Calcium-sensitive regions of GCAP1 as observed by chemical modifications, fluorescence, and EPR spectroscopies

Regulation of retinal membrane guanylyl cyclase (RetGC) by negative calcium feedback and RD3 protein

This article presents a brief overview of the main biochemical and cellular processes involved in regulation of cyclic GMP production in photoreceptors. The main focus is on how the fluctuations of free calcium concentrations in photoreceptors between light and dark regulate the activity of retinal membrane guanylyl cyclase (RetGC) via calcium sensor proteins. The emphasis of the review is on the structure of RetGC and guanylyl cyclase activating proteins (GCAPs) in relation to their functional role in photoreceptors and congenital diseases of photoreceptors. In addition to that, the structure and function of retinal degeneration-3 protein (RD3), which regulates RetGC in a calcium-independent manner, is discussed in detail in connections with its role in photoreceptor biology and inherited retinal blindness.

Neuronal Calcium Sensor GCAP1 Encoded by GUCA1A Exhibits Heterogeneous Functional Properties in Two Cases of Retinitis Pigmentosa

Genetic heterogeneity leading to retinal disorders impairs biological processes by causing, for example, severe disorder of signal transduction in photoreceptor outer segments. A normal balance of the second messenger homeostasis in photoreceptor cells seems to be a crucial factor for healthy and normal photoreceptor function. Genes like GUCY2D coding for guanylate cyclase GC-E and GUCA1A coding for the Ca²⁺-sensor guanylate cyclase-activating protein GCAP1 are critical for a precisely controlled synthesis of the second messenger cGMP. Mutations in GUCA1A frequently correlate in patients with cone dystrophy and cone-rod dystrophy. Here, we report two mutations in the GUCA1A gene that were found in patients diagnosed with retinitis pigmentosa, a phenotype that was rarely detected among previous cases of GUCA1A related retinopathies. One patient was heterozygous for the missense variant c.55C > T (p.H19Y), while the other patient was heterozygous for the missense variant c.479T > G (p.V160G). Using heterologous expression and cell culture systems, we examined the functional and molecular consequences of these point mutations. Both variants showed a dysregulation of guanylate cyclase activity, either a profound shift in Ca²⁺-sensitivity (H19Y) or a nearly complete loss of activating potency (V160G). Functional heterogeneity became also apparent in Ca²⁺/Mg²⁺-binding properties and protein conformational dynamics. A faster progression of retinal dystrophy in the patient carrying the V160G mutation seems to correlate with the more severe impairment of this variant.

Mapping Calcium-Sensitive Regions in GCAPs by Site-Specific Fluorescence Labelling

Signal transduction processes that are under control of changes in cytoplasmic Ca²⁺-concentration involve Ca²⁺-sensor proteins, which often undergo pronounced conformational transitions triggered by Ca²⁺. Consequences of conformational changes can be the structural rearrangement of single amino acids, exposition of small patches of several amino acids, or the movement of whole protein regions or domains. Furthermore, these conformational changes can lead to the exposure or movement of posttranslationally attached acyl groups. These processes could then control the function of target proteins, for example, by Ca²⁺-dependent protein-protein interaction. Fluorescence spectroscopy allows for mapping these Ca²⁺-sensitive regions but needs site-specific fluorescence labelling. We describe the application of a new group of diaminoterephthalate-derived fluorescence probes targeting either cysteines in guanylate cyclase-activating proteins, named GCAPs, or azide moieties in covalently attached acyl groups. By monitoring Ca²⁺-dependent changes in fluorescence emission, we identify Ca²⁺-sensitive protein regions in GCAPs and correlate conformational changes to protein function.

A G86R mutation in the calcium-sensor protein GCAP1 alters regulation of retinal guanylyl cyclase and causes dominant cone-rod degeneration

The guanylyl cyclase-activating protein, GCAP1, activates photoreceptor membrane guanylyl cyclase (RetGC) in the light, when free Ca²⁺ concentrations decline, and decelerates the cyclase in the dark, when Ca²⁺ concentrations rise. Here, we report a novel mutation, G86R, in the GCAP1 (GUCA1A) gene in a family with a dominant retinopathy. The G86R substitution in a "hinge" region connecting EF-hand domains 2 and 3 in GCAP1 strongly interfered with its Ca²⁺-dependent activator-to-inhibitor conformational transition. The G86R-GCAP1 variant activated RetGC at low Ca²⁺ concentrations with higher affinity than did the WT GCAP1, but failed to decelerate the cyclase at the Ca²⁺ concentrations characteristic of dark-adapted photoreceptors. Ca²⁺-dependent increase in Trp⁹⁴ fluorescence, indicative of the GCAP1 transition to its RetGC inhibiting state, was suppressed and shifted to a higher Ca²⁺ range. Conformational changes in G86R GCAP1 detectable by isothermal titration calorimetry (ITC) also became less sensitive to Ca²⁺, and the dose dependence of the G86R GCAP1-RetGC1 complex inhibition by retinal degeneration 3 (RD3) protein was shifted toward higher than normal concentrations. Our results indicate that the flexibility of the hinge region between EF-hands 2 and 3 is required for placing GCAP1-regulated Ca²⁺ sensitivity of the cyclase within the physiological range of intracellular Ca²⁺ at the expense of reducing GCAP1 affinity for the target enzyme. The disease-linked mutation of the hinge Gly⁸⁶, leading to abnormally high affinity for the target enzyme and reduced Ca²⁺ sensitivity of GCAP1, is predicted to abnormally elevate cGMP production and Ca²⁺ influx in photoreceptors in the dark.

Differential Nanosecond Protein Dynamics in Homologous Calcium Sensors

Shaping the temporal response of photoreceptors is facilitated by a well-balanced second messenger cascade, in which two neuronal Ca(2+)-sensor proteins operate in a sequential relay mechanism. Although they share structurally similar sensing units, they differentially activate the same target protein. Here, as a prototypical case in Ca(2+)-mediated signal processing, we investigate differential cellular responsiveness in protein conformational dynamics on a nanosecond time scale. For this, we have site-specifically labeled cysteine residues in guanylate cyclase-activating protein GCAP1 by the fluorescent dye Alexa647 and probed its local environment via time-resolved fluorescence spectroscopy. Fluorescence lifetime and rotational anisotropy measurements reveal a distinct structural movement of the polypeptide chain around position 106 upon release of Ca(2+). This is supported by analyzing the diffusional dye motion in a wobbling-in-a-cone model and by molecular dynamics simulations. We conclude that GCAP1 and its cellular cognate GCAP2 operate by distinctly different switching mechanisms despite their high structural homology.

Retina specific GCAPs in zebrafish acquire functional selectivity in Ca2+-sensing by myristoylation and Mg2+-binding

Zebrafish photoreceptor cells express six guanylate cyclase-activating proteins (zGCAPs) that share a high degree of amino acid sequence homology, but differ in Ca(2+)-binding properties, Ca(2+)-sensitive target regulation and spatial-temporal expression profiles. We here study a general problem in cellular Ca(2+)-sensing, namely how similar Ca(2+)-binding proteins achieve functional selectivity to control finely adjusted cellular responses. We investigated two parameters of critical importance for the trigger and switch function of guanylate cyclase-activating proteins: the myristoylation status and the occupation of Ca(2+)-binding sites with Mg(2+). All zGCAPs can be myristoylated in living cells using click chemistry. Myristoylation does not facilitate membrane binding of zGCAPs, but it significantly modified the regulatory properties of zGCAP2 and zGCAP5. We further determined for all zGCAPs at least two binding sites exhibiting high affinities for Ca(2+) with KD values in the submicromolar range, whereas for other zGCAPs (except zGCAP3) the affinity of the third binding site was in the micromolar range. Mg(2+) either occupied the low affinity Ca(2+)-binding site or it shifted the affinities for Ca(2+)-binding. Hydrodynamic properties of zGCAPs are more influenced by Ca(2+) than by Mg(2+), although to a different extent for each zGCAP. Posttranslational modification and competing ion-binding can tailor the properties of similar Ca(2+)-sensors.

Identification of target binding site in photoreceptor guanylyl cyclase-activating protein 1 (GCAP1)

Retinal guanylyl cyclase (RetGC)-activating proteins (GCAPs) regulate visual photoresponse and trigger congenital retinal diseases in humans, but GCAP interaction with its target enzyme remains obscure. We mapped GCAP1 residues comprising the RetGC1 binding site by mutagenizing the entire surface of GCAP1 and testing the ability of each mutant to bind RetGC1 in a cell-based assay and to activate it in vitro. Mutations that most strongly affected the activation of RetGC1 localized to a distinct patch formed by the surface of non-metal-binding EF-hand 1, the loop and the exiting helix of EF-hand 2, and the entering helix of EF-hand 3. Mutations in the binding patch completely blocked activation of the cyclase without affecting Ca(2+) binding stoichiometry of GCAP1 or its tertiary fold. Exposed residues in the C-terminal portion of GCAP1, including EF-hand 4 and the helix connecting it with the N-terminal lobe of GCAP1, are not critical for activation of the cyclase. GCAP1 mutants that failed to activate RetGC1 in vitro were GFP-tagged and co-expressed in HEK293 cells with mOrange-tagged RetGC1 to test their direct binding in cyto. Most of the GCAP1 mutations introduced into the "binding patch" prevented co-localization with RetGC1, except for Met-26, Lys-85, and Trp-94. With these residues mutated, GCAP1 completely failed to stimulate cyclase activity but still bound RetGC1 and competed with the wild type GCAP1. Thus, RetGC1 activation by GCAP1 involves establishing a tight complex through the binding patch with an additional activation step involving Met-26, Lys-85, and Trp-94.

Structural diversity of neuronal calcium sensor proteins and insights for activation of retinal guanylyl cyclase by GCAP1

Neuronal calcium sensor (NCS) proteins, a sub-branch of the calmodulin superfamily, are expressed in the brain and retina where they transduce calcium signals and are genetically linked to degenerative diseases. The amino acid sequences of NCS proteins are highly conserved but their physiological functions are quite different. Retinal recoverin controls Ca²⁺-dependent inactivation of light-excited rhodopsin during phototransduction, guanylyl cyclase activating proteins 1 and 2 (GCAP1 and GCAP2) promote Ca²⁺-dependent activation of retinal guanylyl cyclases, and neuronal frequenin (NCS-1) modulates synaptic activity and neuronal secretion. Here we review the molecular structures of myristoylated forms of NCS-1, recoverin, and GCAP1 that all look very different, suggesting that the attached myristoyl group helps to refold these highly homologous proteins into different three-dimensional folds. Ca²⁺-binding to both recoverin and NCS-1 cause large protein conformational changes that ejects the covalently attached myristoyl group into the solvent exterior and promotes membrane targeting (Ca²⁺-myristoyl switch). The GCAP proteins undergo much smaller Ca²⁺-induced conformational changes and do not possess a Ca²⁺-myristoyl switch. Recent structures of GCAP1 in both its activator and Ca²⁺-bound inhibitory states will be discussed to understand structural determinants that control their Ca²⁺-dependent activation of retinal guanylyl cyclases.

Structural insights for activation of retinal guanylate cyclase by GCAP1

Guanylyl cyclase activating protein 1 (GCAP1), a member of the neuronal calcium sensor (NCS) subclass of the calmodulin superfamily, confers Ca(2+)-sensitive activation of retinal guanylyl cyclase 1 (RetGC1) upon light activation of photoreceptor cells. Here we present NMR assignments and functional analysis to probe Ca(2+)-dependent structural changes in GCAP1 that control activation of RetGC. NMR assignments were obtained for both the Ca(2+)-saturated inhibitory state of GCAP1 versus a GCAP1 mutant (D144N/D148G, called EF4mut), which lacks Ca(2+) binding in EF-hand 4 and models the Ca(2+)-free/Mg(2+)-bound activator state of GCAP1. NMR chemical shifts of backbone resonances for Ca(2+)-saturated wild type GCAP1 are overall similar to those of EF4mut, suggesting a similar main chain structure for assigned residues in both the Ca(2+)-free activator and Ca(2+)-bound inhibitor states. This contrasts with large Ca(2+)-induced chemical shift differences and hence dramatic structural changes seen for other NCS proteins including recoverin and NCS-1. The largest chemical shift differences between GCAP1 and EF4mut are seen for residues in EF4 (S141, K142, V145, N146, G147, G149, E150, L153, E154, M157, E158, Q161, L166), but mutagenesis of EF4 residues (F140A, K142D, L153R, L166R) had little effect on RetGC1 activation. A few GCAP1 residues in EF-hand 1 (K23, T27, G32) also show large chemical shift differences, and two of the mutations (K23D and G32N) each decrease the activation of RetGC, consistent with a functional conformational change in EF1. GCAP1 residues at the domain interface (V77, A78, L82) have NMR resonances that are exchange broadened, suggesting these residues may be conformationally dynamic, consistent with previous studies showing these residues are in a region essential for activating RetGC1.

Probing the Ca(2+) switch of the neuronal Ca(2+) sensor GCAP2 by time-resolved fluorescence spectroscopy

We report fluorescence lifetime and rotational anisotropy measurements of the fluorescent dye Alexa647 attached to the guanylate cyclase-activating protein 2 (GCAP2), an intracellular myristoylated calcium sensor protein operating in photoreceptor cells. By linking the dye to different protein regions critical for monitoring calcium-induced conformational changes, we could measure fluorescence lifetimes and rotational correlation times as a function of myristoylation, calcium, and position of the attached dye, while GCAP2 was still able to regulate guanylate cyclase in a Ca(2+)-sensitive manner. We observe distinct site-specific variations in the fluorescence dynamics when externally changing the protein conformation. A clear reduction in fluorescence lifetime suggests that in the calcium-free state a dye marker in amino acid position 131 senses a more hydrophobic protein environment than in position 111. Saturating GCAP2 with calcium increases the fluorescence lifetime and hence leads to larger exposure of position 111 to the solvent and at the same time to a movement of position 131 into a hydrophobic protein cleft. In addition, we find distinct, biexponential anisotropy decays reflecting the reorientational motion of the fluorophore dipole and the dye/protein complex, respectively. Our experimental data are well described by a "wobbling-in-a-cone" model and reveal that for dye markers in position 111 of the GCAP2 protein both addition of calcium and myristoylation results in a pronounced increase in orientational flexibility of the fluorophore. Our results provide evidence that the up-and-down movement of an α-helix that is situated between position 111 and 131 is a key feature of the dynamics of the protein-dye complex. Operation of this piston-like movement is triggered by the intracellular messenger calcium.

Calcium-myristoyl Tug is a new mechanism for intramolecular tuning of calcium sensitivity and target enzyme interaction for guanylyl cyclase-activating protein 1: dynamic connection between N-fatty acyl group and EF-hand controls calcium sensitivity

Guanylyl cyclase-activating protein 1 (GCAP1), a myristoylated Ca(2+) sensor in vision, regulates retinal guanylyl cyclase (RetGC). We show that protein-myristoyl group interactions control Ca(2+) sensitivity, apparent affinity for RetGC, and maximal level of cyclase activation. Mutating residues near the myristoyl moiety affected the affinity of Ca(2+) binding to EF-hand 4. Inserting Phe residues in the cavity around the myristoyl group increased both the affinity of GCAP1 for RetGC and maximal activation of the cyclase. NMR spectra show that the myristoyl group in the L80F/L176F/V180F mutant remained sequestered inside GCAP1 in both Ca(2+)-bound and Mg(2+)-bound states. This mutant displayed much higher affinity for the cyclase but reduced Ca(2+) sensitivity of the cyclase regulation. The L176F substitution improved affinity of myristoylated and non-acylated GCAP1 for the cyclase but simultaneously reduced the affinity of Ca(2+) binding to EF-hand 4 and Ca(2+) sensitivity of the cyclase regulation by acylated GCAP1. The replacement of amino acids near both ends of the myristoyl moiety (Leu(80) and Val(180)) minimally affected regulatory properties of GCAP1. N-Lauryl- and N-myristoyl-GCAP1 activated RetGC in a similar fashion. Thus, protein interactions with the central region of the fatty acyl chain optimize GCAP1 binding to RetGC and maximize activation of the cyclase. We propose a dynamic connection (or "tug") between the fatty acyl group and EF-hand 4 via the C-terminal helix that attenuates the efficiency of RetGC activation in exchange for optimal Ca(2+) sensitivity.

Biophysical investigation of retinal calcium sensor function

BACKGROUND

Neuronal calcium sensor proteins represent a subgroup of the family of EF-hand calcium binding proteins. Members of this subgroup are the guanylate cyclase-activating proteins and recoverin, which operate as important calcium sensors in retinal photoreceptor cells. Physiological and biochemical data indicate that these proteins participate in shaping the photoreceptor light response.

SCOPE OF REVIEW

Biophysical methods have been widely applied to investigate the molecular properties of retinal calcium binding proteins like the guanylate cyclase-activating proteins and recoverin. Properties include the determination of calcium affinities by isotope techniques and spectroscopical approaches. Conformational changes are investigated for example by tryptophan fluorescence emission. A special focus of this review is laid on a new experimental approach to study conformational changes in calcium binding proteins by surface plasmon resonance spectroscopy. In addition this technique has been employed for measuring the calcium-dependent binding of calcium sensors to membranes.

MAJOR CONCLUSIONS

Biophysical approaches provide valuable information about key properties of calcium sensor proteins involved in intracellular signalling. Parameters of their molecular properties like calcium binding and conformational changes help to define their physiological role derived from cellular, genetic or physiological studies.

GENERAL SIGNIFICANCE

Calcium is an important second messenger in intracellular signaling. Calcium signals are propagated via calcium binding proteins that are able to discriminate between incremental differences in intracellular calcium and that regulate their targets with high precision and specificity. This article is part of a Special Issue entitled Biochemical, biophysical and genetic approaches to intracellular calcium signalling.

Between promiscuity and specificity: novel roles of EF-hand calcium sensors in neuronal Ca2+ signalling

In recent years, substantial progress has been made towards an understanding of the physiological function of EF-hand calcium sensor proteins of the Calmodulin (CaM) superfamily in neurons. This deeper appreciation is based on the identification of novel target interactions, structural studies and the discovery of novel signalling mechanisms in protein trafficking and synaptic plasticity, in which CaM-like sensor proteins appear to play a role. However, not all interactions are of plausible physiological relevance and in many cases it is not yet clear how the CaM signaling network relates to the proposed function of other EF-hand sensors. In this review, we will summarize these findings and address some of the open questions on the functional role of EF-hand calcium binding proteins in neurons.

Mg2+/Ca2+ cation binding cycle of guanylyl cyclase activating proteins (GCAPs): role in regulation of photoreceptor guanylyl cyclase

Photon absorption by photoreceptors activates hydrolysis of cGMP, which shuts down cGMP-gated channels and decreases free Ca(2+) concentrations in outer segment. Suppression of Ca(2+) influx through the cGMP channel by light activates retinal guanylyl cyclase through guanylyl cyclase activating proteins (GCAPs) and thus expedites photoreceptors recovery from excitation and restores their light sensitivity. GCAP1 and GCAP2, two ubiquitous among vertebrate species isoforms of GCAPs that activate retGC during rod response to light, are myristoylated Ca(2+)/Mg(2+)-binding proteins of the EF-hand superfamily. They consist of one non-metal binding EF-hand-like domain and three other EF-hands, each capable of binding Ca(2+) and Mg(2+). In the metal binding EF-hands of GCAP1, different point mutations can selectively block binding of Ca(2+) or both Ca(2+) and Mg(2+) altogether. Activation of retGC at low Ca(2+) (light adaptation) or its inhibition at high Ca(2+) (dark adaptation) follows a cycle of Ca(2+)/Mg(2+) exchange in GCAPs, rather than release of Ca(2+) and its binding by apo-GCAPs. The Mg(2+) binding in two of the EF-hands controls docking of GCAP1 with retGC1 in the conditions of light adaptation and is essential for activation of retGC. Mg(2+) binding in a C-terminal EF-hand contributes to neither retGC1 docking with the cyclase nor its subsequent activation in the light, but is specifically required for switching the cyclase off in the conditions of dark adaptation by binding Ca(2+). The Mg(2+)/Ca(2+) exchange in GCAP1 and 2 operates within different range of intracellular Ca(2+) concentrations and provides a two-step activation of the cyclase during rod recovery.

High cGMP synthetic activity in carp cones

Cones show briefer light responses than rods and do not saturate even under very bright light. Using purified rod and cone homogenates, we measured the activity of guanylate cyclase (GC), an enzyme responsible for cGMP synthesis and therefore recovery of a light response. The basal GC activity was 36 times higher in cones than in rods: It was mainly caused by higher expression levels of GC in cones (GC-C) than in rods (GC-R). With identification and quantification of GC-activating protein (GCAP) subtypes expressed in rods and cones together with determination of kinetic parameters of GC activation in the presence and absence of GCAP, we estimated the in situ GC activity in rods and cones at low and high Ca(2+) concentrations. It was revealed that the GC activity would be >10 times higher in cones than in rods in both the dark-adapted and the light-adapted states. Electrophysiological estimation of the GC activity measured in the truncated preparations of rod and cone outer segments gave consistent results. Our estimation of the in situ GC activity reasonably explained the rapid recovery and nonsaturating behavior of cone light responses.

Binding of guanylyl cyclase activating protein 1 (GCAP1) to retinal guanylyl cyclase (RetGC1). The role of individual EF-hands

Guanylyl cyclase activating protein 1 (GCAP1), after substitution of Ca(2+) by Mg(2+) in its EF-hands, stimulates photoreceptor guanylyl cyclase, RetGC1, in response to light. We inactivated metal binding in individual EF-hands of GCAP1 tagged with green fluorescent protein to assess their role in GCAP1 binding to RetGC1 in co-transfected HEK293 cells. When expressed alone, GCAP1 was uniformly distributed throughout the cytoplasm and the nuclei of the cells, but when co-expressed with either fluorescently tagged or non-tagged RetGC1, it co-localized with the cyclase in the membranes. The co-localization did not occur when the C-terminal portion of RetGC1, containing its regulatory and catalytic domains, was removed. Mutations that preserved Mg(2+) binding in all three metal-binding EF-hands did not affect GCAP1 association with the cyclase in live cells. Locking EF-hand 4 in its apo-conformation, incapable of binding either Ca(2+) or Mg(2+), had no effect on GCAP1 association with the cyclase. In contrast to EF-hand 4, inactivation of EF-hand 3 reduced the efficiency of the co-localization, and inactivation of EF-hand 2 drastically suppressed GCAP1 binding to the cyclase. These results directly demonstrate that metal binding in EF-hand 2 is crucial for GCAP1 attachment to RetGC1, and that in EF-hand 3 it is less critical, although it enhances the efficiency of the GCAP1 docking on the target enzyme. Metal binding in EF-hand 4 has no role in the primary attachment of GCAP1 to the cyclase, and it only triggers the activator-to-inhibitor functional switch in GCAP1.

Activation and inhibition of photoreceptor guanylyl cyclase by guanylyl cyclase activating protein 1 (GCAP-1): the functional role of Mg2+/Ca2+ exchange in EF-hand domains

Guanylyl cyclase activating protein 1 (GCAP-1), a Ca(2+)/Mg(2+) sensor protein that accelerates retinal guanylyl cyclase (RetGC) in the light and decelerates it in the dark, is inactive in cation-free form. Binding of Mg(2+) in EF-hands 2 and 3 was essential for RetGC activation in the conditions mimicking light adaptation. Mg(2+) binding in EF-hand 2 affected the conformation of a neighboring non-metal binding domain, EF-hand-1, and increased GCAP-1 affinity for RetGC nearly 40-fold compared with the metal-free EF-hand 2. Mg(2+) binding in EF-hand 3 increased GCAP-1 affinity for RetGC 5-fold and its maximal RetGC stimulation 2-fold. Mg(2+) binding in EF-hand 4 affected neither GCAP-1 affinity for RetGC, nor RetGC activation. Inactivation of Ca(2+) binding in EF-hand 4 was sufficient to render GCAP-1 a constitutive activator of RetGC, whereas the EF-hand 3 role in Ca(2+)-dependent deceleration of RetGC was likely to be through the neighboring EF-hand 4. Inactivation of Ca(2+) binding in EF-hand 2 affected cooperativity of RetGC inhibition by Ca(2+), but did not prevent the inhibition. We conclude that 1) Mg(2+) binding in EF-hands 2 and 3, but not EF-hand 4, is essential for the ability of GCAP-1 to activate RetGC in the light; 2) Mg(2+) or Ca(2+) binding in EF-hand 3 and especially in EF-hand 2 is required for high-affinity interaction with the cyclase and affects the conformation of the neighboring EF-hand 1, a domain required for targeting RetGC; and 3) RetGC inhibition is likely to be primarily caused by Ca(2+) binding in EF-hand 4.

Calcium-dependent conformational changes in guanylate cyclase-activating protein 2 monitored by cysteine accessibility

Guanylate cyclase-activating protein 2 (GCAP2) is expressed in vertebrate photoreceptors cells where it regulates the activity of membrane bound guanylate cyclases in a Ca(2+)-dependent manner. The essential trigger step involves a Ca(2+)-induced conformational change in GCAP2. We investigated these Ca(2+)-dependent changes by probing the cysteine accessibility in wild type and mutant GCAP2 forms with the thiol-modifying reagent 5,5'-dithio-bis-(2-nitrobenzoic acid) (DTNB). Cysteine residues in position 35 and 111 displayed a restricted accessibility in the presence of Ca(2+), whereas cysteine in position 131 reacted with DTNB in the presence and absence of Ca(2+). Our data indicate that the Ca(2+)-sensitivity of GCAP2 is significantly controlled by its third Ca(2+)-binding site, EF-hand 3.

Ca2+ and Mg2+ Binding Properties of GCAP-1

Guanylyl cyclase-activating protein 1 (GCAP-1) is an EF-hand protein that activates retinal guanylyl cyclase (RetGC) in photoreceptors at low free Ca2+ in the light and inhibits it in the dark when Ca2+ concentrations rise. We present the first direct evidence that Mg2+-bound form of GCAP-1, not its cation-free form, is the true activator of RetGC-1 under physiological conditions. Of four EF-hand structures in GCAP-1, three bound Ca2+ ions and could exchange Ca2+ for Mg2+. At concentrations of free Ca2+ and Mg2+ typical for the light-adapted photoreceptors, all three metal-binding EF-hands were predominantly occupied by Mg2, and the presence of bound Mg2+ in GCAP-1 was essential for its ability to stimulate RetGC-1. In the Mg2+-bound form of GCAP-1 all three Trp residues became more exposed to the polar environment compared with its apo form. The replacement of Mg2+ by Ca2+ in the EF-hands 2 and 3 further exposed Trp-21 to the solution in a non-metal-binding EF-hand domain 1 that interacts with RetGC. Contrary to that, replacement of Mg2+ by Ca2+ in the EF-hand 4 moved Trp-94 in the entering alpha-helix of the EF-hand 3 back to the non-polar environment. Our results demonstrate that Mg2+ regulates GCAP-1 not only by adjusting its Ca2+ sensitivity to the physiological conditions in photoreceptors but also by creating the conformation required for RetGC stimulation.

The crystal structure of GCAP3 suggests molecular mechanism of GCAP-linked cone dystrophies

Absorption of light by visual pigments initiates the phototransduction pathway that results in degradation of the intracellular pool of cyclic-GMP (cGMP). This hydrolysis promotes the closing of cGMP-gated cation channels and consequent hyperpolarization of rod and cone photoreceptor cell membranes. Guanylate cyclase-activating proteins (GCAPs) are a family of proteins that regulate retinal guanylate cyclase (GC) activity in a Ca2+-dependent manner. At high [Ca2+], typical of the dark-adapted state (approximately 500 nM), GCAPs inhibit retinal GCs. At the low [Ca2+] (approximately 50 nM) that occurs after the closing of cGMP-gated channels, GCAPs activate retinal GCs to replenish dark-state cGMP levels. Here, we report the crystal structure of unmyristoylated human GCAP3 with Ca2+ bound. GCAP3 is an EF-hand Ca2+-binding protein with Ca2+ bound to EF2, 3 and 4, while Ca2+ binding to EF-hand 1 is disabled. GCAP3 contains two domains with the EF-hand motifs arranged in a tandem array similar to GCAP2 and members of the recoverin subfamily of Ca2+-binding proteins. Residues not involved in Ca2+ binding, but conserved in all GCAPs, cluster around EF1 in the N-terminal domain and may represent the interface with GCs. Five point mutations in the closely related GCAP1 have been linked to the etiology of cone dystrophies. These residues are conserved in GCAP3 and the structure suggests important roles for these amino acids. We present a homology model of GCAP1 based on GCAP3 that offers insight into the molecular mechanism underlying the autosomal dominant cone dystrophies produced by GCAP1 mutations.

A novel GCAP1 missense mutation (L151F) in a large family with autosomal dominant cone-rod dystrophy (adCORD)

PURPOSE

To elucidate the phenotypic and biochemical characteristics of a novel mutation associated with autosomal dominant cone-rod dystrophy (adCORD).

METHODS

Twenty-three family members of a CORD pedigree underwent clinical examinations, including visual acuity tests, standardized full-field ERG, and fundus photography. Genomic DNA was screened for mutations in GCAP1 exons using DNA sequencing and single-strand conformational polymorphism (SSCP) analysis. Function and stability of recombinant GCAP1-L151F were tested as a function of [Ca(2+)], and its structure was probed by molecular dynamics.

RESULTS

Affected family members experienced dyschromatopsia, hemeralopia, and reduced visual acuity by the second to third decade of life. Electrophysiology revealed a nonrecordable photopic response with later attenuation of the scotopic response. Affected family members harbored a C-->T transition in exon 4 of the GCAP1 gene, resulting in an L151F missense mutation affecting the EF hand motif 4 (EF4). This change was absent in 11 unaffected family members and in 100 unrelated normal subjects. GCAP1-L151F stimulation of photoreceptor guanylate cyclase was not completely inhibited at high physiological [Ca(2+)], consistent with a lowered affinity for Ca(2+)-binding to EF4.

CONCLUSIONS

A novel L151F mutation in the EF4 hand domain of GCAP1 is associated with adCORD. The clinical phenotype is characterized by early cone dysfunction and a progressive loss of rod function. The biochemical phenotype is best described as persistent stimulation of photoreceptor guanylate cyclase, representing a gain of function of mutant GCAP1. Although a conservative substitution, molecular dynamics suggests a significant change in Ca(2+)-binding to EF4 and EF2 and changes in the shape of L151F-GCAP1.

Diversity of guanylate cyclase-activating proteins (GCAPs) in teleost fish: characterization of three novel GCAPs (GCAP4, GCAP5, GCAP7) from zebrafish (Danio rerio) and prediction of eight GCAPs (GCAP1-8) in pufferfish (Fugu rubripes)

The guanylate cyclase-activating proteins (GCAPs) are Ca(2+)-binding proteins of the calmodulin (CaM) gene superfamily that function in the regulation of photoreceptor guanylate cyclases (GCs). In the mammalian retina, two GCAPs (GCAP 1-2) and two transmembrane GCs have been identified as part of a complex regulatory system responsive to fluctuating levels of free Ca(2+). A third GCAP, GCAP3, is expressed in human and zebrafish (Danio rerio) retinas, and a guanylate cyclase-inhibitory protein (GCIP) has been shown to be present in frog cones. To explore the diversity of GCAPs in more detail, we searched the pufferfish (Fugu rubripes) and zebrafish (Danio rerio) genomes for GCAP-related gene sequences (fuGCAPs and zGCAPs, respectively) and found that at least five additional GCAPs (GCAP4-8) are predicted to be present in these species. We identified genomic contigs encoding fuGCAPl-8, fuGCIP, zGCAPl-5, zGCAP7 and zGCIP. We describe cloning, expression and localization of three novel GCAPs present in the zebrafish retina (zGCAP4, zGCAP5, and zGCAP7). The results show that recombinant zGCAP4 stimulated bovine rod outer segment GC in a Ca(2+)-dependent manner. RT-PCR with zGCAP specific primers showed specific expression of zGCAPs and zGCIP in the retina, while zGCAPl mRNA is also present in the brain. In situ hybridization with anti-sense zGCAP4, zGCAP5 and zGCAP7 RNA showed exclusive expression in zebrafish cone photoreceptors. The presence of at least eight GCAP genes suggests an unexpected diversity within this subfamily of Ca(2+)-binding proteins in the teleost retina, and suggests additional functions for GCAPs apart from stimulation of GC. Based on genome searches and EST analyses, the mouse and human genomes do not harbor GCAP4-8 or GCIP genes.

A novel mutation (I143NT) in guanylate cyclase-activating protein 1 (GCAP1) associated with autosomal dominant cone degeneration

PURPOSE

To identify pathogenic mutations in the guanylate cyclase-activating protein 1 (GCAP1) and GCAP2 genes and to characterize the biochemical effect of mutation on guanylate cyclase (GC) stimulation.

METHODS

The GCAP1 and GCAP2 genes were screened by direct sequencing for mutations in 216 patients and 421 patients, respectively, with various hereditary retinal diseases. A mutation in GCAP1 segregating with autosomal dominant cone degeneration was further evaluated biochemically by employing recombinant proteins, immunoblotting, Ca2+-dependent stimulation of GC, fluorescence emission spectra, and limited proteolysis in the absence and presence of Ca2+.

RESULTS

A novel GCAP1 mutation, I143NT (substitution of Ile at codon 143 by Asn and Thr), affecting the EF4 Ca2+-binding loop, was identified in a heterozygote father and son with autosomal dominant cone degeneration. Both patients had much greater loss of cone function versus rod function; previous histopathologic evaluation of the father's eyes at autopsy (age 75 years) showed no foveal cones but a few, scattered cones remaining in the peripheral retina. Biochemical analysis showed that the GCAP1-I143NT mutant adopted a conformation susceptible to proteolysis, and the mutant inhibited GC only partially at high Ca2+ concentrations. Individual patients with atypical or recessive retinitis pigmentosa (RP) had additional heterozygous GCAP1-T114I and GCAP2 gene changes (V85M and F150C) of unknown pathogenicity.

CONCLUSIONS

A novel GCAP1 mutation, I143NT, caused a form of autosomal dominant cone degeneration that destroys foveal cones by mid-life but spares some cones in the peripheral retina up to 75 years. Properties of the GCAP1-I143NT mutant protein suggested that it is incompletely inactivated by high Ca2+ concentrations as should occur with dark adaptation. The continued activity of the mutant GCAP1 likely results in higher-than-normal scotopic cGMP levels which may, in turn, account for the progressive loss of cones.

Guanylate cyclase-activating proteins: structure, function, and diversity

The guanylate cyclase-activating proteins (GCAPs), Ca2+-binding proteins of the calmodulin gene superfamily, function as regulators of photoreceptor guanylate cyclases. In contrast to calmodulin, which is active in the Ca2+-bound form, GCAPs stimulate GCs in the [Ca2+]-free form and inhibit GCs upon Ca2+ binding. In vertebrate retinas, at least two GCAP1 and two GCs are present, a third GCAP3 is expressed in humans and fish, and at least five additional GCAP4-8 genes have been identified or are predicted in zebrafish and pufferfish. Missense mutations in GCAP1 (Y99C, I143NT, E155G, and P50L) have been associated with autosomal dominant cone dystrophy. Absence of GCAP1/2 in mice delays recovery of the photoresponse, a phenotype consistent with delay in cGMP synthesis. In the absence of GCAP2, GCAP1 supports the generation of wild-type flash responses in both rod and cone cells. Recent progress revealed an unexpected complexity of the GC-GCAP system, pointing, out a number of unsolved questions.

Irregular dimerization of guanylate cyclase-activating protein 1 mutants causes loss of target activation

Guanylate cyclase-activating proteins (GCAPs) are neuronal calcium sensors that activate membrane bound guanylate cyclases (EC 4.6.1.2.) of vertebrate photoreceptor cells when cytoplasmic Ca2+ decreases during illumination. GCAPs contain four EF-hand Ca2+-binding motifs, but the first EF-hand is nonfunctional. It was concluded that for GCAP-2, the loss of Ca2+-binding ability of EF-hand 1 resulted in a region that is crucial for targeting guanylate cyclase [Ermilov, A.N., Olshevskaya, E.V. & Dizhoor, A.M. (2001) J. Biol. Chem.276, 48143-48148]. In this study we tested the consequences of mutations in EF-hand 1 of GCAP-1 with respect to Ca2+ binding, Ca2+-induced conformational changes and target activation. When the nonfunctional first EF-hand in GCAP-1 is replaced by a functional EF-hand the chimeric mutant CaM-GCAP-1 bound four Ca2+ and showed similar Ca2+-dependent changes in tryptophan fluorescence as the wild-type. CaM-GCAP-1 neither activated nor interacted with guanylate cyclase. Size exclusion chromatography revealed that the mutant tended to form inactive dimers instead of active monomers like the wild-type. Critical amino acids in EF-hand 1 of GCAP-1 are cysteine at position 29 and proline at position 30, as changing these to glycine was sufficient to cause loss of target activation without a loss of Ca2+-induced conformational changes. The latter mutation also promoted dimerization of the protein. Our results show that EF-hand 1 in wild-type GCAP-1 is critical for providing the correct conformation for target activation.

Ca(2+)-dependent conformational changes in guanylyl cyclase-activating protein 2 (GCAP-2) revealed by site-specific phosphorylation and partial proteolysis

Guanylyl cyclase-activating proteins (GCAPs) are calcium sensor proteins of the EF-hand superfamily that inhibit retinal photoreceptor membrane guanylyl cyclase (retGC) in the dark when they bind Ca(2+) but activate retGC when Ca(2+) dissociates from GCAPs in response to light stimulus. We addressed the difference in exposure of GCAP-2 structure to protein kinase and a protease as indicators of conformational change caused by binding and release of Ca(2+). We have found that unlike its homolog, GCAP-1, the C terminus of GCAP-2 undergoes phosphorylation by cyclic nucleotide-dependent protein kinases (CNDPK) present in the retinal extract and rapid dephosphorylation by the protein phosphatase PP2C present in the retina. Inactivation of the CNDPK phosphorylation site in GCAP-2 by substitutions S201G or S201D, as well as phosphorylation or thiophosphorylation of Ser(201), had little effect on the ability of GCAP-2 to regulate retGC in reconstituted membranes in vitro. At the same time, Ca(2+) strongly inhibited phosphorylation of the wild-type GCAP-2 by retinal CNDPK but did not affect phosphorylation of a constitutively active Ca(2+)-insensitive GCAP-2 mutant. Partial digestion of purified GCAP-2 with Glu-C protease revealed at least two sites that become exposed or constrained in a Ca(2+)-sensitive manner. The Ca(2+)-dependent conformational changes in GCAP-2 affect the areas around Glu(62) residue in the entering helix of EF-hand 2, the areas proximal to the exiting helix of EF-hand 3, and Glu(136)-Glu (138) between EF-hand 3 and EF-hand 4. These changes also cause the release of the C-terminal Ser(201) from the constraint caused by the Ca(2+)-bound conformation.

Guanylyl cyclase-activating proteins (GCAPs) are Ca2+/Mg2+ sensors: implications for photoreceptor guanylyl cyclase (RetGC) regulation in mammalian photoreceptors

Guanylyl cyclase-activating proteins (GCAP) are EF-hand Ca(2+)-binding proteins that activate photoreceptor guanylyl cyclase (RetGC) in the absence of Ca(2+) and inhibit RetGC in a Ca(2+)-sensitive manner. The reported data for the RetGC inhibition by Ca(2+)/GCAPs in vitro are in disagreement with the free Ca(2+) levels found in mammalian photoreceptors (Woodruff, M. L., Sampath, A. P., Matthews, H. R., Krasnoperova, N. V., Lem, J., and Fain, G. L. (2002) J. Physiol. (Lond.) 542, 843-854). We have found that binding of Mg(2+) dramatically affects both Ca(2+)-dependent conformational changes in GCAP-1 and Ca(2+) sensitivity of RetGC regulation by GCAP-1 and GCAP-2. Lowering free Mg(2+) concentrations ([Mg](f)) from 5.0 mm to 0.5 mm decreases the free Ca(2+) concentration required for half-maximal inhibition of RetGC ([Ca]((1/2))) by recombinant GCAP-1 and GCAP-2 from 1.3 and 0.2 microm to 0.16 and 0.03 microm, respectively. A similar effect of Mg(2+) on Ca(2+) sensitivity of RetGC by endogenous GCAPs was observed in mouse retina. Analysis of the [Ca]((1/2)) changes as a function of [Mg](f) in mouse retina shows that the [Ca]((1/2)) becomes consistent with the range of 23-250 nm free Ca(2+) found in mouse photoreceptors only if the [Mg](f) in the photoreceptors is near 1 mm. Our data demonstrate that GCAPs are Ca(2+)/Mg(2+) sensor proteins. While Ca(2+) binding is essential for cyclase activation and inhibition, Mg(2+) binding to GCAPs is critical for setting the actual dynamic range of RetGC regulation by GCAPs at physiological levels of free Ca(2+).

Guanylate cyclase activating proteins, guanylate cyclase and disease

A range of cone and cone-rod dystrophies (CORD) have been observed in man, caused by mutations in retinal guanylate cyclase 1 (RetGC1) and guanylate cyclase activating protein 1 (GCAP 1). The CORD causing mutations in RetGC1 are located at a mutation "hot spot" within the dimerisation domain, where R838 is the key residue. Three disease causing mutations have been found in human GCAP1, resulting in cone or cone-rod degeneration. All three mutations are dominant in their effect although the mechanism by which the P50L mutation exerts its influence remains unclear although it might act due to a haplo-insufficiency, arising from increased susceptibility to protease activity and increased thermal instability. In contrast, loss of Ca2+ sensitivity appears to be the main cause of the diseased state for the Y99C and E155G mutations. The cone and cone-rod dystrophies that are caused by mutations in RetGC1 or GCAP1 arise from a perturbation of the delicate balance of Ca2+ and cGMP within the photoreceptor cells and it is this disruption that is believed to cause cell death. The diseases caused by mutations in RetGC1 and GCAP1 prominently affect cones, consistent with the higher concentrations of these proteins in cone cells.

GCAPs: Ca2+-sensitive regulators of retGC

Lowered concentration of Ca2+ ions, resulting from illumination of the photoreceptor cell, is the signal for resynthesis of cGMP by retina-specific guanylyl cyclase (retGC). This Ca2+-dependent activation of retGC is mediated by Ca2+-binding proteins named GCAPs (guanylyl cyclase-activating proteins) and contributes to the recovery of photoreceptor cell to the dark state. Three different GCAPs (GCAP1, GCAP2 and GCAP3) are identified in vertebrate retina to date. In this chapter we describe their discovery, methods of purification, properties, and possible modes of action.

Photoreceptor guanylate cyclase variants: cGMP production under control

Changes in the Ca2+ concentration are thought to affect many processes, including signal transduction in a vast number of biological systems. However, only in few cases the molecular mechanisms by which Ca2+ mediates its action are as well understood as in phototransduction. In dark-adapted photoreceptor cells, the equilibrium level of cGMP is maintained by two opposing activities, such as phosphodiesterase (PDE) and guanylate cyclase (GC). Upon absorption of photons, rhodopsin-G-protein-mediated activation of PDE leads to a transient decrease in [cGMP] and subsequently to lowering of [Ca2+]. In turn, lower [Ca2+] increases net production of cGMP by stimulation of GC until dark conditions are re-established. This activation of GC is mediated by Ca2+ -free forms of Ca2+ -binding proteins termed GC-activating proteins (GCAPs). The last decade brought the molecular identification of GCs and GCAPs in the visual system. Recent efforts have been directed toward understanding the properties of GC at the physiological and structural levels. Here, we summarize the recent progress and present a list of topics of ongoing research.

GCAP1 rescues rod photoreceptor response in GCAP1/GCAP2 knockout mice

Visual transduction in retinal photoreceptors operates through a dynamic interplay of two second messengers, Ca(2+) and cGMP. Ca(2+) regulates the activity of guanylate cyclase (GC) and the synthesis of cGMP by acting on a GC-activating protein (GCAP). While this action is critical for rapid termination of the light response, the GCAP responsible has not been identified. To test if GCAP1, one of two GCAPs present in mouse rods, supports the generation of normal flash responses, transgenic mice were generated that express only GCAP1 under the control of the endogenous promoter. Paired flash responses revealed a correlation between the degree of recovery of the rod a-wave and expression levels of GCAP1. In single cell recordings, the majority of the rods generated flash responses that were indistinguishable from wild type. These results demonstrate that GCAP1 at near normal levels supports the generation of wild-type flash responses in the absence of GCAP2.

Soluble fusion proteins between single transmembrane photoreceptor guanylyl cyclases and their activators

Among single-spanning transmembrane receptors (sTMRs), two guanylyl cyclase receptors, GC1 and GC2, are critically important during phototransduction in vertebrate retinal photoreceptor cells. Ca(2+)-free forms of guanylyl cyclase-activating proteins (GCAPs) stimulate GCs intracellularly by a molecular mechanism that is not fully understood. To gain further insight into the mechanism of activation and specificity among these proteins, for the first time, several soluble and active truncated GCs and fusion proteins between intracellular domains of GCs and full-length GCAPs were generated. The GC activity of myristoylated GCAP--(437-1054)GC displayed typical [Ca(2+)] dependence, and was further enhanced by ATP and inhibited by guanylyl cyclase inhibitor protein (GCIP). The myristoyl group of GCAP1 appeared to be critical for the inhibition of GCs at high [Ca(2+)], even without membranes. In contrast, calmodulin (CaM)--(437-1054)GC1 fusion protein was inactive, but could be stimulated by exogenous GCAP1. In a series of experiments, we showed that the activation of GCs by linked GCAPs involved intra- and intermolecular mechanisms. The catalytically productive GCAP1--(437-1054)GC1 complex can dissociate, allowing binding and stimulation of the GC1 fusion protein by free GCAP1. This suggests that the intramolecular interactions within the fusion protein have low affinity and are mimicking the native system. We present evidence that the mechanism of GC activation by GCAPs involves a dimeric form of GCs, involves direct interaction between GCs and GCAPs, and does not require membrane components. Thus, fusion proteins may provide an important advance for further structural studies of photoreceptor GCs and other sTMRs with and without different forms of regulatory proteins.

Characterization of retinal guanylate cyclase-activating protein 3 (GCAP3) from zebrafish to man

Calmodulin-like neuronal Ca2+-binding proteins (NCBPs) are expressed primarily in neurons and contain a combination of four functional and nonfunctional EF-hand Ca2+-binding motifs. The guanylate cyclase-activating proteins 1-3 (GCAP1-3), the best characterized subgroup of NCBPs, function in the regulation of transmembrane guanylate cyclases 1-2 (GC1-2). The pairing of GCAPs and GCs in vivo depends on cell expression. Therefore, we investigated the expression of these genes in retina using in situ hybridization and immunocytochemistry. Our results demonstrate that GCAP1, GCAP2, GC1 and GC2 are expressed in human rod and cone photoreceptors, while GCAP3 is expressed exclusively in cones. As a consequence of extensive modification, the GCAP3 gene is not expressed in mouse retina. However, this lack of evolutionary conservation appears to be restricted to only some species as we cloned all three GCAPs from teleost (zebrafish) retina and localized them to rod cells, short single cones (GCAP1-2), and all subtypes of cones (GCAP3). Furthermore, sequence comparisons and evolutionary trace analysis coupled with functional testing of the different GCAPs allowed us to identify the key conserved residues that are critical for GCAP structure and function, and to define class-specific residues for the NCBP subfamilies.