How photons start vision

Photoreceptor renewal: a role for peripherin/rds

Visual transduction begins with the detection of light within the photoreceptor cell layer of the retina. Within this layer, specialized cells, termed rods and cones, contain the proteins responsible for light capture and its transduction to nerve impulses. The phototransductive proteins reside within an outer segment region that is connected to an inner segment by a thin stalk rich in cytoskeletal elements. A unique property of the outer segments is the presence of an elaborate intracellular membrane system that holds the phototransduction proteins and provides the requisite lipid environment. The maintenance of normal physiological function requires that these postmitotic cells retain the unique structure of the outer segment regions--stacks of membrane saccules in the case of rods and a continuous infolding of membrane in the case of cones. Both photoreceptor rod and cone cells achieve this through a series of coordinated steps. As new membranous material is synthesized, transported, and incorporated into newly forming outer segment membranes, a compensatory shedding of older membranous material occurs, thereby maintaining the segment at a constant length. These processes are collectively referred to as ROS (rod outer segment) or COS (cone outer segment) renewal. We review the cellular and molecular events responsible for these renewal processes and present the recent but compelling evidence, drawn from molecular genetic, biochemical, and biophysical approaches, pointing to an essential role for a unique tetraspanning membrane protein, called peripherin/rds, in the processes of disk morphogenesis.

Factors that affect regulation of cGMP synthesis in vertebrate photoreceptors and their genetic link to human retinal degeneration

Cyclic GMP is essential for the ability of rods and cones to respond to the light stimuli. Light triggers hydrolysis of cGMP and stops the influx of sodium and calcium through the cGMP-gated ion channels. The consequence of this event is 2-fold: first, the decrease in the inward sodium current plays the major role in an abrupt hyperpolarization of the cellular membrane; secondly, the decrease in the Ca2+ influx diminishes the free intracellular Ca2+ concentration. While the former constitutes the essence of the phototransduction pathway in rods and cones, the latter gives rise to a potent feedback mechanism that accelerates photoreceptor recovery and adaptation to background light. One of the most important events by which Ca2+ feedback controls recovery and light adaptation is synthesis of cGMP by guanylyl cyclase. Two isozymes of membrane photoreceptor guanylyl cyclase (retGC) have been identified in rods and cones that are regulated by Ca2+-binding proteins, GCAPs. At low intracellular concentrations of Ca2+ typical for light-adapted rods and cones GCAPs activate RetGC, but concentrations above 500 nM typical for dark-adapted photoreceptors turn them into inhibitors of retGC. A variety of mutations found in GCAP and retGC genes have been linked to several forms of human congenital retinal diseases, such as dominant cone degeneration, cone-rod dystrophy and Leber congenital amaurosis.

Instead of binding calcium, one of the EF-hand structures in guanylyl cyclase activating protein-2 is required for targeting photoreceptor guanylyl cyclase

Guanylyl cyclase activator proteins (GCAPs) are calcium-binding proteins closely related to recoverin, neurocalcin, and many other neuronal Ca(2+)-sensor proteins of the EF-hand superfamily. GCAP-1 and GCAP-2 interact with the intracellular portion of photoreceptor membrane guanylyl cyclase and stimulate its activity by promoting tight dimerization of the cyclase subunits. At low free Ca(2+) concentrations, the activator form of GCAP-2 associates into a dimer, which dissociates when GCAP-2 binds Ca(2+) and becomes inhibitor of the cyclase. GCAP-2 is known to have three active EF-hands and one additional EF-hand-like structure, EF-1, that deviates form the EF-hand consensus sequence. We have found that various point mutations within the EF-1 domain can specifically affect the ability of GCAP-2 to interact with the target cyclase but do not hamper the ability of GCAP-2 to undergo reversible Ca(2+)-sensitive dimerization. Point mutations within the EF-1 region can interfere with both the activation of the cyclase by the Ca(2+)-free form of GCAP-2 and the inhibition of retGC basal activity by the Ca(2+)-loaded GCAP-2. Our results strongly indicate that evolutionary conserved and GCAP-specific amino acid residues within the EF-1 can create a contact surface for binding GCAP-2 to the cyclase. Apparently, in the course of evolution GCAP-2 exchanged the ability of its first EF-hand motif to bind Ca(2+) for the ability to interact with the target enzyme.

Low amplification and fast visual pigment phosphorylation as mechanisms characterizing cone photoresponses

Vertebrate cone photoreceptors are known to show lower light sensitivity and briefer photoresponses than rod photoreceptors. To understand the molecular mechanisms characterizing cone photoresponses, we compared some of the reactions in the phototransduction cascade between rods and cones. For this purpose, rods and cones were obtained in quantities large enough to do biochemical studies. The cells were purified from the retina of carp (Cyprinus carpio) with a stepwise Percoll gradient. The purified rod fraction contained almost no other kinds of cells besides rods, and the purified cone fraction contained a mixture of red-, green-, and blue-sensitive cones in the ratio 3: approximately 1: approximately 1. We prepared membrane preparations from the rod and the cone fraction, and in these membranes, we measured activation efficiencies of the reactions in the phototransduction cascade. The results showed that the signal amplification is lower in the cone membranes, which accounts for the lower light sensitivity in cones. Furthermore, we measured the time courses of visual pigment phosphorylation. The result showed that the phosphorylation is much faster in the cone membranes, which also explains the lower light sensitivity and, in addition, the briefer photoresponse in cones.

Rhodopsin: structural basis of molecular physiology

The crystal structure of rod cell visual pigment rhodopsin was recently solved at 2.8-A resolution. A critical evaluation of a decade of structure-function studies is now possible. It is also possible to begin to explain the structural basis for several unique physiological properties of the vertebrate visual system, including extremely low dark noise levels as well as high gain and color detection. The ligand-binding pocket of rhodopsin is remarkably compact, and several apparent chromophore-protein interactions were not predicted from extensive mutagenesis or spectroscopic studies. The transmembrane helices are interrupted or kinked at multiple sites. An extensive network of interhelical interactions stabilizes the ground state of the receptor. The helix movement model of receptor activation, which might apply to all G protein-coupled receptors (GPCRs) of the rhodopsin family, is supported by several structural elements that suggest how light-induced conformational changes in the ligand-binding pocket are transmitted to the cytoplasmic surface. The cytoplasmic domain of the receptor is remarkable for a carboxy-terminal helical domain extending from the seventh transmembrane segment parallel to the bilayer surface. Thus the cytoplasmic surface appears to be approximately the right size to bind to the transducin heterotrimer in a one-to-one complex. Future high-resolution structural studies of rhodopsin and other GPCRs will form a basis to elucidate the detailed molecular mechanism of GPCR-mediated signal transduction.

Identification and functional consequences of a new mutation (E155G) in the gene for GCAP1 that causes autosomal dominant cone dystrophy

Mutations in the gene for guanylate cyclase-activating protein-1 (GCAP1) (GUCA1A) have been associated with autosomal dominant cone dystrophy (COD3). In the present study, a severe disease phenotype in a large white family was initially shown to map to chromosome 6p21.1, the location of GUCA1A. Subsequent single-stranded conformation polymorphism analysis and direct sequencing revealed an A464G transition, causing an E155G substitution within the EF4 domain of GCAP1. Modeling of the protein structure shows that the mutation eliminates a bidentate amino acid side chain essential for Ca2+ binding. This represents the first disease-associated mutation in GCAP1, or any neuron-specific calcium-binding protein within an EF-hand domain, that directly coordinates Ca2+. The functional consequences of this substitution were investigated in an in vitro assay of retinal guanylate cyclase activation. The mutant protein activates the cyclase at low Ca2+ concentrations but fails to inactivate at high Ca2+ concentrations. The overall effect of this would be the constitutive activation of guanylate cyclase in photoreceptors, even at the high Ca2+ concentrations of the dark-adapted state, which may explain the dominant disease phenotype.

Confronting complexity: the interlink of phototransduction and retinoid metabolism in the vertebrate retina

Absorption of light by rhodopsin or cone pigments in photoreceptors triggers photoisomerization of their universal chromophore, 11-cis-retinal, to all-trans-retinal. This photoreaction is the initial step in phototransduction that ultimately leads to the sensation of vision. Currently, a great deal of effort is directed toward elucidating mechanisms that return photoreceptors to the dark-adapted state, and processes that restore rhodopsin and counterbalance the bleaching of rhodopsin. Most notably, enzymatic isomerization of all-trans-retinal to 11-cis-retinal, called the visual cycle (or more properly the retinoid cycle), is required for regeneration of these visual pigments. Regeneration begins in rods and cones when all-trans-retinal is reduced to all-trans-retinol. The process continues in adjacent retinal pigment epithelial cells (RPE), where a complex set of reactions converts all-trans-retinol to 11-cis-retinal. Although remarkable progress has been made over the past decade in understanding the phototransduction cascade, our understanding of the retinoid cycle remains rudimentary. The aim of this review is to summarize recent developments in our current understanding of the retinoid cycle at the molecular level, and to examine the relevance of these reactions to phototransduction.

Effects of differential sulfhydryl group-specific labeling on the rhodopsin and guanine nucleotide binding activities of transducin

The role of transducin sulfhydryl groups was examined by chemical modification with four different reagents: 4-acetamido-4'-maleimidyl-stilbene-2, 2' disulfonic acid (AMDA); 4-vinyl pyridine (VP); 2-nitro-5-thiocyano benzoic acid (NTCBA); and 2, 5-dimethoxystilbene-4'-maleimide (DM). All these compounds rapidly inhibited the [3H]GMPpNp-binding activity of transducin stimulated by photoexcited rhodopsin (R*). Sedimentation experiments showed that the labeling of transducin with AMDA or VP hindered its binding to R* while NTCBA-modified transducin was capable of interacting with the photoreceptor protein. In contrast, DM-labeled transducin precipitated even in the absence of R*. Photoactivated rhodopsin was capable of protecting against the observed AMDA and NTCBA inhibition in transducin function, but not against the inactivation caused by VP or DM. These results suggest the existence of different functional cysteines on transducin that are located in the proximity of the interaction site with the photoreceptor protein, near the guanine nucleotide binding site, or in regions involved in the structural changes taking place upon protein activation. With the use of these reagents, transducin appears to be "frozen" in various conformational stages of its cycle, providing conditions for studying two of the initial steps of the visual process: the light-dependent binding of transducin to rhodopsin and the transducin guanine nucleotide exchange reaction.

Na(+) action potentials in human photoreceptors

Mammalian photoreceptors are hyperpolarized by a light stimulus and are commonly thought to be nonspiking neurons. We used the whole-cell patch-clamp technique on surgically excised human retina to examine whether human photoreceptors can elicit action potentials. We discovered that human rod photoreceptors express voltage-gated Na(+) channels, and generate Na(+) action potentials, in response to membrane depolarization from membrane potentials of -60 or -70 mV. Na(+) spikes in human rods were elicited at the termination of a light response that hyperpolarized the potential well below -50 mV. This served to amplify the release of a neurotransmitter when a bright light is turned off, and thus selectively amplify the off response to the light signal.

Rethinking the role of phosducin: light-regulated binding of phosducin to 14-3-3 in rod inner segments

Phosducin (Pd), a small protein found abundantly in photoreceptors, is widely assumed to regulate light sensitivity in the rod outer segment through interaction with the heterotrimeric G protein transducin. But, based on histochemistry and Western blot analysis, Pd is found almost entirely in the inner segment in both light and dark, most abundantly near the rod synapse. We report a second small protein, 14-3-3, in the rod with a similar distribution. By immunoprecipitation, phospho-Pd is found to interact with 14-3-3 in material from dark-adapted retina, and this interaction is markedly diminished by light, which dephosphorylates Pd. Conversely, unphosphorylated Pd binds to inner segment G protein(s) in the light. From these results and reported functions of 14-3-3, we have constructed a hypothesis for the regulation of light sensitivity at the level of rod synapse. By dissociating the Pd/14-3-3 complex, light enables both proteins to function in this role.

Signals from cone photoreceptors to L-type horizontal cells are differentially modulated by low calcium in carp retina

Ca2+ plays crucial roles in both phototransduction and calcium-dependent glutamate release from the photoreceptor terminal. Modulation, by lowering extracellular Ca2+, of red-sensitive (R-) and short wavelength-sensitive (S-) cone-driven light responses of L-type horizontal cells (LHCs) was studied in the isolated superfused carp retina using intracellular recording techniques. Low Ca2+ (nominally Ca2+-free) Ringer's reduced responses of LHCs to both green (500 nm) and red (680 nm) flashes in darkness, with the former being suppressed more substantially than the latter. This differential suppression became more significant when contribution of R-cones to the green-light-induced responses was diminished by a moderate red (680 nm) background light. Application of IBMX, an inhibitor of phosphodiesterase (PDE), increased LHC responses to both red and green flashes equally, resembling the effect of low Ca2+ on phototransduction. In addition, photopic electroretinographic P III responses, reflecting the activity of cones, to red flashes were more potentiated by low Ca2+, compared to those to green flashes, whilst they were both equally potentiated by IBMX. Furthermore, low Ca2+ caused a more pronounced suppression of LHC responses to red flashes than those to green flashes in the presence of IBMX. It is postulated that reduction of LHC responses in low Ca2+ may be due to the 'saturation suppression' caused by the increased glutamate release from the photoreceptor terminal and the differential modulation may reflect a consequence of the dual action of low Ca2+ on the PDE activity in the photoreceptor outer segment and the synaptic strength between cones and LHCs.

Adaptation in vertebrate photoreceptors

When light is absorbed within the outer segment of a vertebrate photoreceptor, the conformation of the photopigment rhodopsin is altered to produce an activated photoproduct called metarhodopsin II or Rh(*). Rh(*) initiates a transduction cascade similar to that for metabotropic synaptic receptors and many hormones; the Rh(*) activates a heterotrimeric G protein, which in turn stimulates an effector enzyme, a cyclic nucleotide phosphodiesterase. The phosphodiesterase then hydrolyzes cGMP, and the decrease in the concentration of free cGMP reduces the probability of opening of channels in the outer segment plasma membrane, producing the electrical response of the cell. Photoreceptor transduction can be modulated by changes in the mean light level. This process, called light adaptation (or background adaptation), maintains the working range of the transduction cascade within a physiologically useful region of light intensities. There is increasing evidence that the second messenger responsible for the modulation of the transduction cascade during background adaptation is primarily, if not exclusively, Ca(2+), whose intracellular free concentration is decreased by illumination. The change in free Ca(2+) is believed to have a variety of effects on the transduction mechanism, including modulation of the rate of the guanylyl cyclase and rhodopsin kinase, alteration of the gain of the transduction cascade, and regulation of the affinity of the outer segment channels for cGMP. The sensitivity of the photoreceptor is also reduced by previous exposure to light bright enough to bleach a substantial fraction of the photopigment in the outer segment. This form of desensitization, called bleaching adaptation (the recovery from which is known as dark adaptation), seems largely to be due to an activation of the transduction cascade by some form of bleached pigment. The bleached pigment appears to activate the G protein transducin directly, although with a gain less than Rh(*). The resulting decrease in intracellular Ca(2+) then modulates the transduction cascade, by a mechanism very similar to the one responsible for altering sensitivity during background adaptation.

Cell biology. Actin' up with Rac1

The elegant architecture of photoreceptor cells in the retina is dependent on organization of the actin cytoskeleton during eye development. But what drives this organization? In an equally elegant Perspective, Colley explains new findings in fruit flies (Chang and Ready) that point to the photopigment rhodopsin and its signaling molecule the Rho GTPase Drac1 as the orchestrators of actin organization and the consequent assembly of the sensory membrane in the photoreceptor cell.

Engineering aspects of enzymatic signal transduction: photoreceptors in the retina

Identifying the basic module of enzymatic amplification as an irreversible cycle of messenger activation/deactivation by a "push-pull" pair of opposing enzymes, we analyze it in terms of gain, bandwidth, noise, and power consumption. The enzymatic signal transduction cascade is viewed as an information channel, the design of which is governed by the statistical properties of the input and the noise and dynamic range constraints of the output. With the example of vertebrate phototransduction cascade we demonstrate that all of the relevant engineering parameters are controlled by enzyme concentrations and, from functional considerations, derive bounds on the required protein numbers. Conversely, the ability of enzymatic networks to change their response characteristics by varying only the abundance of different enzymes illustrates how functional diversity may be built from nearly conserved molecular components.

Structure of the GAF domain, a ubiquitous signaling motif and a new class of cyclic GMP receptor

GAF domains are ubiquitous motifs present in cyclic GMP (cGMP)-regulated cyclic nucleotide phosphodiesterases, certain adenylyl cyclases, the bacterial transcription factor FhlA, and hundreds of other signaling and sensory proteins from all three kingdoms of life. The crystal structure of the Saccharomyces cerevisiae YKG9 protein was determined at 1.9 A resolution. The structure revealed a fold that resembles the PAS domain, another ubiquitous signaling and sensory transducer. YKG9 does not bind cGMP, but the isolated first GAF domain of phosphodiesterase 5 binds with K:(d) = 650 nM. The cGMP binding site of the phosphodiesterase GAF domain was identified by homology modeling and site-directed mutagenesis, and consists of conserved Arg, Asn, Lys and Asp residues. The structural and binding studies taken together show that the cGMP binding GAF domains form a new class of cyclic nucleotide receptors distinct from the regulatory domains of cyclic nucleotide-regulated protein kinases and ion channels.

Cyclic nucleotide analogs as biochemical tools and prospective drugs

Cyclic AMP (cAMP) and cyclic GMP (cGMP) are key second messengers involved in a multitude of cellular events. From the wealth of synthetic analogs of cAMP and cGMP, only a few have been explored with regard to their therapeutic potential. Some of the first-generation cyclic nucleotide analogs were promising enough to be tested as drugs, for instance N(6),O(2)'-dibutyryl-cAMP and 8-chloro-cAMP (currently in clinical Phase II trials as an anticancer agent). Moreover, 8-bromo and dibutyryl analogs of cAMP and cGMP have become standard tools for investigations of biochemical and physiological signal transduction pathways. The discovery of the Rp-diastereomers of adenosine 3',5'-cyclic monophosphorothioate and guanosine 3',5'-cyclic monophosphorothioate as competitive inhibitors of cAMP- and cGMP-dependent protein kinases, as well as subsequent development of related analogs, has proven very useful for studying the molecular basis of signal transduction. These analogs exhibit a higher membrane permeability, increased resistance against degradation, and improved target specificity. Furthermore, better understanding of signaling pathways and ligand/protein interactions has led to new therapeutic strategies. For instance, Rp-8-bromo-adenosine 3',5'-cyclic monophosphorothioate is employed against diseases of the immune system. This review will focus mainly on recent developments in cyclic nucleotide-related biochemical and pharmacological research, but also highlights some historical findings in the field.

Bias in phylogenetic reconstruction of vertebrate rhodopsin sequences

Two spurious nodes were found in phylogenetic analyses of vertebrate rhodopsin sequences in comparison with well-established vertebrate relationships. These spurious reconstructions were well supported in bootstrap analyses and occurred independently of the method of phylogenetic analysis used (parsimony, distance, or likelihood). Use of this data set of vertebrate rhodopsin sequences allowed us to exploit established vertebrate relationships, as well as the considerable amount known about the molecular evolution of this gene, in order to identify important factors contributing to the spurious reconstructions. Simulation studies using parametric bootstrapping indicate that it is unlikely that the spurious nodes in the parsimony analyses are due to long branches or other topological effects. Rather, they appear to be due to base compositional bias at third positions, codon bias, and convergent evolution at nucleotide positions encoding the hydrophobic residues isoleucine, leucine, and valine. LogDet distance methods, as well as maximum-likelihood methods which allow for nonstationary changes in base composition, reduce but do not entirely eliminate support for the spurious resolutions. Inclusion of five additional rhodopsin sequences in the phylogenetic analyses largely corrected one of the spurious reconstructions while leaving the other unaffected. The additional sequences not only were more proximal to the corrected node, but were also found to have intermediate levels of base composition and codon bias as compared with neighboring sequences on the tree. This study shows that the spurious reconstructions can be corrected either by excluding third positions, as well as those encoding the amino acids Ile, Val, and Leu (which may not be ideal, as these sites can contain useful phylogenetic signal for other parts of the tree), or by the addition of sequences that reduce problems associated with convergent evolution.

Effects of xanthine derivatives on electroretinographic responsiveness

In view of the use of synthetic propentofylline (PPF) as a protective agent in brain ischemia, its possible side effects on vision capacities have been explored by electroretinography in comparative experiments with theophylline. We used eyecup preparations of small-spotted dogfish sharks and of European eels, particularly suitable for long-lasting experiments. The drug exerted profound but reversible modifications of ERG records: (1) a dose-dependent increase of the amplitude and duration of the chemically isolated late receptor potential (LRP), (2) a partial unmasking of LRP, (3) a strong potentiation of the LRP-unmasking effect of low temperature, (4) a potentiation of light adaptation effects, and (5) a strong potentiation of the post-illumination hyperexcitability. The effects were explicable as due to a strong phosphodiesterase (PDE) inhibiting, cyclic guanosine monophosphate (cGMP) promoting, action of the drug. The effects were considerably stronger, or even of opposite sign, in comparison to those of the chemically related theophylline. PPF did not seriously affect the ERG c-wave originating in the pigment epithelium. The results suggested that the effects of PPF on vision may not seriously hamper the therapeutic use of the drug. They indicated, on the other hand, that PPF was a retinoactive drug of potential usefulness in the exploration of the complex biochemical events underlying visual transduction.

Phospholipase C and termination of G-protein-mediated signalling in vivo

In Drosophila photoreceptors, phospholipase C (PLC) and other signalling components form multiprotein structures through the PDZ scaffold protein INAD. Association between PLC and INAD is important for termination of responses to light; the underlying mechanism is, however, unclear. Here we report that the maintenance of large amounts of PLC in the signalling membranes by association with INAD facilitates response termination, and show that PLC functions as a GTPase-activating protein (GAP). The inactivation of the G protein by its target, the PLC, is crucial for reliable production of single-photon responses and for the high temporal and intensity resolution of the response to light.

Localization of two distinct type III phosphatidylinositol 4-kinase enzyme mRNAs in the rat

Inositol lipid kinases generate polyphosphoinositides, important regulators of several cellular functions. We have recently cloned two distinct phosphatidylinositol (PI) 4-kinase enzymes, the 210-kDa PI4KIIIalpha and the 110-kDa PI4KIIIbeta, from bovine tissues. In the present study, the distribution of mRNAs encoding these two enzymes was analyzed by in situ hybridization histochemistry in the rat. PI4KIIIalpha was found predominantly expressed in the brain, with low expression in peripheral tissues. PI4KIIIbeta was more uniformly expressed being also present in various peripheral tissues. Within the brain, PI4KIIIbeta showed highest expression in the gray matter, especially in neurons of the olfactory bulb and the hippocampus, but also gave a signal in the white matter indicating its presence in glia. PI4KIIIalpha was highly expressed in neurons, but lacked a signal in the white matter and the choroid plexus. Both enzymes showed expression in the pigment layer and nuclear layers as well as in the ganglion cells of the retina. In a 17-day-old rat fetus, PI4KIIIbeta was found to be more widely distributed and PI4KIIIalpha was primarily expressed in neurons. These results indicate that PI4KIIIbeta is more widely expressed than PI4KIIIalpha, and that the two enzymes are probably coexpressed in many neurons. Such expression pattern and the conservation of these two proteins during evolution suggest their nonredundant functions in mammalian cells.

Loss of the effector function in a transducin-alpha mutant associated with Nougaret night blindness

A missense mutation, G38D, was found in the rod transducin alpha subunit (Galpha(t)) in individuals with the Nougaret form of dominant stationary night blindness. To elucidate the mechanism of Nougaret night blindness, we have examined the key functional properties of the mutant transducin. Our data show that the G38D mutation does not alter the interaction between Galpha(t) and Gbetagamma(t) or activation of transducin by photoexcited rhodopsin (R*). The mutant Galpha(t) has only a modestly (approximately 2.5-fold) reduced k(cat) value for GTP hydrolysis. The GTPase activity of Galpha(t)G38D can be accelerated by photoreceptor regulator of G protein signaling, RGS9. Analysis of the Galpha(t)G38D interaction with cGMP phosphodiesterase revealed marked impairment of the mutant effector function. Galpha(t)G38D completely fails to bind the inhibitory PDE gamma subunit and activate the enzyme. Altogether, our results demonstrate a novel molecular mechanism in dominant stationary night blindness. In contrast to known forms of the disease caused by constitutive activation of the visual cascade, the Nougaret form has its origin in attenuated visual signaling due to loss of effector function by transducin G38D mutant.

Olfaction and odor discrimination are mediated by the C. elegans guanylyl cyclase ODR-1

Animals in complex environments must discriminate between salient and uninformative sensory cues. Caenorhabditis elegans uses one pair of olfactory neurons called AWC to sense many different odorants, yet the animal can distinguish each odorant from the others in discrimination assays. We demonstrate that the transmembrane guanylyl cyclase ODR-1 is essential for responses to all AWC-sensed odorants. ODR-1 appears to be a shared signaling component downstream of odorant receptors. Overexpression of ODR-1 protein indicates that ODR-1 can influence odor discrimination and adaptation as well as olfaction. Adaptation to one odorant, butanone, is disrupted by ODR-1 overexpression. Olfactory discrimination is also disrupted by ODR-1 overexpression, probably by overproduction of the shared second messenger cGMP. We propose that AWC odorant signaling pathways are insulated to permit odor discrimination.

Molecular cloning and functional characterization of a new modulatory cyclic nucleotide-gated channel subunit from mouse retina

Cyclic nucleotide-gated (CNG) channels play a key role in olfactory and visual transduction. Native CNG channels are heteromeric complexes consisting of the principal alpha subunits (CNG1-3), which can form functional channels by themselves, and the modulatory beta subunits (CNG4-5). The individual alpha and beta subunits that combine to form the CNG channels in rod photoreceptors (CNG1 + CNG4) and olfactory neurons (CNG2 + CNG4 + CNG5) have been characterized. In contrast, only an alpha subunit (CNG3) has been identified so far in cone photoreceptors. Here we report the molecular cloning of a new CNG channel subunit (CNG6) from mouse retina. The cDNA of CNG6 encodes a peptide of 694 amino acids with a predicted molecular weight of 80 kDa. Among the CNG channel subunits, CNG6 has the highest overall similarity to the CNG4 beta subunit (47% sequence identity). CNG6 transcripts are present in a small subset of retinal photoreceptor cells and also in testis. Heterologous expression of CNG6 in human embryonic kidney 293 cells did not lead to detectable currents. However, when coexpressed with the cone photoreceptor alpha subunit, CNG6 induced a flickering channel gating, weakened the outward rectification in the presence of extracellular Ca(2+), increased the sensitivity for L-cis diltiazem, and enhanced the cAMP efficacy of the channel. Taken together, the data indicate that CNG6 represents a new CNG channel beta subunit that may associate with the CNG3 alpha subunit to form the native cone channel.

Molecular cloning and functional characterization of a new modulatory cyclic nucleotide-gated channel subunit from mouse retina

Cyclic nucleotide-gated (CNG) channels play a key role in olfactory and visual transduction. Native CNG channels are heteromeric complexes consisting of the principal alpha subunits (CNG1-3), which can form functional channels by themselves, and the modulatory beta subunits (CNG4-5). The individual alpha and beta subunits that combine to form the CNG channels in rod photoreceptors (CNG1 + CNG4) and olfactory neurons (CNG2 + CNG4 + CNG5) have been characterized. In contrast, only an alpha subunit (CNG3) has been identified so far in cone photoreceptors. Here we report the molecular cloning of a new CNG channel subunit (CNG6) from mouse retina. The cDNA of CNG6 encodes a peptide of 694 amino acids with a predicted molecular weight of 80 kDa. Among the CNG channel subunits, CNG6 has the highest overall similarity to the CNG4 beta subunit (47% sequence identity). CNG6 transcripts are present in a small subset of retinal photoreceptor cells and also in testis. Heterologous expression of CNG6 in human embryonic kidney 293 cells did not lead to detectable currents. However, when coexpressed with the cone photoreceptor alpha subunit, CNG6 induced a flickering channel gating, weakened the outward rectification in the presence of extracellular Ca(2+), increased the sensitivity for L-cis diltiazem, and enhanced the cAMP efficacy of the channel. Taken together, the data indicate that CNG6 represents a new CNG channel beta subunit that may associate with the CNG3 alpha subunit to form the native cone channel.

Molecular mechanism of visual transduction

Our vision renders an incredible wealth of information about the external environment presented in the form of light of different wavelengths and intensities. To operate in a wide range of light intensities, our visual system has developed several mechanisms that allow an adjustment of its sensitivity to light. Immense progress has been made in understanding how light is captured and activates visual phototransduction cascade within photoreceptor cells; however, much less is known about desensitization. It has been known for some time, that many of these processes rely on Ca2+ as the principal modifier of phototransduction. Ca(2+)-binding proteins (CBPs) are specifically poised to take advantage of transient changes in [Ca2+] to act as enzymatic regulators. Some other CBPs are capable of changing the intracellular Ca2+ buffering capacity. Various retinal CBP proteins have been identified, including recoverin, GCAP1, GCAP2, GCAP3, GCIP, CBP1, CBP3 and CBP4. Although these numerous CBPs were identified, functions can be ascribed to only a few of them. Recently, genetic, physiological and biochemical analyses of retinal diseases have yielded additional insights into the role of many phototransduction proteins, including CBPs. Understanding the properties and the functions of these CBPs will pave the way for a more complete picture of visual transduction and accompanying desensitization processes.

Amino acid residues controlling the properties and functions of rod and cone visual pigments

The visual transduction processes in rod and cone photoreceptor cells are initiated by photon absorption by the different types of visual pigments. In relation to the functional difference between these cells, cone visual pigments in chicken retinas exhibit faster regeneration from 11-cis-retinal and opsin and faster decay of physiologically active intermediate (Meta II) than rod visual pigment, rhodopsin. Replacement of the amino acid residue at position 122 of chicken rhodopsin by the residues present in the respective cone pigments dramatically changes both the decay rate of Meta II and the rate of regeneration into those of the cone pigment-type, indicating that the residue at this position is a major determinant controlling these properties. Thus, the single replacement of amino acid residue at this position would be one of the key steps of the divergence into twilight and daylight vision.

Dimerization of guanylyl cyclase-activating protein and a mechanism of photoreceptor guanylyl cyclase activation

Ca(2+)-binding guanylyl cyclase-activating proteins (GCAPs) stimulate photoreceptor membrane guanylyl cyclase (retGC) in the light when the free Ca(2+) concentrations in photoreceptors decrease from 600 to 50 nM. RetGC activated by GCAPs exhibits tight dimerization revealed by chemical cross-linking (Yu, H., Olshevskaya, E., Duda, T., Seno, K., Hayashi, F., Sharma, R. K., Dizhoor, A. M., and Yamazaki, A. (1999) J. Biol. Chem. 274, 15547-15555). We have found that the Ca(2+)-loaded GCAP-2 monomer undergoes reversible dimerization upon dissociation of Ca(2+). The ability of GCAP-2 and its several mutants to activate retGC in vitro correlates with their ability to dimerize at low free Ca(2+) concentrations. A constitutively active GCAP-2 mutant E80Q/E116Q/D158N that stimulates retGC regardless of the free Ca(2+) concentrations forms dimers both in the absence and in the presence of Ca(2+). Several GCAP-2/neurocalcin chimera proteins that cannot efficiently activate retGC in low Ca(2+) concentrations are also unable to dimerize in the absence of Ca(2+). Additional mutation that restores normal activity of the GCAP-2 chimera mutant also restores its ability to dimerize in the absence of Ca(2+). These results suggest that dimerization of GCAP-2 can be a part of the mechanism by which GCAP-2 regulates the photoreceptor guanylyl cyclase. The Ca(2+)-free GCAP-1 is also capable of dimerization in the absence of Ca(2+), but unlike GCAP-2, dimerization of GCAP-1 is resistant to the presence of Ca(2+).

Initiating ocular proteomics for cataloging bovine retinal proteins: microanalytical techniques permit the identification of proteins derived from a novel photoreceptor preparation

Though some mechanisms of photoreception have been well characterized, others remain obscure. Presumably, most, if not all, of the major players in photoreceptor-specific functions are present in large amounts in the photoreceptor layer, and a catalog of these proteins will prove a useful tool for vision researchers. As a first step toward a complete catalog of photoreceptor cells, we have developed a novel method for isolating the photoreceptor cell monolayer from bovine retina. Electron microscopic studies of both the photoreceptor layer and the residual retina from which the photoreceptor layer had been removed, indicate that the preparation contains the photoreceptor outer segments and the majority of the inner segments. Proteins were extracted from the isolated photoreceptor cell layer as well as the rest of the retina with isoelectric focusing lysis buffer, and the protein components were separated by two-dimensional gel electrophoresis. The obtained protein maps reveal several classes of proteins that appear to be expressed more abundantly or specifically in the photoreceptor layer than in the rest of the retina. Four of these protein spots were excised and in-gel digested with trypsin, and the digests were extracted with solvent. The mixture of peptides digested from each protein was analyzed by high performance liquid chromatography interfaced with electrospray ionization tandem quadrupole mass spectrometry or by matrix-assisted laser desorption ionization time-of-flight mass spectrometry. Some of the peptides were isolated and their sequences were determined by gas phase Edman degradation. RNA transcripts extracted from the photoreceptor layer or the whole retina were subjected to Northern blot analysis as well as to reverse transcriptase-polymerase chain reaction amplification of probes for the successful selection of cDNA clones. These data permit both the identification of virtually any protein detectable on a two-dimensional gel, and also enable the corresponding cDNA clone to be selected. We have validated this approach by identifying aspartate aminotransferase and creatine kinase from the populations of abundant photoreceptor layer proteins. Both aspartate aminotransferase and creatine kinase are of mitochondrial origin and are thought to play crucial roles in photoreceptor functions by producing glutamate and ATP, respectively. We also identified two photoreceptor layer specific proteins: an acidic and high molecular weight protein, interphotoreceptor retinoid-binding protein, and an acidic and small molecular weight protein, recoverin.The technique presented here will allow vision researchers to discover and identify the proteins that are expressed specifically or abundantly in the photoreceptor cell as well as the proteins that undergo post-translational modification or modulation in expression under a defined biological condition. With the use of this technology, we anticipate that a researcher who knows only the 2-D gel position of a protein of interest can identify the protein, isolate a cDNA clone, and move into molecular genetic studies. Moreover, this streamlined technology will enable one to assemble a catalog of photoreceptor proteins using a minute amount of materials in a short period of time. We believe that such a catalog will serve as a valuable resource for vision investigators and will accelerate the rate of research progress.

Conformational changes in guanylyl cyclase-activating protein 1 (GCAP1) and its tryptophan mutants as a function of calcium concentration

Guanylyl cyclase-activating proteins (GCAPs are 23-kDa Ca2+-binding proteins belonging to the calmodulin superfamily. Ca2+-free GCAPs are responsible for activation of photoreceptor guanylyl cyclase during light adaptation. In this study, we characterized GCAP1 mutants in which three endogenous nonessential Trp residues were replaced by Phe residues, eliminating intrinsic fluorescence. Subsequently, hydrophobic amino acids adjacent to each of the three functional Ca2+-binding loops were replaced by reporter Trp residues. Using fluorescence spectroscopy and biochemical assays, we found that binding of Ca2+ to GCAP1 causes a major conformational change especially in the region around the EF3-hand motif. This transition of GCAP1 from an activator to an inhibitor of GC requires an activation energy Ea = 9.3 kcal/mol. When Tyr99 adjacent to the EF3-hand motif was replaced by Cys, a mutation linked to autosomal dominant cone dystrophy in humans, Cys99 is unable to stabilize the inactive GCAP1-Ca2+ complex. Stopped-flow kinetic measurements indicated that GCAP1 rapidly loses its bound Ca2+ (k-1 = 72 s-1 at 37 degrees C) and was estimated to associate with Ca2+ at a rate (k1 > 2 x 10(8) M-1 s-1) close to the diffusion limit. Thus, GCAP1 displays thermodynamic and kinetic properties that are compatible with its involvement early in the phototransduction response.

Three-dimensional structure of guanylyl cyclase activating protein-2, a calcium-sensitive modulator of photoreceptor guanylyl cyclases

Guanylyl cyclase activating protein-2 (GCAP-2) is a Ca2+-sensitive regulator of phototransduction in retinal photoreceptor cells. GCAP-2 activates retinal guanylyl cyclases at low Ca2+ concentration (<100 nM) and inhibits them at high Ca2+ (>500 nM). The light-induced lowering of the Ca2+ level from approximately 500 nM in the dark to approximately 50 nM following illumination is known to play a key role in visual recovery and adaptation. We report here the three-dimensional structure of unmyristoylated GCAP-2 with three bound Ca2+ ions as determined by nuclear magnetic resonance spectroscopy of recombinant, isotopically labeled protein. GCAP-2 contains four EF-hand motifs arranged in a compact tandem array like that seen previously in recoverin. The root mean square deviation of the main chain atoms in the EF-hand regions is 2.2 A in comparing the Ca2+-bound structures of GCAP-2 and recoverin. EF-1, as in recoverin, does not bind calcium because it contains a disabling Cys-Pro sequence. GCAP-2 differs from recoverin in that the calcium ion binds to EF-4 in addition to EF-2 and EF-3. A prominent exposed patch of hydrophobic residues formed by EF-1 and EF-2 (Leu24, Trp27, Phe31, Phe45, Phe48, Phe49, Tyr81, Val82, Leu85, and Leu89) may serve as a target-binding site for the transmission of calcium signals to guanylyl cyclase.

Selective loss of cone function in mice lacking the cyclic nucleotide-gated channel CNG3

Two types of photoreceptors, rods and cones, coexist in the vertebrate retina. An in-depth analysis of the retinal circuitry that transmits rod and cone signals has been hampered by the presence of intimate physical and functional connections between rod and cone pathways. By deleting the cyclic nucleotide-gated channel CNG3 we have generated a mouse lacking any cone-mediated photoresponse. In contrast, the rod pathway is completely intact in CNG3-deficient mice. The functional loss of cone function correlates with a progressive degeneration of cone photoreceptors but not of other retinal cell types. CNG3-deficient mice provide an animal model to dissect unequivocally the contribution of rod and cone pathways for normal retinal function.

Variability in the time course of single photon responses from toad rods: termination of rhodopsin's activity

We examined the responses of toad rod photoreceptors to single photons of light. To minimize the effects of variability in the early rising phase, we selected sets of responses that closely matched the rise of the mean single photon response. Responses selected in this way showed substantial variations in kinetics, appearing to peel off from a common time course after different delays. Following incorporation of the calcium buffer BAPTA, the time to peeling off was retarded. Our analysis indicates that it is not necessary to invoke a long series of reaction steps to explain the shutoff of rhodopsin activity. Instead, our results suggest that the observed behavior is explicable by the presently known shutoff reactions of activated rhodopsin, modulated by feedback.

Adenovirus-mediated expression of an olfactory cyclic nucleotide-gated channel regulates the endogenous Ca2+-inhibitable adenylyl cyclase in C6-2B glioma cells

Previous studies have established that Ca2+-sensitive adenylyl cyclases, whether endogenously or heterologously expressed, are preferentially regulated by capacitative Ca2+ entry, compared with other means of elevating cytosolic Ca2+ (Chiono, M., Mahey, R., Tate, G., and Cooper, D. M. F. (1995) J. Biol. Chem. 270, 1149-1155; Fagan, K. A., Mahey, R., and Cooper, D. M. F. (1996) J. Biol. Chem. 271, 12438-12444; Fagan, K. A., Mons, N., and Cooper, D. M. F. (1998) J. Biol. Chem. 273, 9297-9305). These findings led to the suggestion that adenylyl cyclases and capacitative Ca2+ entry channels were localized in the same functional domain of the plasma membrane. In the present study, we have asked whether a heterologously expressed Ca2+-permeable channel could regulate the Ca2+-inhibitable adenylyl cyclase of C6-2B glioma cells. The cDNA coding for the rat olfactory cyclic nucleotide-gated channel was inserted into an adenovirus construct to achieve high levels of expression. Electrophysiological measurements confirmed the preservation of the properties of the expressed olfactory channel. Stimulation of the channel with cGMP analogs yielded a robust elevation in cytosolic Ca2+, which was associated with an inhibition of cAMP accumulation, comparable with that elicited by capacitative Ca2+ entry. These findings not only extend the means whereby Ca2+-sensitive adenylyl cyclases may be regulated, they also suggest that in tissues where they co-exist, cyclic nucleotide-gated channels and Ca2+-sensitive adenylyl cyclases may reciprocally modulate each other's activity.

Mapping functional domains of the guanylate cyclase regulator protein, GCAP-2

Guanylate cyclase regulator protein (GCAP)-2 is a Ca2+-binding protein that regulates photoreceptor outer segment membrane guanylate cyclase (RetGC) in a Ca2+-sensitive manner. GCAP-2 activates RetGC at free Ca2+ concentrations below 100 nM, characteristic of light-adapted photoreceptors, and inhibits RetGC when free Ca2+ concentrations are above the 500 nM level, characteristic of dark-adapted photoreceptors. We have mapped functional domains in GCAP-2 by using deletion mutants and chimeric proteins in which parts of GCAP-2 were substituted with corresponding fragments of other closely related recoverin-like proteins that do not regulate RetGC. We find that in addition to the EF-hand Ca2+-binding centers there are three regions that contain GCAP-2-specific sequences essential for regulation of RetGC. 1) The region between Phe78 and Asp113 determines whether GCAP-2 activates outer segment RetGC in low or high Ca2+ concentrations. Substitution of this domain with the corresponding region from neurocalcin causes a paradoxical behavior of the chimeric proteins. They activate RetGC only at high and not at low Ca2+ concentrations. 2) The amino acid sequence of GCAP-2 between Lys29 and Phe48 that includes the EF-hand-related motif EF-1 is essential both for activation of RetGC at low Ca2+ and inhibition at high Ca2+ concentrations. Most of the remaining N-terminal region can be substituted with recoverin or neurocalcin sequences without loss of GCAP-2 function. 3) Region Val171-Asn189, adjacent to the C-terminal EF-4 contributes to activation of RetGC, but it is not essential for the ability of Ca2+-loaded GCAP-2 to inhibit RetGC. Other regions of the molecule can be substituted with the corresponding fragments from neurocalcin or recoverin, or even partially deleted without preventing GCAP-2 from regulating RetGC. Substitution of these three domains in GCAP-2 with corresponding neurocalcin sequences also affects activation of individual recombinant RetGC-1 and RetGC-2 expressed in HEK293 cells.

cAMP-dependent induction of PDE5 expression in murine neuroblastoma cell differentiation

The present study demonstrates, in both hybrid NG108-15 and mouse neuroblastoma N18TG2 cells, the presence and regulation of PDE5 mRNA during cell differentiation. PDE5 cDNA probes in Northern blot analysis recognize a approximately 9 kb transcript in bovine lung as well as in mouse neuroblastoma cells. Hybridization on total RNA extracted from dibutyryl-cAMP-treated NG108-15 cells shows a 5-fold increase of PDE5 9 kb mRNA: such an increase is not observed in N18TG2 although we observed a similar increase in the enzymatic activity of both cell lines. Our data demonstrate that PDE5 gene expression can be regulated by cAMP and suggest the existence of a complex regulatory system for PDE5 activity.

Noise and its effects on photoreceptor temporal contrast sensitivity at low light levels

We studied photoreceptors in the locust (Schistocerca americanus) visual system to determine the extent to which quantal noise and intrinsic neural noise limit temporal sensitivity. Typical computational models of the temporal contrast sensitivity function are deterministic, reflect only filter characteristics, and lack explicit noise sources [J. Opt. Soc. Am. 58, 1133 (1968); Vision Res. 32, 1373 (1992)]. We report here that the temporal contrast sensitivity function, at low light levels, is not simply the reflection of a filter function. Our evidence suggests that, at low backgrounds, noise, in conjunction with temporal filtering, plays a role in shaping the temporal contrast sensitivity function. At a given low adaptation level, quantal noise limits sensitivity at low temporal frequencies, while intrinsic noise limits sensitivity at relatively higher temporal frequencies.

Regulation of squid visual phospholipase C by activated G-protein alpha

Phospholipase C (PLC) is the key enzyme in the phototransduction cascade of invertebrate rhabdomeric photoreceptors. In addition to 130 kDa PLC, a 95 kDa protein recognized by antibody against the catalytic site of PLC was found in the squid retina. The PLC-like 95 kDa protein (95 kDa PLC) was produced from 130 kDa PLC by an intrinsic protease in the presence of calcium. The 130 kDa PLC was stimulated by the active form of Gq-class G-protein alpha (Gq alpha), but the 95 kDa PLC was not, although their PLC activity was similar. A 35 kDa fragment, the counterpart of 95 kDa PLC, was not recognized by antibodies against catalytic site or N-terminal site of the 130 kDa PLC, indicating that the cleavage site is on the C-terminal side beyond the catalytic site. In the presence of a large excess of the 35 kDa fragment, 95 kDa PLC was stimulated by Gq alpha to a similar extent as intact 130 kDa PLC. These results indicate that the C-terminal polypeptide of PLC is necessary for regulation of its enzyme activity by Gq alpha. The uncoupling of PLC from Gq alpha, caused by limited proteolysis, is therefore a candidate regulatory mechanism of the phototransduction cascade in rhabdomeric photoreceptors.

Pharmacological characterization of the effects of taurine on calcium uptake in the rat retina

Taurine is known to increase ATP-dependent calcium ion (Ca2+) uptake in retinal membrane preparations and in isolated rod outer segments (ROS) under low calcium conditions (10 microM) (Pasantes-Morales and Ordóñez, 1982; Lombardini, 1991). In this report, ATP-dependent Ca2+ uptake in retinal membrane preparations was found to be inhibited by 5 microM cadmium (Cd2+), suggesting the involvement of cation channel activation. The activation of cGMP-gated cation channels, which are found in the ROS, is a crucial step in the phototransduction process. An inhibitor of cGMP-gated channels, LY83583, was found to inhibit taurine-stimulated ATP-dependent Ca2+ uptake but had no effect on ATP-dependent Ca2+ uptake in the absence of taurine, indicating that taurine may be increasing ATP-dependent Ca2+ uptake through a mechanism of action involving the opening of cGMP-gated channels. The activation of cGMP-gated channels with dibutyryl-cGMP and with phosphodiesterase inhibition using zaprinast caused an increase in ATP-dependent Ca2+ uptake in isolated ROS, but not in taurine-stimulated ATP-dependent Ca2+ uptake. LY83583 had the same effects in isolated ROS as in retinal membrane preparations. Another inhibitor of cGMP-gated channels, Rp-8-Br-PET-cGMPS, produced the same pattern of inhibition in isolated ROS as LY83583. Thus, there appears to be a causal link between taurine and the activation of the cGMP-gated channels in the ROS under conditions of low calcium concentration, a connection that suggests an important role for taurine in the visual signalling function of the retina.

Retinitis pigmentosa: defined from a molecular point of view

Retinitis pigmentosa (RP) denotes a group of hereditary retinal dystrophies, characterized by the early onset of night blindness followed by a progressive loss of the visual field. The primary defect underlying RP affects the function of the rod photoreceptor cell, and, subsequently, mostly unknown molecular and cellular mechanisms trigger the apoptotic degeneration of these photoreceptor cells. Retinitis pigmentosa is very heterogeneous, both phenotypically and genetically. In this review we propose a tentative classification of RP based on the functional systems affected by the mutated proteins. This classification connects the variety of phenotypes to the mutations and segregation patterns observed in RP. Current progress in the identification of the molecular defects underlying RP reveals that at least three distinct functional mechanisms may be affected: 1) the daily renewal and shedding of the photoreceptor outer segments, 2) the visual transduction cascade, and 3) the retinol (vitamin A) metabolism. The first group includes the rhodopsin and peripherin/RDS genes, and mutations in these genes often result in a dominant phenotype. The second group is predominantly associated with a recessive phenotype that results, as we argue, from continuous inactivation of the transduction pathway. Disturbances in the retinal metabolism seem to be associated with equal rod and cone involvement and the presence of deposits in the retinal pigment epithelium.

Nitric oxide and cyclic GMP modulate photoreceptor cell responses in the visual system of the locust

Nitric oxide (NO) is a membrane-permeant messenger molecule which activates the cyclic GMP (cGMP)-synthesizing enzyme soluble guanylyl cyclase. Using cytochemical techniques, we recently reported NO-induced cGMP immunoreactivity in the photoreceptor cells of the compound eye of the locust Schistocerca gregaria and also detected NADPH diaphorase staining, a marker of NO synthase, in a subset of the monopolar cells of the lamina. By recording the corneal electroretinogram (ERG), we found that the application of neurochemicals that raise NO/cGMP levels in the optic lobe increased the ERG amplitude, whereas the experimental reduction of NO levels caused a decrease in the response to light. An increase in the light response was also found in intracellular recordings after application of a NO donor, suggesting that the NO-induced changes in the ERG are not caused by changes in the resistive isolation of the retina. Our cytochemical and electrophysiological data are both consistent with the hypothesis that NO synthesized in monopolar cells is a retrograde messenger to the presynaptic photoreceptor neurones.

A Ca2+/calmodulin-dependent protein kinase modulates Drosophila photoreceptor K+ currents: a role in shaping the photoreceptor potential

Light activation of Drosophila photoreceptors leads to the generation of a depolarizing receptor potential via opening of transient receptor potential and transient receptor potential-like cationic channels. Counteracting the light-activated depolarizing current are two voltage-gated K+ conductances, IA and IK, that are expressed in these sensory neurons. Here we show that Drosophila photoreceptors IA and IK are regulated by calcium-calmodulin (Ca2+/calmodulin) via a Ca2+/calmodulin-dependent protein kinase (CaM kinase), with IK being far more sensitive than IA. Inhibition of Ca2+/calmodulin by N-(6 aminohexyl)-5-chloro-1-naphthalenesulfonamide or trifluoperazine markedly reduced the K+ current amplitudes. Likewise, inhibition of CaM kinases by KN-93 potently depressed IK and accelerated its C-type inactivation kinetics. The effect of KN-93 was specific because its structurally related but functionally inactive analog KN-92 was totally ineffective. In Drosophila photoreceptor mutant ShKS133, which allows isolation of IK, we demonstrate by current-clamp recording that inhibition of IK by quinidine or tetraethylammonium increased the amplitude of the photoreceptor potential, depressed light adaptation, and slowed down the termination of the light response. Similar results were obtained when CaM kinases were blocked by KN-93. These findings place photoreceptor K+ channels as an additional target for Ca2+/calmodulin and suggest that IK is well suited to act in concert with other components of the signaling machinery to sharpen light response termination and fine tune photoreceptor sensitivity during light adaptation.