nucleotides through their activation of cAMP-dependent protein kinase. The regulation of phosphodiesterase activity by the noncatalytic cGMP binding sites as well as Ca2+/calmodulin dependent regulation of cGMP binding to the cation channel are also discussed.
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103
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Long term potentiation and CaM-sensitive adenylyl cyclase: Long-term prospects. Behav Brain Sci 1995. [DOI: 10.1017/s0140525x00039327] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
Abstract
AbstractThe type I CaM-sensitive adenylyl cyclase is in a position to integrate signals from multiple inputs, consistent with the requirements for mediating long term potentiation (LTP). Biochemical and genetic evidence supports the idea that this enzyme plays an important role inc LTP. However, more work is needed before we will be certain of the role that CaM-sensitive adenylyl cyclases play in LTP.
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104
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Modulation of the cGMP-gated channel by calcium. Behav Brain Sci 1995. [DOI: 10.1017/s0140525x00039431] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
Abstract
AbstractCalcium acting through calmodulin has been shown to regulate the affinity of cyclic nucleotide-gated channels expressed in cell lines. But is calmodulin the Ca-sensor that normally regulates these channels?
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105
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How many light adaptation mechanisms are there? Behav Brain Sci 1995. [DOI: 10.1017/s0140525x00039546] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
Abstract
AbstractThe generally positive response to our target article indicates that most of the commentators accept our contention that light adaptation consists of multiple and possibly redundant mechanisms. The commentaries fall into three general categories. The first deals with putative mechanisms that we chose not to emphasize. The second is a more extended discussion of the role of calcium in adaptation. Finally, additional aspects of cGMP involvement in adaptation are considered. We discuss each of these points in turn.
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106
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Gene therapy, regulatory mechanisms, and protein function in vision. Behav Brain Sci 1995. [DOI: 10.1017/s0140525x00039388] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
Abstract
AbstractHereditary retinal degeneration due to mutations in visual genes may be amenable to therapeutic interventions that modulate, either positively or negatively, the amount of protein product. Some of the proteins involved in phototransduction are rapidly moved by a lightdependent mechanism between the inner segment and the outer segment in rod photoreceptor cells, and this phenomenon is important in phototransduction.
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107
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A novel protein family of neuronal modulators. Behav Brain Sci 1995. [DOI: 10.1017/s0140525x00039479] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
Abstract
AbstractA number of proteins homologous to recoverin have been identified in the brains of the several vertebrate species. The brainderived members originally contain four EF-hand domains, but NH2- terminal domain is aberrant. Many of these proteins inhibited light-induced rhodopsin phosphorylation at high [Ca2+], suggesting that the brain-derived members may act as a Ca2+-sensitive modulator of receptor phosphorylation, as recoverin does.
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108
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The structure of rhodopsin and mechanisms of visual adaptation. Behav Brain Sci 1995. [DOI: 10.1017/s0140525x00039273] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/05/2022]
Abstract
AbstractRapidly advancing studies on rhodopsin have focused on new strategies for crystallization of this integral membrane protein for x-ray analysis and on alternative methods for structural determination from nuclear magnetic resonance data. Functional studies of the interactions between the apoprotein and its chromophore have clarified the role of the chromophore in deactivation of opsin and in photoactivation of the pigment.
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109
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Crucial steps in photoreceptor adaptation: Regulation of phosphodiesterase and guanylate cyclase activities and Ca 2+-buffering. Behav Brain Sci 1995. [DOI: 10.1017/s0140525x00039364] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
Abstract
AbstractThis commentary discusses the balance of phosphodiesterase and guanylate cyclase activities in vertebrate photoreceptors at moderate light intensities. The rate of cGMP hydrolysis and synthesis seem to equal each other. Ca2+ as regulator of both enzyme activities is also effectively buffered in photoreceptor cells by cytoplasmic buffer components.
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110
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The atomic structure of visual rhodopsin: How and when? Behav Brain Sci 1995. [DOI: 10.1017/s0140525x00039297] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
Abstract
AbstractStrong arguments are presented by Hargrave suggesting that the crystallization of visual rhodopsin for high resolution analysis by X-ray crystallography or electron microscopy is feasible. However, the effort needed to achieve this goal will most likely exceed the resources of a single laboratory and a concerted approach to the research is necessary.
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111
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Molecular insights gained from covalently tethering cGMP to the ligand-binding sites of retinal rod cGMP-gated channels. Behav Brain Sci 1995. [DOI: 10.1017/s0140525x00039261] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
Abstract
AbstractA photoaffinity analog of cGMP has been used to biochemically identify a new ligand-binding subunit of the retinal rod cGMP-activated ion channel, as well as amino acids in contact with cGMP in the original subunit. Covalent tethering of this probe to channels in excised menbrane patches has revealed a functional heteogeneity in the ligand-binding sites that may arise from the two biochemically identified subunits.
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112
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Abstract
AbstractRecent findings emphasize the complexity, both genetic and functional, of the manifold genes and mutations causing inherited retinal degeneration in humans. Knowledge of the genetic bases of these diseases can contribute to design of rational therapy, as well as elucidating the function of each gene product in normal visual processes.
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113
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Channel structure and divalent cation regulation of phototransduction. Behav Brain Sci 1995. [DOI: 10.1017/s0140525x00039339] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
Abstract
AbstractThe identification of additional subunits of the cGMP-gated cation channel suggests exciting questions about their regulatory roles and about structure/functional relationships. How do the different subunits interact? How is the complex assembled into the plasma membrane? Divalent cations have been implicated in the regulation of adaptation. One often overlooked cation is magnesium. Could this ion play a role in phototransduction?
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114
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Structure of the cGMP-gated channel. Behav Brain Sci 1995. [DOI: 10.1017/s0140525x0003939x] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
Abstract
AbstractThe subunit structure of the cGMP-gated cation channel of rod photoreceptors is rapidly being defined, and in the process the mode of regulation by Ca2+-calmodulin unraveled. Intriguingly, early results suggest that additional subunits of unknown function are associated with the channel and remain to be identified.
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115
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Linking genotypes with phenotypes in human retinal degenerations: Implications for future research and treatment. Behav Brain Sci 1995. [DOI: 10.1017/s0140525x00039340] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
Abstract
AbstractAlthough undoubtedly it will be incomplete by the time it is published, the target article by Daiger et al. organizes mutations in genes that produce retinal degenerations in humans into categories of clinically relevant phenotypes. Such classifications should help us understand the link between altered photoreceptor cell proteins and subsequent cell death, and they may yield insight into methods for preventing consequent blindness.
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116
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Genetic and clinical heterogeneity in tapetal retinal dystrophies. Behav Brain Sci 1995. [DOI: 10.1017/s0140525x0003925x] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/05/2022]
Abstract
AbstractLarge scale DNA-mutation screening in patients with hereditary retinal diseases greatly enhances our knowledge about retinal function and diseases. Scientists, clinicians, patients, and families involved with retinal disorders may directly benefit from these developments. However, certain aspects of this expanding knowledge, such as the correlation between genotype and phenotype, may be much more complicated than we expect at present.
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117
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The determination of rhodopsin structure may require alternative approaches. Behav Brain Sci 1995. [DOI: 10.1017/s0140525x00039236] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/05/2022]
Abstract
AbstractThe structure of rhodopsin is a subject of intense interest. Solving the structure by traditional methods has proved exceedingly challenging. It may therefore be useful to confront the problem by a combination of alternate techniques. These include FTIR (Fourier transform infrared spectroscopy) and AFM (atomic force microscopy) on the intact protein. Furthermore, additional insights may be gained through structural investigations of discrete rhodopsin domains.
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118
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Na-Ca + K exchanger and Ca 2+ homeostasis in retinal rod outer segments: Inactivation of the Ca 2+ efflux mode and possible involvement of intracellular Ca 2+ stores in Ca 2+ homeostasis. Behav Brain Sci 1995. [DOI: 10.1017/s0140525x00039455] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/05/2022]
Abstract
AbstractInactivation of the Ca2+ extrusion mode of the retinal rod Na- Ca + K exchanger is suggested to be the mechanism that prevents lowering of cytosolic free Ca2+ to < 1 nM when rod cells are saturated for a prolonged time under bright light conditions. Under these conditions, Ca2+ fluxes across disk membranes can contribute significantly to Ca2+ homeostasis in rods.
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119
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Nuclear magnetic resonance studies on the structure and function of rhodopsin. Behav Brain Sci 1995. [DOI: 10.1017/s0140525x00039467] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
Abstract
AbstractMagic angle spinning (MAS) NMR methods provide a means of obtaining high resolution structural data on rhodopsin and its photoin termediates. Current work has focused on the structure of the retinal chromophore and its interactions with surrounding protein charges. The recent development of MAS NMR methods for measuring internuclear distances with a resolution of ∼0.2 will complement diffraction methods for addressing key mechanistic questions.
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120
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Glutamate accumulation in the photoreceptor-presumed final common path of photoreceptor cell death. Behav Brain Sci 1995. [DOI: 10.1017/s0140525x00039480] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
Abstract
AbstractGenetic abnormalities of three factors related to the photoreceptor mechanism have been reported in both animal models and humans. Apoptotic mechanism has also been suggested as a final common pathway of photoreceptor cell death. Our findings of increased level of glutamate in photoreceptor cells in rds mice suggest that amino acid might mediate between these two pathological mechanisms.
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121
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Unique lipids and unique properties of retinal proteins. Behav Brain Sci 1995. [DOI: 10.1017/s0140525x00039443] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
Abstract
AbstractAmino-terminal heteroacylation has been identified in retinal proteins including recoverin and α subunit of G-protein, transducin. The tissue-specific modification seems to mediate not only a proteinmembrane interaction but also a specific protein-protein interaction. The mechanism generating the heterogeneity and its physiological role are still unclear, but an interesting idea for the latter postulates a fine regulation of the signal transduction pathway by distinct N-acyl groups.
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122
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Further insight into the structural and regulatory properties of the cGMP-gated channel. Behav Brain Sci 1995. [DOI: 10.1017/s0140525x00039571] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
Abstract
AbstractRecent studies from several different laboratories have provided further insight into structure-function relationships of cyclic nucleotide-gated channel and in particular the cCMPgated channel of rod photoreceptors. Site-directed mutagenesis and rod-olfactory chimeria constructs have defined important amino acids and peptide segments of the channel that are important in ion blockage, ligand specificity, and gating properties. Molecular cloning studies have indicated that cyclic nucleotide-gated channels consist of two subunits that are required to reproduce the properties of the native channels. Biochemical analysis of the cGMP-gated channel of rodcells have indicated that the 240 kDa protein that co-purifies with the 63 kDa channel subunit contains both the previously cloned second subunit of the channel and a glutamic acid-rich protein. The regulatory properties of the cGMP-gated channel from rod cells has also been studied in more detail. Studies indicate that the beta subunit of the cGMP-gated channel of rod cells contains the binding site for calmodulin. Interaction of calmodulin with the channel alters the apparent affinity of the channel for cGMP in all in vitro systems that have been studied. The significance of these recent studies are discussed in relation to the commentaries on the target article.
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123
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Unsolved issues in S-modulin/recoverin study. Behav Brain Sci 1995. [DOI: 10.1017/s0140525x00039352] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/05/2022]
Abstract
AbstractS-Modulin is a frog homolog of recoverin. The function and the underlying mechanism of the action of these proteins are now understood in general. However, there remain some unsolved issues including; two distinct effects of S-modulin; Ca2+-dependent binding of S-modulin to membranes and a possible target protein; S-modulin-like proteins in other neurons. These issues are considered in this commentary.
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124
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Mechanisms of photoreceptor degenerations. Behav Brain Sci 1995. [DOI: 10.1017/s0140525x00039248] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
Abstract
AbstractThe candidate gene approach has identified many causes of photoreceptor rod cell death in retinitis pigmentosa. Some mutations lead to increased cyclicGMP concentrations in rods. Rod photoreceptors are also particularly susceptible to some mutations in housekeeping genes. Although many more cases of macular degeneration than retinitis pigmentosa occur each year, there is much less known about both genetic and sporadic forms of this disease.
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125
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Reduced cytoplasmic calcium concentration may be both necessary and sufficient for photoreceptor light adaptation. Behav Brain Sci 1995. [DOI: 10.1017/s0140525x00039376] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
Abstract
AbstractLight adaptation is modulated almost exclusively by changes in intracellular Ca2+ concentration, and other Ca2+-independent mechanisms are likely to play only a minor role. Changes in Ca2+i may be not only necessary for light adaptation to take place but sufficient to cause it.
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126
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The genetic kaleidoscope of vision. Behav Brain Sci 1995. [DOI: 10.1017/s0140525x00039492] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
Abstract
AbstractSite-specific phenotypic effects of the 73 known alleles in the rhodopsin gene that cause retinal degeneration are difficult to interpret because most alleles are documented in only one case or one family, which means variation in effects could actually arise from interactions with other loci. However, sample sizes necessary to detect epistatic interaction may place an answer to this question beyond our grasp.
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127
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Evidence that the type I adenylyl cyclase may be important for neuroplasticity: Mutant mice deficient in the gene for type I adenylyl cyclase show altered behavior and LTP. Behav Brain Sci 1995. [DOI: 10.1017/s0140525x0003956x] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
Abstract
AbstractThe regulatory properties of the neurospecific, type I adenylyl cyclase and its distribution within brain have suggested that this enzyme may be important for neuroplasticity. To address this issue, the murine, Ca2+ -stimulated adenylyl cyclase (type I), was inactivated by targeted mutagenesis. Ca2+ -stimulated adenylyl cyclase activity was reduced 40% to 60% in the hippocampus, neocortex, and cerebellum. Long term potentiation in the CA1 region of the hippocampus from mutants was perturbed relative to controls. Both the initial slope and maxim um extent of changes in synaptic response were reduced. Although mutant mice learned to find a hidden platform normally in the Morris water task, they did not display a preference for the region where the platform had been when it was removed. The behavioral phenotype of these mice is very similar to that exhibited by mice which have been surgically lesioned in the hippocampus. These results indicate that disruption of the gene for the type I adenylyl cyclase produces changes in spatial memory and indicate that the cAMP signal transduction pathway may play an important role for synaptic plasticity.
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128
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Calcium/calmodulin-sensitive adenylyl cyclase as an example of a molecular associative integrator. Behav Brain Sci 1995. [DOI: 10.1017/s0140525x00039224] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
Abstract
AbstractEvidence suggests that the Ca2+/calmodulin-sensitive adenylyl cyclase may play a key role in neural plasticity and learning in Aplysia, Drosophila, and mammals. This dually-regulated enzyme has been proposed as a possible site of stimulus convergence during associative learning. This commentary discusses the evidence that is required to demonstrate that a protein in a second messenger cascade actually functions as a molecular site of associative integration. It also addresses the issue of how a dually-regulated protein could contribute to the temporal pairing requirements of classical conditioning: that relationship between stimuli display both temporal contiguity and predictability.
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129
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The key to rhodopsin function lies in the structure of its interface with transducin. Behav Brain Sci 1995. [DOI: 10.1017/s0140525x00039285] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
Abstract
AbstractLight activated rhodopsin functions by catalyzing the exchange of GTP for GDP on numerous copies of transducin. Peptide mapping has shown that at least six regions, three on rhodopsin and three on the transducin alpha subunit, are involved in the active interface between the two proteins. The most informative structural studies of rhodopsin should include focus on the transducin interaction.
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130
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Abstract
A variety of different cyclic nucleotide gated ion channels have recently been identified using molecular cloning and electrophysiological techniques. Current research is focussed on the specific molecular determinants that endow these channels with their distinctive character of gating, selectivity and modulation. In some cases, it has been possible to identify the specific physiological roles of different cyclic nucleotide gated channels. Their interactions with Ca2+ and calmodulin are particularly important, and determine the specific functions these channels subserve in distinct cells.
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Affiliation(s)
- U B Kaupp
- Institut für Biologische Informationsverarbeitung, Forschungszentrum Jülich, Germany
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131
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Gordon SE, Downing-Park J, Zimmerman AL. Modulation of the cGMP-gated ion channel in frog rods by calmodulin and an endogenous inhibitory factor. J Physiol 1995; 486 ( Pt 3):533-46. [PMID: 7473217 PMCID: PMC1156544 DOI: 10.1113/jphysiol.1995.sp020832] [Citation(s) in RCA: 91] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/25/2023] Open
Abstract
1. Outer segment patches excised in the light were used to investigate the effects of exogenous calmodulin and an endogenous inhibitory factor on the cGMP-gated channel of frog rods. 2. Calmodulin shifted to the right the dose-response relation for activation of the channels by 8-Br-cGMP, but did not change the maximum current or the form of the relation. Reversal of this effect by removal of calmodulin was accelerated by brief exposure to saturating [8-Br-cGMP]. Inhibition by calmodulin required calcium and gave as much as a 5-fold decrease in current for an [8-Br-cGMP] functionally comparable to the presumed physiological [cGMP]. 3. Exposure to low [Ca2+]i (tens of nanomolar) appeared to irreversibly remove or inactivate an endogenous channel inhibitory factor from the patches, increasing the current at low [8-Br-cGMP]. Like calmodulin, this factor slowed the voltage-dependent channel-gating kinetics and did not change the maximum current. However, unlike calmodulin, the endogenous factor remained stably associated with the patches at high [Ca2+]i (1 microM), even with exposure to saturating [8-Br-cGMP]. 4. After the low-Ca2+ treatment increased the current, calmodulin reduced the current to about the same level as it had before the low-Ca2+ treatment, giving a larger fractional suppression. Furthermore, patches with high initial sensitivity to 8-Br-cGMP had small low-Ca2+ effects and large calmodulin effects, while the reverse was true for patches with low initial agonist sensitivity. 5. Application of trypsin to the intracellular surface of the patch prevented the responses to calmodulin and to low [Ca2+]i, suggesting involvement of a cytoplasmic portion of the channel. However, trypsin also reduced the total agonist-induced patch current. 6. Our results are consistent with a model in which calmodulin and an endogenous calcium-binding protein compete for the same site, inhibiting channel opening or cGMP binding. The tight association of the endogenous factor with the channel even at relatively low [Ca2+]i suggests that in the transducing rod it may inhibit the channels most of the time in darkness and in dim light, preventing any potential inhibitory effects of calmodulin. The endogenous factor would be expected to leave the channel only in bright or prolonged light, when the [Ca2+]i is thought to be very low.
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Affiliation(s)
- S E Gordon
- Section of Physiology, Brown University, Providence, RI 02912, USA
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132
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Brown RL, Gramling R, Bert RJ, Karpen JW. Cyclic GMP contact points within the 63-kDa subunit and a 240-kDa associated protein of retinal rod cGMP-activated channels. Biochemistry 1995; 34:8365-70. [PMID: 7541241 DOI: 10.1021/bi00026a018] [Citation(s) in RCA: 21] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/25/2023]
Abstract
Ion channels from retinal rods and a variety of other cells are directly gated by cyclic nucleotides. The rod channel is known to contain a 63-kDa subunit, and there is molecular genetic evidence for the existence, in human retina, of a second subunit with a deduced molecular mass of about 100 kDa. When purified from bovine rods, the channel consists of the 63-kDa subunit and a 240-kDa associated protein that has been shown recently to contain a version of the cloned second subunit as part of a larger complex. We had previously shown that a photoaffinity analog of cGMP, 8-(p-azidophenacylthio)-[32P]cGMP, specifically labels both the 63- and 240-kDa proteins. Here the analog was used to identify cGMP-binding regions and amino acid contact points within these proteins. The specific labeling of the 63-kDa subunit was localized to a 66 amino acid fragment (Tyr-515-Met-580) that is contained entirely within a 110 amino acid region proposed to be the cGMP-binding site on the basis of homology with other cyclic nucleotide-binding proteins. Within this fragment, amino acid residues Val-524, Val-525, and Ala-526 were found to contain label. These residues are part of a larger hydrophobic cluster that appears to line the binding pocket. The results also indicate that the 240-kDa protein contains a similar cGMP-binding site. Sequencing of a specifically labeled 8-kDa fragment through 16 amino acid residues indicated that the fragment was derived from the portion of the 240-kDa complex that contains the second subunit.(ABSTRACT TRUNCATED AT 250 WORDS)
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Affiliation(s)
- R L Brown
- Department of Physiology, University of Colorado School of Medicine, Denver 80262, USA
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133
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Abstract
Recent studies have revealed that cyclic nucleotide gated channels have a variety of forms and functions. These channels are now thought to be heteromultimers of at least two kinds of subunits and to undergo functional modulation. Ion permeation involves at least two ion-binding sites, and recent work on the alpha subunit suggests that many structural regions are involved in the control of channel gating. The continued use of both molecular and physiological approaches promises to further our understanding of how these channels work and how they are involved in cellular function.
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Affiliation(s)
- A L Zimmerman
- Department of Physiology, Brown University, Providence, Rhode Island 02912, USA
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134
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Nakatani K, Koutalos Y, Yau KW. Ca2+ modulation of the cGMP-gated channel of bullfrog retinal rod photoreceptors. J Physiol 1995; 484 ( Pt 1):69-76. [PMID: 7541463 PMCID: PMC1157922 DOI: 10.1113/jphysiol.1995.sp020648] [Citation(s) in RCA: 117] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/25/2023] Open
Abstract
1. The outer segment of an isolated rod photoreceptor from the bullfrog retina was drawn into a pipette containing choline solution for recording membrane current. The rest of the cell was sheared off with a glass probe to allow internal dialysis of the outer segment with a bath potassium solution ('truncated rod outer segment' preparation). The potential between the inside and the outside of the pipette was held at 0 mV. 2. Application of bath cGMP, in the presence of 3-isobutyl-1-methylxanthine (IBMX), gave rise to an outward membrane current. At saturating cGMP concentrations, this current was insensitive to intracellular Ca2+ at concentrations between 0 and 10 microM. At subsaturating cGMP concentrations, however, this current was inhibited by intracellular Ca2+. This sensitivity to Ca2+ declined after dialysis with a low-Ca2+ solution, suggesting the involvement of a soluble factor. 3. At low (nominally 0) Ca2+, the half-maximal activation constant and Hill coefficient for the activation of the cGMP-gated current by cGMP were 27 microM and 2.0, respectively. At high (ca 10 microM) Ca2+, the corresponding values were 40 microM cGMP and 2.4. 4. The inhibition of the current by Ca2+ was characterized at 20 microM cGMP. Ca2+ inhibited the current by up to 60%, with half-maximal inhibition at 48 nM Ca2+ and a Hill coefficient of 1.6.
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Affiliation(s)
- K Nakatani
- Institute of Biological Sciences, University of Tsukuba, Ibaraki, Japan
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135
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Liu M, Chen TY, Ahamed B, Li J, Yau KW. Calcium-calmodulin modulation of the olfactory cyclic nucleotide-gated cation channel. Science 1994; 266:1348-54. [PMID: 7526466 DOI: 10.1126/science.266.5189.1348] [Citation(s) in RCA: 256] [Impact Index Per Article: 8.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/25/2023]
Abstract
Although several ion channels have been reported to be directly modulated by calcium-calmodulin, they have not been conclusively shown to bind calmodulin, nor are the modulatory mechanisms understood. Study of the olfactory cyclic nucleotide-activated cation channel, which is modulated by calcium-calmodulin, indicates that calcium-calmodulin directly binds to a specific domain on the amino terminus of the channel. This binding reduces the effective affinity of the channel for cyclic nucleotides, apparently by acting on channel gating, which is tightly coupled to ligand binding. The data reveal a control mechanism that resembles those underlying the regulation of enzymes by calmodulin. The results also point to the amino-terminal part of the olfactory channel as an element for gating, which may have general significance in the operation of ion channels with similar overall structures.
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Affiliation(s)
- M Liu
- Howard Hughes Medical Institute, Baltimore, MD
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