Molecular cloning and characterization of retinal photoreceptor guanylyl cyclase-activating protein

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+).

Biochemical analysis of a dimerization domain mutation in RetGC-1 associated with dominant cone-rod dystrophy

Mutations in the photoreceptor membrane guanylyl cyclase RetGC-1 have been linked to autosomal dominant cone-rod dystrophy. Three mutations were identified that alter strictly conserved residues within the RetGC-1 dimerization domain, a region predicted to form an amphipathic alpha-helical coil. Here we report on a biochemical characterization of one of the mutations, a substitution of cysteine for arginine at residue 838. We generated this mutation in vitro and measured its catalytic activity and sensitivity to guanylyl cyclase activating protein 1 (GCAP-1) and GCAP-2. The R838C substitution has several effects. It reduces the overall catalytic ability of RetGC-1 and dramatically reduces stimulation by GCAP-2, although GCAP-2 still appears to interact with the protein. The R838C substitution also increases the apparent affinity of RetGC-1 for GCAP-1 and alters the Ca(2+) sensitivity of the GCAP-1 response, allowing the mutant to be stimulated by GCAP-1 at higher Ca(2+) concentrations than wild type. The diminished response to GCAP-2, which we propose is not likely the cause of cone-rod degeneration in these patients, is interesting mechanistically because it separates the ability to bind a specific GCAP from the ability to be stimulated by it, and it also discriminates between the mechanisms of activation of GCAP-1 vs. GCAP-2. We suggest that the gain-of-function effects of R838C on RetGC-1 stimulated by GCAP-1, which are dominant in vitro and may cause an abnormal increase in cGMP synthesis in dark-adapted photoreceptors, may be the cause of the cone-rod degeneration.

Disruption of a retinal guanylyl cyclase gene leads to cone-specific dystrophy and paradoxical rod behavior

One of two orphan photoreceptor guanylyl cyclases that are highly conserved from fish to mammals, GC-E (or retGC1) was eliminated by gene disruption. Expression of the second retinal cyclase (GC-F) as well as the numbers and morphology of rods remained unchanged in GC-E null mice. However, rods isolated from such mice, despite having a normal dark current, recovered from a light flash markedly faster. Unexpectedly, the a- and b-waves of electroretinograms (ERG) from dark-adapted null mice were suppressed markedly. Cones, initially present in normal numbers in the retina, disappeared by 5 weeks, based on ERG and histology. Thus, the GC-E-deficient mouse defines a model for cone dystrophy, but it also demonstrates that morphologically normal rods display paradoxical behavior in their responses to light.

S100B and S100A1 proteins in bovine retina:their calcium-dependent stimulation of a membrane-bound guanylate cyclase activity as investigated by ultracytochemistry

The Ca2(+)-binding proteins of the EF-hand type, S100B and S100A1, were detected in the outer segment of bovine retina photoreceptors where they are localized to disc membranes, as investigated by immunofluorescence and immunogold cytochemistry. S100B and S100A1 stimulate a membrane-bound guanylate cyclase activity associated with photoreceptor disc membranes in dark-adapted retina in a Ca2(+)-dependent manner, although with different Ca2+ requirements, as investigated by an ultracytochemical approach. Other retinal cell types express S100B and S100A1 as well. S100B is detected in the outer limiting membrane, fine cell processes in the outer nuclear layer and the outer plexiform layer, cell bodies in the inner nuclear layer and the ganglion cell layer, and the inner limiting membrane, whereas S100A1 has a more discrete distribution. S100B and S100A1 also stimulate a membrane-bound guanylate cyclase activity in photoreceptor cell bodies and Muller cells, but their effect appears independent of the light- or dark-adapted state of the retina and is observed at relatively high Ca2+ concentrations. These data represent the ultrastructural counterpart of recent biochemical observations implicating S100B and, possibly, S100A1 in the Ca2(+)-dependent stimulation of a photoreceptor membrane-bound guanylate cyclase activity [T. Duda, R. M. Goraczniak and R. K. Sharma (1996) Molecular characterization of S100A1-S1000B protein in retina and its activation mechanism of bovine photoreceptor guanylate cyclast. Biochemistry 35, 6263-6266; A. Margulis, N. Pozdnyakov and A. Sitaramayya (1996) Activation of bovine photoreceptor guanylate cyclast by S100 proteins. Biochem. Biophys. Res. Commun. 218, 243-247]. Our data suggest that at least S100B may take part in the regulation of a membrane-bound guanylate cyclase-based signalling pathway in both photoreceptors and Muller cells.

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.

Ultracytochemical study of guanylate cyclases A and B in light- and dark-adapted retinas

The ultracytochemical localization of guanylate cyclases A and B activity has been studied after stimulation with atrial natriuretic peptide and C-type natriuretic peptide in light- and dark-adapted retinas and pigmented epithelium. The results showed that both peptides stimulated guanylate cyclases A and B activity in light-adapted retinas only. Guanylate cyclases A and B activity was detected on plasma membrane of body of photoreceptors, bipolar, horizontal and ganglion cells, on plasma membranes of interneuronal connections at plexiform layers and on the plasma membrane of fibres at the nerve fibres layer. Independently of the light-or dark-adapted state, the pigmented epithelium also presented guanylate cyclases A and B activity on basal and lateral plasma membranes.

Activation of retinal guanylyl cyclase-1 by Ca2+-binding proteins involves its dimerization

Retinal guanylyl cyclase-1 (retGC-1), a key enzyme in phototransduction, is activated by guanylyl cyclase-activating proteins (GCAPs) if [Ca2+] is less than 300 nM. The activation is believed to be essential for the recovery of photoreceptors to the dark state; however, the molecular mechanism of the activation is unknown. Here, we report that dimerization of retGC-1 is involved in its activation by GCAPs. The GC activity and the formation of a 210-kDa cross-linked product of retGC-1 were monitored in bovine rod outer segment homogenates, GCAPs-free bovine rod outer segment membranes and recombinant bovine retGC-1 expressed in COS-7 cells. In addition to recombinant bovine GCAPs, constitutively active mutants of GCAPs that activate retGC-1 in a [Ca2+]-independent manner and bovine brain S100b that activates retGC-1 in the presence of approximately 10 microM [Ca2+] were used to investigate whether these activations take place through a similar mechanism, and whether [Ca2+] is directly involved in the dimerization. We found that a monomeric form of retGC-1 ( approximately 110 kDa) was mainly observed whenever GC activity was at basal or low levels. However, the 210-kDa product was increased whenever the GC activity was stimulated by any Ca2+-binding proteins used. We also found that [Ca2+] did not directly regulate the formation of the 210-kDa product. The 210-kDa product was detected in a purified GC preparation and did not contain GCAPs even when the formation of the 210-kDa product was stimulated by GCAPs. These data strongly suggest that the 210-kDa cross-linked product is a homodimer of retGC-1. We conclude that inactive retGC-1 is predominantly a monomeric form, and that dimerization of retGC-1 may be an essential step for its activation by active forms of GCAPs.

Molecular modeling of single polypeptide chain of calcium-binding protein p26olf from dimeric S100B(betabeta)

P26olf from olfactory tissue of frog, which may be involved in olfactory transduction or adaptation, is a Ca2+-binding protein with 217 amino acids. The p26olf molecule contains two homologous parts consisting of the N-terminal half with amino acids 1-109 and the C-terminal half with amino acids 110-217. Each half resembles S100 protein with about 100 amino acids and contains two helix-loop-helix Ca2+-binding structural motifs known as EF-hands: a normal EF-hand at the C-terminus and a pseudo EF-hand at the N-terminus. Multiple alignment of the two S100-like domains of p26olf with 18 S100 proteins indicated that the C-terminal putative EF-hand of each domain contains a four-residue insertion when compared with the typical EF-hand motifs in the S100 protein, while the N-terminal EF-hand is homologous to its pseudo EF-hand. We constructed a three-dimensional model of the p26olf molecule based on results of the multiple alignment and NMR structures of dimeric S100B(betabeta) in the Ca2+-free state. The predicted structure of the p26olf single polypeptide chain satisfactorily adopts a folding pattern remarkably similar to dimeric S100B(betabeta). Each domain of p26olf consists of a unicornate-type four-helix bundle and they interact with each other in an antiparallel manner forming an X-type four-helix bundle between the two domains. The two S100-like domains of p26olf are linked by a loop with no steric hindrance, suggesting that this loop might play an important role in the function of p26olf. The circular dichroism spectral data support the predicted structure of p26olf and indicate that Ca2+-dependent conformational changes occur. Since the C-terminal putative EF-hand of each domain fully keeps the helix-loop-helix motif having a longer Ca2+-binding loop, regardless of the four-residue insertion, we propose that it is a new, novel EF-hand, although it is unclear whether this EF-hand binds Ca2+. P26olf is a new member of the S100 protein family.

Age-related changes in expression of hippocalcin and NVP2 in rat brain

Expression of hippocalcin and neural visinin-like calcium-binding protein 2 (NVP2) in aging rat brain was investigated by immunoblot and immunohistochemical analyses. In 3-month old rats, hippocalcin and NVP2 were present at high concentrations in hippocampal and cerebral pyramidal cells and dentate granule cells, with hippocalcin protein levels being five to ten times higher than NVP2 levels. Hippocalcin levels in hippocampus and cerebral cortex decreased by approximately 20% at 24 months. While the number of hippocalcin-positive cells in CA3, dentate gyrus and cerebral cortex were preserved, staining intensity decreased. In contrast, the number and staining intensity of hippocalcin-positive cells in CA1 were maintained. NVP2 levels in hippocampus and cerebral cortex decreased by approximately 30% at 24 months. In cerebral cortex, the number and intensity of NVP2-positive cells decreased. In CA1 through CA3 and in dentate gyrus, NVP2-positive cell numbers were preserved, but staining intensity decreased. In summary, the loss of hippocalcin and NVP2 in aging rat brain may be associated with age-related impairment of postsynaptic functions.

Mapping sites in guanylyl cyclase activating protein-1 required for regulation of photoreceptor membrane guanylyl cyclases

Guanylyl cyclase activating protein (GCAP)-1 regulates photoreceptor membrane guanylyl cyclase, RetGC, in a Ca2+-sensitive manner. It contains four Ca2+-binding motifs, EF-hands, three of which are capable of binding Ca2+. GCAP-1 activates RetGC in low Ca2+ and inhibits it in high Ca2+. In this study we used deletion and substitution analysis to identify regions of GCAP-1 sequence that are specifically required for inhibition and activation. A COOH-terminal sequence within Met157 to Arg182 is required for activation but not for inhibition of RetGC. We localized one essential stretch to 5 residues from Arg178 to Arg182. Another sequence essential for activation is within the N-terminal residues Trp21 to Thr27. The region between EF-hands 1 and 3 of GCAP-1 also contains elements needed for activation of RetGC. Finally, we found that inhibition of RetGC requires the first 9 amino-terminal residues of GCAP-1, but none of the residues from Gln33 to the COOH-terminal Gly205 are specifically required for inhibition. The ability of GCAP-1 mutants to regulate RetGC was tested on total guanylyl cyclase activity present in rod outer segments. In addition, the key mutants were also shown to produce similar effects on recombinant bovine outer segment cyclases GC1 and GC2.

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.

Identification of a domain in guanylyl cyclase-activating protein 1 that interacts with a complex of guanylyl cyclase and tubulin in photoreceptors

The membrane-bound guanylyl cyclase in rod photoreceptors is activated by guanylyl cyclase-activating protein 1 (GCAP-1) at low free [Ca2+]. GCAP-1 is a Ca2+-binding protein and belongs to the superfamily of EF-hand proteins. We created an oligopeptide library of overlapping peptides that encompass the entire amino acid sequence of GCAP-1. Peptides were used in competitive screening assays to identify interaction regions in GCAP-1 that directly bind the guanylyl cyclase in bovine photoreceptor cells. We found four regions in GCAP-1 that participate in regulating guanylyl cyclase. A 15-amino acid peptide located adjacent to the second EF-hand motif (Phe73-Lys87) was identified as the main interaction domain. Inhibition of GCAP-1-stimulated guanylyl cyclase activity by the peptide Phe73-Lys87 was completely relieved when an excess amount of GCAP-1 was added. An affinity column made from this peptide was able to bind a complex of photoreceptor guanylyl cyclase and tubulin. Using an anti-GCAP-1 antibody, we coimmunoprecipitated GCAP-1 with guanylyl cyclase and tubulin. Complex formation between GCAP-1 and guanylyl cyclase was observed independent of [Ca2+]. Our experiments suggest that there exists a tight association of guanylyl cyclase and tubulin in rod outer segments.

Molecular characterization of a third member of the guanylyl cyclase-activating protein subfamily

The mammalian retina contains at least two guanylyl cyclases (GC1 and GC2) and two guanylyl cyclase-activating proteins (GCAP1 and GCAP2). Here we present evidence of the presence of a new photoreceptor-specific GCAP, termed GCAP3, which is closely related to GCAP1. The sequence similarity of GCAP3 with GCAP1 and GCAP2 is 57 and 49%, respectively. Recombinant GCAP3 and GCAP2 stimulate GC1 and GC2 in low [Ca2+]free and inhibit GCs when [Ca2+]free is elevated, unlike GCAP1, which only stimulates GC1. GCAP3 is encoded by a distinct gene present in other mammalian species but could not be detected by genomic Southern blotting in rodents, amphibians, and lower vertebrates. The intron/exon arrangement of the GCAP3 gene is identical to that of the other GCAP genes. While the GCAP1 and GCAP2 genes are arranged in a tail-to-tail array on chromosome 6p in human, the GCAP3 gene is located on 3q13.1, suggesting an ancestral gene duplication/translocation event. The identification of multiple Ca2+-binding proteins that interact with GC is suggestive of complex regulatory mechanisms for photoreceptor GC.

Ca2+-dependent conformational changes in bovine GCAP-2

GCAP-2, a mammalian photoreceptor-specific protein, is a Ca2+-dependent regulator of the retinal membrane guanylyl cyclases (Ret-GCs). Sensing the fall in intracellular free Ca2+ after photo-excitation, GCAP-2 stimulates the activity of Ret-GC leading to cGMP production. Like other members of the recoverin superfamily, GCAP-2 is a small N-myristoylated protein containing four EF-hand consensus motifs. In this study, we demonstrate that like recoverin and neurocalcin, GCAP-2 alters its conformation in response to Ca2+-binding as measured by a Ca2+-dependent change in its far UV CD spectrum. Differences in the conformation of the Ca2+-bound and Ca2+-free forms of GCAP-2 were also observed by examining their relative susceptibility to V8 protease. In contrast to recoverin, we do not observe proteolytic cleavage of the myristoylated N-terminus of Ca2+-bound GCAP-2. NMR spectra also show that, in contrast to recoverin, the chemical environment of the N-terminus of GCAP-2 is not dramatically altered by Ca2+ binding. Despite the similarity of GCAP-2 and recoverin, the structural consequences of Ca2+-binding for these two proteins are significantly dissimilar.

Purification and molecular cloning of a novel calcium-binding protein, p26olf, in the frog olfactory epithelium

Olfactory adaptation requires the change of intracellular calcium concentration during stimuli. To contribute in the study of the molecular mechanism of calcium-dependent regulations in olfactory receptor cells, we isolated a novel 26-kDa Ca2+-binding protein named p26olf from the frog olfactory epithelium after four chromatographical steps. Based on the partial amino acid sequences of the proteolysed fragments of p26olf, we obtained a cDNA clone that encodes p26olf. The analysis of its amino acid sequence revealed that p26olf consists of two S-100-like regions aligned sequentially with a calculated molecular mass of 24,493. Northern blot analysis showed that p26olf is expressed in the frog olfactory epithelium and also in other tissues. Immunoreactivity against p26olf was detected in the cilia layer of the olfactory epithelium. These results suggest that p26olf is a dimeric form of S-100 proteins and is involved in the olfactory transduction or adaptation.

Light adaptation and sensitivity controlling mechanisms in vertebrate photoreceptors

The human visual system can discriminate increment and decrement light stimuli over a wide range of ambient illumination; from moonlight to bright sunlight. Several mechanisms contribute to this property but the major ones reside in the retina and more specifically within the photoreceptors themselves. Numerous studies in retinae from cold- and warm-blooded vertebrates have demonstrated the ability of the photoreceptors to respond in a graded manner to light increments and decrements even if these are applied during a background illumination that is expected to saturate the cells. In all photoreceptors regardless of type and species, three cellular mechanisms have been identified that contribute to background desensitization and light adaptation. These gain controlling mechanisms include; response-compression due to the non-linearity of the intensity-response function, biochemical modulation of the phototransduction process and pigment bleaching. The overall ability of a photoreceptor to adapt to background lights reflects the relative contribution of each of these mechanisms and the light intensity range over which they operate. In rods of most species, response-compression tends to dominate these mechanisms at light levels too weak to cause significant pigment bleaching and therefore, rods exhibit saturation. In contrast, cones are characterized by powerful background-induced modulation of the phototransduction process at moderate to bright background intensities where pigment bleaching becomes significant.Therefore, cones do not exhibit saturation even when the level of ambient illumination is raised by 6-7 log units.

Constitutive activation of photoreceptor guanylate cyclase by Y99C mutant of GCAP-1. Possible role in causing human autosomal dominant cone degeneration

Photoreceptor membrane guanylate cyclases (RetGC) are regulated by calcium-binding proteins, GCAP-1 and GCAP-2. At Ca2+ concentrations below 100 nM, characteristic of light-adapted photoreceptors, guanylate cyclase-activating protein (GCAPs) activate RetGC, and at free Ca2+ concentrations above 500 nM, characteristic of dark-adapted photoreceptors, GCAPs inhibit RetGC. A mutation, Y99C, in human GCAP-1 was recently found to be linked to autosomal dominant cone dystrophy in a British family (Payne, A. M., Downes, S. M., Bessant, D. A. R., Taylor, R., Holder, G. E., Warren, M. J., Bird, A. C., and Bhattachraya, S. S. (1998) Hum. Mol. Genet. 7, 273-277). We produced recombinant Y99C GCAP-1 mutant and tested its ability to activate RetGC in vitro at various free Ca2+ concentrations. The Y99C mutation does not decrease the ability of GCAP-1 to activate RetGC. However, RetGC stimulated by the Y99C GCAP-1 remains active even at Ca2+ concentration above 1 microM. Hence, the cyclase becomes constitutively active within the whole physiologically relevant range of free Ca2+ concentrations. We have also found that the Y99C GCAP-1 can activate RetGC even in the presence of Ca2+-loaded nonmutant GCAPs. This is consistent with the fact that cone degeneration was dominant in human patients who carried such mutation (Payne, A. M., Downes, S. M., Bessant, D. A. R. , Taylor, R., Holder, G. E., Warren, M. J., Bird, A. C., and Bhattachraya, S. S. (1998) Hum. Mol. Genet. 7, 273-277). A similar mutation, Y104C, in GCAP-2 results in a different phenotype. This mutation apparently does not affect Ca2+ sensitivity of GCAP-2. Instead, the Y104C GCAP-2 stimulates RetGC less efficiently than the wild-type GCAP-2. Our data indicate that cone degeneration associated with the Y99C mutation in GCAP-1 can be a result of constitutive activation of cGMP synthesis.

GCAP1 (Y99C) mutant is constitutively active in autosomal dominant cone dystrophy

GCAP1 stimulates photoreceptor guanylate cyclase (GC) in bleached vertebrate photoreceptors when [Ca2+]free decreases but is inactivated when cytoplasmic [Ca2+]free increase after dark adaptation. A Y99C mutation in GCAP1 has recently been found to be associated with autosomal dominant cone dystrophy. We show that the GCAP1(Y99C) mutant and native GCAP1 are highly effective in stimulation of photoreceptor GC1. The Ca2+ sensitivity of the mutant GCAP1, however, is markedly altered, causing reduced but persistent stimulation of GC1 under physiological dark conditions. These results are consistent with a model in which enhanced GC activity in dark-adapted cones leads to elevated levels of cytoplasmic cGMP. Alterations in physiological cGMP levels are also associated with other retinal degenerations, including Leber's congenital amaurosis.

Structure-function relationships and localization of the Na/Ca-K exchanger in rod photoreceptors

The structural and functional properties of the bovine rod photoreceptor Na/Ca-K exchanger and its distribution in vertebrate photoreceptor cells were studied using a panel of monoclonal antibodies. Antibodies that bind to distinct epitopes along the large hydrophilic N-terminal segment of the exchanger labeled the extracellular surface of the rod outer segment plasma membrane, whereas antibodies against a large hydrophilic loop between the two membrane domains labeled the intracellular side. Enzymatic deglycosylation studies indicated that the exchanger primarily contains O-linked sialo-oligosaccharides located within the N-terminal domain. Removal of the extracellular domain with trypsin or the large intracellular domain with kallikrein did not alter the Na+- or K+-dependent Ca2+ efflux activity of the exchanger when reconstituted into lipid vesicles. Anti-exchanger antibodies were also used to visualize the distribution of the exchanger in the retina by light and electron microscopy. The exchanger was localized to the plasma membrane of rod outer segments. No labeling was observed in the disk membranes, cone photoreceptor cells, or other retinal neurons, and only faint staining was seen in the rod inner segment. These results indicate that the O-linked glycosylated rod Na/Ca-K exchanger is specifically targeted to the plasma membrane of rod photoreceptors and has a topological organization similar to that reported for the cardiac Na/Ca exchanger. The large intracellular and extracellular domains do not directly function in the transport of ions across the rod outer segment plasma membrane, but instead may play a role in protein-protein interactions that maintain the spatial organization of the exchanger in the plasma membrane or possibly regulate transport activity of the exchanger.

Two amino acid substitutions convert a guanylyl cyclase, RetGC-1, into an adenylyl cyclase

Guanylyl cyclases (GCs) and adenylyl cyclases (ACs) have fundamental roles in a wide range of cellular processes. Whereas GCs use GTP as a substrate to form cGMP, ACs catalyze the analogous conversion of ATP to cAMP. Previously, a model based on the structure of adenylate cyclase was used to predict the structure of the nucleotide-binding pocket of a membrane guanylyl cyclase, RetGC-1. Based on this model, we replaced specific amino acids in the guanine-binding pocket of GC with their counterparts from AC. A change of two amino acids, E925K together with C995D, is sufficient to completely alter the nucleotide specificity from GTP to ATP. These experiments strongly validate the AC-derived RetGC-1 structural model and functionally confirm the role of these residues in nucleotide discrimination.

Calcium-sensitive particulate guanylyl cyclase as a modulator of cAMP in olfactory receptor neurons

The second messengers cAMP and inositol-1,4,5-triphosphate have been implicated in olfaction in various species. The odorant-induced cGMP response was investigated using cilia preparations and olfactory primary cultures. Odorants cause a delayed and sustained elevation of cGMP. A component of this cGMP response is attributable to the activation of one of two kinetically distinct cilial receptor guanylyl cyclases by calcium and a guanylyl cyclase-activating protein (GCAP). cGMP thus formed serves to augment the cAMP signal in a cGMP-dependent protein kinase (PKG) manner by direct activation of adenylate cyclase. cAMP, in turn, activates cAMP-dependent protein kinase (PKA) to negatively regulate guanylyl cyclase, limiting the cGMP signal. These data demonstrate the existence of a regulatory loop in which cGMP can augment a cAMP signal, and in turn cAMP negatively regulates cGMP production via PKA. Thus, a small, localized, odorant-induced cAMP response may be amplified to modulate downstream transduction enzymes or transcriptional events.

Rhodopsin phosphorylation and its role in photoreceptor function

Light-stimulated phosphorylation of rhodopsin was first described 25 years ago. This paper reviews the progress that has been made towards (i) understanding the nature of the enzymes that phosphorylate and dephosphorylate rhodopsin (ii) identifying the sites of phosphorylation on rhodopsin and (iii) understanding the physiological importance of rhodopsin phosphorylation. Many important questions related to rhodopsin phosphorylation remain unanswered and new strategies and methods are needed to address issues such as the roles of Ca2+ and recoverin. We present one such method that uses mass spectrometry to quantitate rhodopsin phosphorylation in intact mouse retinas.

Functional heterogeneity of transducin alpha subunits

The N-terminal glycine of transducin alpha subunits is acylated by lauroyl (C12:0), myristoyl (C14:0), (cis-delta5)-tetradecaenoyl (C14:1) or (cis,cis-delta5,delta8)-tetradecadienoyl (C14:2) fatty acyl groups. We examined functional heterogeneity of transducin by sequentially eluting it from bleached outer segments using increasing concentrations of GTP then identifying the N-terminal acyl groups on the eluted alpha subunits. C14:2 acylated transducin eluted at low GTP concentrations followed by C12:0, C14:1 and C14:0 transducin at higher GTP concentrations. This suggests functional heterogeneity in the different forms of transducin alpha subunits.

Third calcium-modulated rod outer segment membrane guanylate cyclase transduction mechanism

Ca2+-modulated rod outer segment membrane guanylate cyclase (ROS-GC1) has been cloned and reconstituted to show that it is regulated by two processes: one inhibitory, the other stimulatory. The inhibitory process is consistent with its linkage to phototransduction; the physiology of the stimulatory process is probably linked to neuronal transmission. In both regulatory processes, calcium modulation of the cyclase takes place through the calcium binding proteins; guanylate cyclase activating proteins (GCAP1 and GCAP2) in the case of the phototransduction process and calcium-dependent GCAP (CD-GCAP) in the case of the stimulatory process. The cyclase domains involved in the two processes are located at two different sites on the ROS-GC1 intracellular region. The GCAP1-modulated domain resides within the aa 447-730 segment of ROS-GC1 and the CD-GCAP-modulated domain resides within the aa 731-1054 segment. In the present study the GCAP2-dependent Ca2+ modulation of the cyclase activity has been reconstituted using recombinant forms of GCAP2 and ROS-GC1, and its mutants. The results indicate that consistent to phototransduction, GCAP2 at low Ca2+ concentration (10 nM) maximally stimulates the cyclase activity of the wild-type and its mutants: ext (deleted aa 8-408), kin (deleted aa 447-730) and hybrid consisting of the ext, transmembrane and kin domains of ANF-RGC and the C-terminal domain, aa 731-1054, of ROS-GC1. In all cases, it inhibits the cyclase activity with an IC50 of about 140 nM. A previous study has shown that under identical conditions the kin and the hybrid mutant are at best only minimally stimulated. Thus, the GCAP1 and GCAP2 signal transduction mechanisms are different, occurring through different modules of ROS-GC1. These findings also demonstrate that the intracellular region of ROS-GC1 is composed of multiple modules, each designed to mediate a particular calcium-specific signalling pathway.

Onset of feedback reactions underlying vertebrate rod photoreceptor light adaptation

Light adaptation in vertebrate photoreceptors is thought to be mediated through a number of biochemical feedback reactions that reduce the sensitivity of the photoreceptor and accelerate the kinetics of the photoresponse. Ca2+ plays a major role in this process by regulating several components of the phototransduction cascade. Guanylate cyclase and rhodopsin kinase are suggested to be the major sites regulated by Ca2+. Recently, it was proposed that cGMP may be another messenger of light adaptation since it is able to regulate the rate of transducin GTPase and thus the lifetime of activated cGMP phosphodiesterase. Here we report measurements of the rates at which the changes in Ca2+ and cGMP are followed by the changes in the rates of corresponding enzymatic reactions in frog rod outer segments. Our data indicate that there is a temporal hierarchy among reactions that underlie light adaptation. Guanylate cyclase activity and rhodopsin phosphorylation respond to changes in Ca2+ very rapidly, on a subsecond time scale. This enables them to accelerate the falling phase of the flash response and to modulate flash sensitivity during continuous illumination. To the contrary, the acceleration of transducin GTPase, even after significant reduction in cGMP, occurs over several tens of seconds. It is substantially delayed by the slow dissociation of cGMP from the noncatalytic sites for cGMP binding located on cGMP phosphodiesterase. Therefore, cGMP-dependent regulation of transducin GTPase is likely to occur only during prolonged bright illumination.

Calcium binding to recoverin: implications for secondary structure and membrane association

Recoverin is an EF-hand calcium-binding protein reportedly involved in the transduction of light by vertebrate photoreceptor cells. It also is an autoantigen in a cancer-associated degenerative disease of the retina. Measurements by circular dichroism presented here demonstrate that the binding of calcium to recoverin causes large structural changes. increasing the alpha-helical content of the protein and decreasing its beta-turn, beta-sheet and 'other' structures. The maximum helical content (67%) was observed at 100 microM free calcium and, unlike calmodulin, decreased as the calcium concentration was modulated in either direction from this value. Fluorescence measurements indicated that recoverin may aggregate or undergo structural changes independent of calcium binding as the calcium concentration is increased above 100 microM. EGTA also appeared to affect the structure of recoverin independent of its chelation of calcium. While calcium-induced conformational changes have been proposed to alter the membrane binding of recoverin through association of its myristoylated amino terminus, in the experiments presented here the partitioning of recoverin between the cytoplasmic and membrane compartments of the rod photoreceptor outer segment was unaffected by the concentration of calcium, therefore it appears unlikely that a calcium-myristoyl switch acts alone to anchor recoverin directly to the membrane. These experiments were conducted with native recoverin which is heterogeneously acylated, but mass spectrometry confirmed that simple chromatographic methods could be devised to isolate the different forms of recoverin for further studies.

Calcium binding, but not a calcium-myristoyl switch, controls the ability of guanylyl cyclase-activating protein GCAP-2 to regulate photoreceptor guanylyl cyclase

Guanylyl cyclase-activating protein 2 (GCAP-2) is a recoverin-like calcium-binding protein that regulates photoreceptor guanylyl cyclase (RetGC) (Dizhoor, A. M., and Hurley, J. B. (1996) J. Biol. Chem. 271, 19346-19350). It was reported that myristoylation of a related protein, GCAP-1, was critical for its affinity for RetGC (Frins, S., Bonigk, W., Muller, F., Kellner, R., and Koch, K.-W. (1996) J. Biol. Chem. 271, 8022-8027). We demonstrate that the N terminus of GCAP-2, like those of other members of the recoverin family of Ca2+-binding proteins, is fatty acylated. However, unlike other proteins of this family, more GCAP-2 is present in the membrane fraction at low Ca2+ than at high Ca2+ concentrations. We investigated the role of the N-terminal fatty acyl residue in the ability of GCAP-2 to regulate RetGCs. Myristoylated or nonacylated GCAP-2 forms were expressed in Escherichia coli. Wild-type GCAP-2 and the Gly2 --> Ala2 GCAP-2 mutant, which is unable to undergo N-terminal myristoylation, were also expressed in mammalian HEK293 cells. We found that compartmentalization of GCAP-2 in photoreceptor outer segment membranes is Ca2+- and ionic strength-sensitive, but it does not require the presence of the fatty acyl group and does not necessarily directly reflect GCAP-2 interaction with RetGC. The lack of myristoylation does not significantly affect the ability of GCAP-2 to stimulate RetGC. Nor does it affect the ability of the Ca2+-loaded form of GCAP-2 to compete with the GCAP-2 mutant that constitutively activates RetGC. We conclude that while Ca2+ binding plays a major regulatory role in GCAP-2 function, it does not operate through a calcium-myristoyl switch similar to the one found in recoverin.

Localization of guanylate cyclase-activating protein 2 in mammalian retinas

Guanylate cyclase-activating proteins (GCAP1 and GCAP2) are thought to mediate the intracellular stimulation of guanylate cyclase (GC) by Ca2+, a key event in recovery of the dark state of rod photoreceptors after exposure to light. GCAP1 has been localized to rod and cone outer segments, the sites of phototransduction, and to photoreceptor synaptic terminals and some cone somata. We used in situ hybridization and immunocytochemistry to localize GCAP2 in human, monkey, and bovine retinas. In human and monkey retinas, the most intense immunolabeling with anti-GCAP2 antibodies was in the cone inner segments, somata, and synaptic terminals and, to a lesser degree, in rod inner segments and inner retinal neurons. In bovine retina, the most intense immunolabeling was in the rod inner segments, with weaker labeling of cone myoids, somata, and synapses. By using a GCAP2-specific antibody in enzymatic assays, we confirmed that GCAP1 but not GCAP2 is the major component that stimulates GC in bovine rod outer segment homogenates. These results suggest that although GCAP1 is involved in the Ca2+-sensitive regulation of GC in rod and cone outer segments, GCAP2 may have non-phototransduction functions in photoreceptors and inner retinal neurons.

Monitoring calcium-induced conformational changes in recoverin by electrospray mass spectrometry

Recoverin is a calcium-binding protein that regulates the vertebrate photoresponse by inhibiting rhodopsin kinase in response to high calcium concentrations. It is heterogeneously N-acylated by myristoyl and related fatty acyl residues that are thought to act as "calcium-myristoyl switches," whereby, in the presence of Ca2+, the N-terminal acyl group is extended away from recoverin and, in the absence of calcium, it is more closely associated with the protein. Here we use electrospray ionization mass spectrometry (ESI/MS) to examine hydrogen isotopic exchange rates for specific regions of both acylated and nonacylated recoverin in the presence and absence of calcium. The deuterium exchange rates of three regions in the hydrophobic myristoyl binding pocket of acylated recoverin decreased in the absence of calcium. This effect is most likely due to the closer association of the acyl group with the protein under these conditions. In contrast, rates of deuterium incorporation increased in the absence of calcium for other regions, including the two functional calcium-binding sites. In addition to supporting the calcium-myristoyl switch hypothesis, a comparison of the behavior of acylated and unacylated recoverin revealed that the N-acyl group (N-lauroyl or N-myristoyl) exerts a significant stabilizing influence on the dynamics of recoverin. We demonstrate that the new technique of monitoring hydrogen isotopic exchange by ESI/MS can be used to obtain useful information concerning protein structures in solution using smaller amounts of protein and under more physiologically relevant conditions than is typically possible with NMR or X-ray crystallography.

The human GCAP1 and GCAP2 genes are arranged in a tail-to-tail array on the short arm of chromosome 6 (p21.1)

GCAP1 and GCAP2 are related Ca(2+)-binding proteins that activate photoreceptor guanylate cyclase(s). We showed previously that the human GCAP1 gene, consisting of four exons, is located at 6p21.1 (locus designation GUCA). To identify the chromosomal location of the GCAP2 gene, we first cloned its cDNA and determined its intron-exon distribution by PCR analysis. The results show that the introns of the GCAP2 gene are positioned exactly as in the GCAP1 gene and are nearly double in size. Sequence similarity between the two genes, however, is limited to portions of exons 1 and 2. The GCAP1 and GCAP2 genes are transcribed into single mRNA species (1.7 and 2.2 kb, respectively) and are detectable only in the retina by Northern blotting. The GCAP2 gene was found by somatic human-hamster hybrid panel analysis and FISH to reside at GUCA in a region indistinguishable from that of GCAP1. PCR analysis with exon 4-specific primers showed that the genes are in a tail-to-tail array less than 5 kb apart and altogether span less than 20 kb of genomic DNA. The identical gene structures and loci of GCAP1 and GCAP2, and the identical function of the gene products, are consistent with gene duplication event.

Understanding covalent modifications of proteins by lipids: where cell biology and biophysics mingle

Much effort has been expended on the in vitro characterization of enzymes that covalently attach lipids to proteins. Less information is available about properties conferred on modified proteins by their attached lipid groups, but biophysical studies of simple model systems have begun to shed light on this issue. Recent evidence suggests that the specificity of lipid modifications may be dependent upon the intracellular compartmentalization of the lipid and protein substrates of lipidating enzymes. The function and targeting of their lipidated products appear to be regulated dynamically through addition or subtraction of lipid moieties, other covalent or noncovalent modifications, as well as several devices that at this point can only be inferred. This field of research illustrates the necessity of integrating cell-biological and biophysical perspectives.

Turned on by Ca2+! The physiology and pathology of Ca(2+)-binding proteins in the retina

Vertebrate photoreceptor cells can signal the absorption of a single photon and then modulate their response as the intensity of the light and the intensity of the background illumination vary, and it has long been recognized that Ca2+ ions contribute to the underlying processes. Recently, several Ca(2+)-binding proteins of the EF-hand family were identified that mediate the actions of Ca2+ during the response to light. Molecular interactions between these Ca(2+)-binding proteins and their cellular targets are amenable to study owing in part to the unique features of phototransduction. In addition, two of the proteins, recoverin and guanylate cyclase activating protein (GCAP), appear to be involved in separate degenerative diseases of the retina that arise in humans and in animal models of human disease. Information obtained from these studies should also be relevant to the growing number of homologous proteins found in other neural tissues.

Modulation of the calcium sensitivity of bovine retinal rod outer segment guanylyl cyclase by sodium ions and protein kinase A

Guanylyl cyclases (GC, EC 4.6.1.2) serve as receptors that produce cGMP in response to ligand binding. The production of cGMP is essential for the ability of retinal photoreceptor cells to restore the dark state after photoexcitation. GC activity is enhanced in rod outer segments (ROS) by a decrease in the cytosolic free Ca2+ concentration. We recently developed a new real-time assay to measure initial rates of ROS GC activity with much improved precision [Wolbring, G. & P. P. M. Schnetkamp (1995) Biochemistry 34, 4689-4695]. With this assay we examined the Ca2+ sensitivity of ROS GC, and we report here that protein kinase A-mediated phosphorylation and Na+ cause significant shifts in the IC50 for Ca2+ of the particulate guanylyl cyclase from bovine retinal rod outer segments. The IC50 for Ca2+ ranged between 30 and 270 nM Ca2+ dependent on the presence of Na+, choline, cAMP, cGMP, 8-bromo-cAMP, 8-bromo-cGMP, or the catalytic subunit of protein kinase A.

Inactivation of EF-hands makes GCAP-2 (p24) a constitutive activator of photoreceptor guanylyl cyclase by preventing a Ca2+-induced "activator-to-inhibitor" transition

Guanylyl cyclase activator proteins GCAP-1 and GCAP-2 (Dizhoor et al. , 1995, Gorczyca et al., 1995) are members of a recently identified subclass of EF-hand type Ca2+-binding proteins that respond to Ca2+ differently than any other known members of the EF-hand superfamily. GCAPs acquire an activating conformation only in their Ca2+-free form. Free Ca2+ concentrations corresponding to levels in dark-adapted vertebrate photoreceptors inhibit the ability of GCAPs to activate photoreceptor guanylyl cyclases (RetGCs). We studied the effects of mutations that block binding of Ca2+ to the EF-hands of GCAP-2. Unlike other EF-hand proteins, which fail to activate their target when their EF-hands are inactivated by mutations, GCAP-2 with any single EF-hand inactivated remains active and is 3-6 times less sensitive to the inhibitory effect of Ca2+. Inactivation of any two or all three EF-hands produces active forms of GCAP-2 that are insensitive to inhibition by physiological intracellular concentrations of Ca2+. Unexpectedly we also found that activation of RetGCs by a Ca2+-insensitive mutant is inhibited by Ca2+-loaded wild type GCAP-2. We propose the following. 1) GCAP-2 can exist in two extreme functional forms: an apo form that activates RetGCs and a Ca2+-loaded form that blocks activation of RetGCs. 2) All three EF-hands of GCAP-2 contribute to the inhibitory effect of Ca2+. 3) Inactivation of two or three EF-hands is sufficient to shift the "activator-inhibitor" transition outside the physiological range of intracellular free Ca2+.

The mechanisms of vertebrate light adaptation: speeded recovery versus slowed activation

Light adaptation in vertebrate photoreceptors is commonly attributed to a feedback mechanism that reduces the amplitude of the receptor potential by speeding the inactivation of the transduction cascade and hastening the recovery process. Recent studies have challenged this model and suggest instead that desensitization originates mainly from changes in the activation phase rather than the recovery phase of the response. This has important implications for understanding the molecular mechanisms that underlie the control of sensitivity in this G-protein-coupled, signal-transduction pathway.

Calcium modulation of bovine photoreceptor guanylate cyclase

Bovine photoreceptor guanylate cyclase (ROS-GC) consists of a single transmembrane polypeptide chain with extracellular and intracellular domains. In contrast to non-photoreceptor guanylate cyclases (GCs) which are activated by hormone peptides, ROS-GC is modulated in low Ca2+ by calmodulin-like Ca(2+)-binding proteins termed GCAPs (guanylate cyclase-activating proteins). In this communication we show that, like the native system, ROS-GC expressed in COS cells is activated 4-6-fold by recombinant GCAP1 at 10 nM Ca2+ and that the reconstituted system is inhibited at physiological levels of Ca2+ (1 microM). A mutant ROS-GC in which the extracellular domain was deleted was stimulated by GCAP1 indistinguishable from native ROS-GC indicating that this domain is not involved in Ca2+ modulation. Deletion of the intracellular kinase-like domain diminished the stimulation by GCAP1, indicating that this domain is at least in part involved in Ca2+ modulation. Replacement of the catalytic domain in a non-photoreceptor GC by the catalytic domain of ROS-GC yielded a chimeric GC that was sensitive to ANF/ATP and to a lesser extent to GCAP1. The results establish that GCAP1 acts at an intracellular domain, suggesting a mechanism of photoreceptor GC stimulation fundamentally distinct from hormone peptide stimulation of other cyclase receptors.

Distribution of guanylate cyclase within photoreceptor outer segments

Guanylate cyclases play an essential role in the recovery of vertebrate photoreceptor cells after light activation. Here, we have investigated how one such guanylate cyclase, RetGC-1, is distributed within light- and dark-adapted rod photoreceptor cells. Guanylate cyclase activity partitioned with the photoreceptor outer segment (OS) cytoskeleton in a light-sensitive manner. RetGC-1 was found to bind actin filaments in actin blot overlays, suggesting a mechanism for its association with the OS cytoskeleton. In retinal sections, this enzyme was immunodetected only in the OSs, where it appeared to be distributed throughout the disk membranes.

Expression of GCAP1 and GCAP2 in the retinal degeneration (rd) mutant chicken retina

We cloned the guanylate cyclase activating proteins, GCAP1 and GCAP2, from chicken retina and examined their expression in normal and predegenerate rdlrd chicken retina. Northern analyses show that the amounts of the single transcripts encoding GCAP1 and GCAP2 are reduced to about 70% of normal levels in rdlrd retina. Western analyses reveal that GCAP2 levels appear normal in this retina, while GCAP1 levels are reduced by more than 90%. The specific downregulation of GCAP1 in rdlrd retina is consistent with a model for this disease in which activation of guanylate cyclase in the photoreceptors is abnormal, resulting in low levels of cGMP and an absence of phototransduction.

Drosophila neurocalcin, a fatty acylated, Ca2+-binding protein that associates with membranes and inhibits in vitro phosphorylation of bovine rhodopsin

Neurocalcins belong to a family of neuronal specific EF hand Ca2+-binding proteins defined by recoverin. Previously, we reported the cloning and initial characterization of neurocalcin in Drosophila melanogaster (Teng, D. H.-F., Chen, C.-K., and Hurley, J. B. (1994) J. Biol. Chem. 269, 31900-31907). We showed that the Drosophila neurocalcin protein (DrosNCa) is expressed in neurons and that bacterially expressed recombinant DrosNCa (rDrosNCa) can be myristoylated. Here, we present two lines of evidence that DrosNCa is fatty acylated in vivo. First, the mobility of affinity-purified native DrosNCa on two-dimensional gel electrophoresis is identical to that of myristoylated rDrosNCa and distinct from that of nonacylated rDrosNCa. Second, the membrane binding properties of native DrosNCa are similar to those of myristoylated rDrosNCa; both of these proteins bind to membranes at 0.2 mM Ca2+, whereas nonacylated rDrosNCa always remains soluble. It has been shown that recoverin inhibits the phosphorylation of rhodopsin when Ca2+ is present (Kawamura et al., 1993) and that a dependent recoverin/rhodopsin kinase interaction underlies the inhibitory effect of recoverin (Chen et al., 1995). Given the similarities between recoverin and neurocalcin, we examined the effect of DrosNCa on rhodopsin phosphorylation. We find that rDrosNCa is capable of inhibiting bovine rhodopsin phosphorylation in vitro in a Ca2+-dependent manner. The inhibitory activity of rDrosNCa is enhanced by myristoylation, and the potency of its effect is similar to that of recoverin. Two other related EF hand proteins, guanylate cyclase-activating protein-2 and calmodulin, are only poor inhibitors in these phosphorylation assays. in vitro inhibition of rhodopsin phosphorylation therefore appears to be an assayable property of a subset of recoverin-like proteins.

Dual regulation of a chimeric plant serine/threonine kinase by calcium and calcium/calmodulin

A chimeric Ca2+/calmodulin-dependent protein kinase (CCaMK) gene characterized by a catalytic domain, a calmodulin-binding domain, and a neural visinin-like Ca2+-binding domain was recently cloned from plants (Patil, S., Takezawa, D., and Poovaiah, B. W. (1995) Proc. Natl. Acad. Sci. U. S. A. 92, 4797-4801). The Escherichia coli-expressed CCaMK phosphorylates various protein and peptide substrates in a Ca2+/calmodulin-dependent manner. The calmodulin-binding region of CCaMK has similarity to the calmodulin-binding region of the alpha-subunit of multifunctional Ca2+/calmodulin-dependent protein kinase (CaMKII). CCaMK exhibits basal autophosphorylation at the threonine residue(s) (0.098 mol of 32P/mol) that is stimulated 3.4-fold by Ca2+ (0.339 mol of 32P/mol), while calmodulin inhibits Ca2+-stimulated autophosphorylation to the basal level. A deletion mutant lacking the visinin-like domain did not show Ca2+-stimulated autophosphorylation activity but retained Ca2+/calmodulin-dependent protein kinase activity at a reduced level. Ca2+-dependent mobility shift assays using E. coli-expressed protein from residues 358 520 revealed that Ca2+ binds to the visinin-like domain. Studies with site-directed mutants of the visinin-like domain indicated that EF-hands II and III are crucial for Ca2+-induced conformational changes in the visinin-like domain. Autophosphorylation of CCaMK increases Ca2+/calmodulin-dependent protein kinase activity by about 5-fold, whereas it did not affect its Ca2+-independent activity. This report provides evidence for the existence of a protein kinase in plants that is modulated by Ca2+ and Ca2+/calmodulin. The presence of a visinin-like Ca2+-binding domain in CCaMK adds an additional Ca2+-sensing mechanism not previously known to exist in the Ca2+/calmodulin-mediated signaling cascade in plants.

Functional characterization of a guanylyl cyclase-activating protein from vertebrate rods. Cloning, heterologous expression, and localization

The membrane-bound guanylyl cyclase in vertebrate photoreceptor cells is one of the key enzymes in visual transduction. It is highly sensitive to the free calcium concentration ([Ca2+]). The activation process is cooperative and mediated by a novel calcium-binding protein named GCAP (guanylyl cyclase-activating protein). We isolated GCAP from bovine rod outer segments, determined amino acid sequences of proteolytically obtained peptides, and cloned its gene. The Ca2+-bound form of native GCAP has an apparent molecular mass of 20.5 kDa and the Ca2+-free form of 25 kDa as determined by SDS-polyacrylamide gel electrophoresis. Recombinant GCAP was functionally expressed in Escherichia coli. Activation of guanylyl cyclase in vertebrate photoreceptor cells by native acylated GCAP was half-maximal at 100 nM free [Ca2+] with a Hill coefficient of 2.5. Activation by recombinant nonacylated GCAP showed a lower degree of cooperativity (n = 2.0), and half-maximal activation was shifted to 261 nM free [Ca2+]. Immunocytochemically we localized GCAP only in rod and cone cells of a bovine retina.

Biosynthesis of the unsaturated 14-carbon fatty acids found on the N termini of photoreceptor-specific proteins

In the vertebrate retina, a number of proteins involved in signal transduction are known to be N-terminal acylated with the unusual 14 carbon fatty acids 14:1n-9 and 14:2n-6. We have explored possible pathways for producing these fatty acids in the frog retina by incubation in vitro with candidate precursor fatty acids bearing radiolabels, including [3H]14:0, [3H]18:1n-9, [3H]18:2n-6, and [3H]18:3n-3. Rod outer segments were prepared from the radiolabeled retinas for analysis of protein-linked fatty acids, and total lipids were extracted from the remaining retinal pellet. Following saponification of extracted lipids, fatty acid phenacyl esters were prepared and analyzed by high pressure liquid chromatography (HPLC) with detection by continuous scintillation counting. Transducin, whose alpha-subunit (Gt alpha) is known to bear N-terminal acyl chains, was extracted from the rod outer segments and subjected to SDS-polyacrylamide gel electrophoresis and fluorography to detect radiolabeled proteins. Gt alpha was also subjected to methanolysis, and the resulting fatty acyl methyl esters were analyzed by HPLC. The identities of HPLC peaks coinciding with unsaturated species of both phenacyl esters and methyl esters were confirmed by reanalyzing them after catalytic hydrogenation. The results showed that 14:1n-9 can be derived in the retina from 18:1n-9 and 14:2n-6 from 18:2n-6, most likely by two rounds of beta-oxidation, but that neither is produced in detectable amounts from 14:0. Retroconversion of unsaturated 18 carbon fatty acids to the corresponding 14 carbon species showed specificity, in that 18:3n-3 was not converted to 14 carbon fatty acids in detectable amounts. Myristic acid (14:0), 14:1n-9, and 14:2n-6 were all incorporated into Gt alpha. A much less efficient incorporation of 18:1n-9 into Gt alpha was also observed, but no radiolabeling of Gt alpha was observed in retinas incubated with 18:3n-3. Thus, retroconversion by limited beta-oxidation of longer chain unsaturated fatty acids appears to be the most likely metabolic source of the unusual fatty acids found on the N termini of signal transducing proteins in the retina.

Regulation of sensitivity in vertebrate rod photoreceptors by calcium

Over the past decade and a half, there have been great advances in our understanding of how light is transduced into electrical signals by the retinal rod and cone photoreceptors in vertebrates. One essential feature of these sensory neurons is their ability to adapt to background illumination, which allows them to function over a broad range of light intensities. This adaptation appears to arise mostly from negative feedback on phototransduction that is mediated by calcium ions. Recent work has suggested that this feedback is fairly complex, and involves several pathways directed at different components of phototransduction. From direct measurements of these feedback pathways in rods, it is possible to evaluate their relative contributions to the overall sensitivity of the cell. At the same time, these feedback mechanisms, as currently known, appear to be sufficient for explaining the change in sensitivity of rods during adaptation to light.

How photons start vision

Recent studies have elucidated how the absorption of a photon in a rod or cone cell leads to the generation of the amplified neural signal that is transmitted to higher-order visual neurons. Photoexcited visual pigment activates the GTP-binding protein transducin, which in turn stimulates cGMP phosphodiesterase. This enzyme hydrolyzes cGMP, allowing cGMP-gated cationic channels in the surface membrane to close, hyperpolarize the cell, and modulate transmitter release at the synaptic terminal. The kinetics of reactions in the cGMP cascade limit the temporal resolution of the visual system as a whole, while statistical fluctuations in the reactions limit the reliability of detection of dim light. Much interest now focuses on the processes that terminate the light response and dynamically regulate amplification in the cascade, causing the single photon response to be reproducible and allowing the cell to adapt in background light. A light-induced fall in the internal free Ca2+ concentration coordinates negative feedback control of amplification. The fall in Ca2+ stimulates resynthesis of cGMP, antagonizes rhodopsin's catalytic activity, and increases the affinity of the light-regulated cationic channel for cGMP. We are using physiological methods to study the molecular mechanisms that terminate the flash response and mediate adaptation. One approach is to observe transduction in truncated, dialyzed photoreceptor cells whose internal Ca2+ and nucleotide concentrations are under experimental control and to which exogenous proteins can be added. Another approach is to observe transduction in transgenic mouse rods in which specific proteins within the cascade are altered or deleted.

The development of neural visinin-like Ca(2+)-binding protein 2 immunoreactivity in the rat neocortex and hippocampus

Neural visinin-like Ca(2+)-binding protein 2 (NVP2) immunoreactivity in the rat neocortex and hippocampus was barely detectable by immunoblot analysis on postnatal day 1 (P1), but increased during postnatal weeks 2-3, reaching a plateau on P28. Immunohistochemical analysis revealed moderate immunoreactivity firstly on P7 in some subsets of the hippocampal interneurons and in the hippocampal pyramidal cells and dentate granule cells. Immunoreactivity of the interneurons decreased during postnatal weeks 2-3 and disappeared by P28. In contrast, immunoreactivity of the cortical and hippocampal pyramidal cells and dentate granule cells abruptly increased during postnatal week 2. The distinctly immunoreactive cells were distributed throughout the neocortex, especially in the cortical plate, and the stratum pyramidale of Ammon's horn and granular layer of the dentate gyrus on P14. Immunoreactivity was homogeneously concentrated in the cell bodies and proximal dendrites at this stage, whereas thereafter immunoreactivity in the neuropil gradually increased, and underwent a relative decrease in the cell bodies. By P28, the higher and granular immunoreactivity in the neuropil covered whole layers of the neocortex, Ammon's horn and the dentate gyrus, the same as in adults. Differential expression of NVP2 in different neuron populations may reflect the differential functional consequences for neuronal development.

Cloning, sequencing, and expression of a 24-kDa Ca(2+)-binding protein activating photoreceptor guanylyl cyclase

Two vertebrate photoreceptor-specific membrane guanylyl cyclases, RetGC-1 and RetGC-2, are activated by a soluble 24-kDa retinal protein, p24, in a Ca(2+)-sensitive manner (Dizhoor, A.M., Lowe, D.G., Olshevskaya, E.V., Laura, R.P., and Hurley, J.B. (1994) Neuron 12, 1345-1352; Lowe, D.G., Dizhoor, A.M., Liu, K., Gu, O., Laura, R., Lu, L., and Hurley, J.B. (1995) Proc. Natl. Acad. Sci. U.S.A. 92, 5535-5539). The primary structure of bovine p24 has been derived from peptide sequencing and from its cDNA. p24 is a new EF-hand-type Ca(2+)-binding protein, related but not identical to another guanylyl cyclase-activating protein, GCAP (Palczewski, K., Subbaraya, I., Gorczyca, W.A., Helekar, B.S., Ruiz, C.C., Ohguro, H. Huang, J., Zhao, X., Crabb, J.W., Johnson, R.S., Walsh, K.A., Gray-Keller, M.P., Detwiler, P.B., and Baehr, W. (1994) Neuron 13, 395-404) and other members of the recovering family of Ca(2+)-binding proteins. Antibodies against a truncated fusion protein and against a p24-specific synthetic peptide specifically recognize retinal p24 on immunoblot. Both antibodies inhibit activation of photoreceptor membrane guanylyl cyclase by purified p24. p24 is found only in retina, and it copurifies with outer segment membranes. Immunocytochemical analysis shows that it is present in rod photoreceptor cells. An immobilized antibody column was used to purify p24 from a heat-treated retinal extract. Purified p24 appears on SDS-polyacrylamide gel electrophoresis as a homogeneous protein not contaminated with GCAP, and it activates photoreceptor guanylyl cyclase in vitro at submicromolar concentrations. Ca2+ inhibits this activation with an EC50 near 200 nM and a Hill coefficient of 1.7. Recombinant p24 expressed in 293 cells effectively stimulates photoreceptor guanylyl cyclase. These findings demonstrate that p24, like GCAP, imparts Ca2+ sensitivity to photoreceptor membrane guanylyl cyclase. We propose that p24 be referred to as GCAP-2 and that GCAP be referred to as GCAP-1.

Control of photoreceptor proteins by Ca2+

A decrease of cytoplasmic Ca(2+)-concentration in vertebrate photoreceptor cells after illumination is necessary for light adaptation. Although the mechanisms of adaptation is not completely understood, several Ca(2+)-dependent cellular processes have been discovered. Some involve calcium-binding proteins like recoverin, guanylyl cyclase-activating protein and calmodulin, and their target proteins rhodopsin kinase, guanylyl cyclase, the cGMP-gated channel, and NO synthase. The activity of several enzymes or channels is directly controlled by Ca2+ and does not involve calcium-binding proteins. These proteins are pyrophosphatase, protein kinase C and the cGMP-gated channel.