P26-calcium binding protein from bovine retinal photoreceptor cells

Structural basis for calcium-induced inhibition of rhodopsin kinase by recoverin

Recoverin, a member of the neuronal calcium sensor branch of the EF-hand superfamily, serves as a calcium sensor that regulates rhodopsin kinase (RK) activity in retinal rod cells. We report here the NMR structure of Ca(2+)-bound recoverin bound to a functional N-terminal fragment of rhodopsin kinase (residues 1-25, called RK25). The overall main-chain structure of recoverin in the complex is similar to structures of Ca(2+)-bound recoverin in the absence of target (<1.8A root-mean-square deviation). The first eight residues of recoverin at the N terminus are solvent-exposed, enabling the N-terminal myristoyl group to interact with target membranes, and Ca(2+) is bound at the second and third EF-hands of the protein. RK25 in the complex forms an amphipathic helix (residues 4-16). The hydrophobic face of the RK25 helix (Val-9, Val-10, Ala-11, Ala-14, and Phe-15) interacts with an exposed hydrophobic groove on the surface of recoverin lined by side-chain atoms of Trp-31, Phe-35, Phe-49, Ile-52, Tyr-53, Phe-56, Phe-57, Tyr-86, and Leu-90. Residues of recoverin that contact RK25 are highly conserved, suggesting a similar target binding site structure in all neuronal calcium sensor proteins. Site-specific mutagenesis and deletion analysis confirm that the hydrophobic residues at the interface are necessary and sufficient for binding. The recoverin-RK25 complex exhibits Ca(2+)-induced binding to rhodopsin immobilized on concanavalin-A resin. We propose that Ca(2+)-bound recoverin is bound between rhodopsin and RK in a ternary complex on rod outer segment disk membranes, thereby blocking RK interaction with rhodopsin at high Ca(2+).

Functional analysis of the human recoverin gene promoter

PURPOSE

Following photoactivation of rhodopsin, recoverin inhibits rhodopsin kinase activity in retinal photoreceptors by reducing the binding of arrestin to rhodopsin and therefore prolonging the termination of the phototransduction cascade. To identify potential cis-elements that may be involved in understanding the mechanisms that determine the cell-specific expression of recoverin in retinal and cancerous cells, the promoter region of the human recoverin gene was studied in cultured human Y79 retinoblastoma cells.

METHODS

A 2.5 kb EcoRI fragment of a 9.4 kb cosmid that contains the 5' non-coding region of the human recoverin gene was sequenced and cloned into an expression vector upstream of a luciferase gene. Cultured Y79 human retinoblastoma cells were transfected with DNA constructs using lipofection. Deletion mutants were generated by site-directed mutagenesis, cloned into the expression vector, and transfected into Y79 cells. Reporter gene activity was measured with a luciferase assay, and normalized to beta-beta-galactosidase activity resulting from a co-transfected SV-beta-galactosidase SV40 vector.

RESULTS

Reporter gene expression in the transfected Y79 cells demonstrated an increase in activity between 232 and 620 bp from the translational start site of the recoverin gene. There was a decrease in the reporter gene expression between 900 and 1200 bp from the start site, followed by an increase between 1200 and 1440 bp.

CONCLUSIONS

This analysis suggests that there are cis-acting elements in the 5' non-coding region of the recoverin gene that are involved in the activation and suppression of gene expression.

Subsets of retinal progenitors display temporally regulated and distinct biases in the fates of their progeny

Cell fate determination in the developing vertebrate retina is characterized by the sequential generation of seven classes of cells by multipotent progenitor cells. Despite this order of genesis, more than one cell type is generated at any time; for example, in the rat, several cell types are born during the prenatal period, while others are born postnatally. In order to examine whether there are classes of progenitor cells with distinct developmental properties contributing to this developmental progression, we examined antigen expression in progenitor cells during rat retinal development. Two markers of amacrine and horizontal cells, the VC1.1 epitope and syntaxin, were found to be expressed on a subset of progenitors in a temporally regulated manner that closely paralleled the birthdays of these cell types. In order to investigate which cell types were produced by the progenitors expressing these markers, fluorescent latex microspheres covalently coupled to VC1.1 antibodies were used to indelibly label VC1.1+ progenitor cells and their progeny. Early in retinal development, VC1.1+ progenitors generated a high percentage of amacrine and horizontal cells, but no cone photoreceptors. During this same period, a comparable number of cone photoreceptors were generated by VC1.1- progenitors. In the late embryonic and early postnatal period, VC1.1+ progenitors continued to generate predominantly amacrine cells, but also gave rise to an increasing number of rod photoreceptors. These findings demonstrate that expression of these two markers by progenitors is highly correlated with a bias towards the production of amacrine and horizontal cells. The fact that subsets of progenitors with temporally regulated and distinct biases are intermingled within the retinal neuroepithelium provides a basis for understanding how different cell types are generated both simultaneously and in a particular order by multipotent progenitors during retinal development.

Analysis of protein modifications: recent advances in detection, characterization and mapping

The past year has seen several contributions, both in methods for determining and characterizing chemical modifications of proteins and in related technologies used to map peptides. These contributions mainly involve improvements in capillary zone electrophoresis and in various applications of mass spectrometry.

Distribution of hippocalcin mRNA and immunoreactivity in rat brain

Distribution of hippocalcin in rat brain was analysed by in situ hybridization and immunohistochemical methods. Hippocalcin mRNA and immunoreactivity were expressed more intensely in the pyramidal cells of the hippocampus, intensely in the Purkinje cells of the cerebellum, moderately in the dentate granule cells and pyramidal cells of cerebral cortex layers II-VI and weakly in the large neuronal cells of the caudate-putamen. Some discrepancies in the localization of hippocalcin mRNA and immunoreactivity were noted in the mamillary nuclei, anterior part of the thalamus and the septal nuclei. In most cell types, hippocalcin immunoreactivity was localized in the cytoplasm and plasma membrane of cell bodies and dendrites.

The effect of recoverin-like calcium-binding proteins on the photoresponse of retinal rods

The rod photoresponse is triggered by an enzyme cascade that stimulates cGMP hydrolysis. The resulting fall in cGMP leads to a decrease in Ca2+, which promotes photoresponse recovery by activating guanylate cyclase, causing cGMP resynthesis. In vitro biochemical studies suggest that Ca2+ activation of guanylate cyclase is medicated by recoverin, a 26 kd Ca(2+)-binding protein. To evaluate this, exogenous bovine recoverin and two other homologous Ca(2+)-binding proteins from chicken and Gecko retina were dialyzed into functionally intact Gecko rods using whole-cell recording. All three proteins prolonged the rising phase of the photoresponse without affecting the kinetics of response recovery. These results suggest that recoverin-like proteins affect termination of the transduction cascade, rather than mediate Ca(2+)-sensitive activation of guanylate cyclase.

On the activation of phosphodiesterase by a 26 kDa protein

The effects of a 26 kDa protein isolated from vertebrate retina rod outer segments (ROS) and its reconstituted analog on the phosphodiesterase (PDE) activity and cGMP-dependent conductance have been studied [Nature 313 (1985) 310-313]. Using the patch-clamp technique it was shown that the 26 kDa protein in concentrations up to 1 microM accelerates hydrolysis of cGMP by near-membrane PDE by 1-2 orders of magnitude. This process is suggested to be mediated by some intracellular agent. At the same concentrations the 26 kDa protein was shown to inhibit cGMP-dependent conductance of the photoreceptor membrane. A possible role of these effects in the processes of phototransduction and adaptation is discussed.

Recoverin, a novel calcium-binding protein from vertebrate photoreceptors

Photoreceptor guanylyl cyclase activity is modulated by an endogenous calcium-binding protein called recoverin. A modified isolation procedure for recoverin using gel-filtration chromatography instead of a heat denaturation step is presented. The elution volume of recoverin corresponds to a monomer. Recoverin exhibits a calcium-dependent mobility shift in a native gel electrophoresis. Isoelectric focusing revealed a pI of 5.25. No subspecies of recoverin were detected.

Light-sensitivity modulating protein in frog rods

Cyclic GMP is the second messenger in the phototransduction mechanism in rod photoreceptors. Light-induced activation of cGMP phosphodiesterase (PDE), the hydrolyzing enzyme of cGMP, reduces cytoplasmic cGMP concentration to close the cGMP-activated channel and thereby causes a hyperpolarizing light response. Ca2+ concentration decreases during light-adaptation and this decrease is thought to be at least one of the underlying mechanisms of light-adaptation. Our previous electrophysiological work suggested that PDE in frog rod photoreceptors is regulated by this Ca2+ concentration decrease. In the present work, we isolated a protein that binds to disk membranes at high Ca2+ concentrations. In the presence of this protein (a 26 kDa protein), PDE light sensitivity becomes high at high Ca2+ concentrations. The effect was observed at physiological ranges of Ca2+ concentrations. Thus we could explain high light-sensitivity of photoreceptors under the dark-adapted condition. According to its function, we termed the 26 kDa protein 'sensitivity-modulating protein' or 'S-modulin'. During the purification we noticed that there are additional mechanisms present that may contribute to light-adaptation in frog rod photoreceptors.

Molecular cloning of hippocalcin, a novel calcium-binding protein of the recoverin family exclusively expressed in hippocampus

We have isolated a cDNA clone encoding a novel calcium-binding protein of the recoverin family from rat brain cDNA library. This clone (PCB11) has 588 nucleotides in the open reading frame including the termination codon, 174 nucleotides of the 5' leader and 800 nucleotides of the 3' noncoding region. The complete amino acid sequence deduced from the cDNA is composed of 195 residues, has a calculated molecular mass of 22,574 Daltons, and contains three putative calcium-binding domains of the EF-hand structure. The deduced amino acid sequence has a striking sequence homology to those of the retinal recoverin family (recoverin, visinin, P26, 23kD protein, S-modulin) and the brain-derived recoverin family (P23k, 21-kDa CaBP and neurocalcin). Northern blot, in situ hybridization, immunoblot and immunohistochemical analyses revealed that the protein is exclusively expressed in pyramidal layer of the hippocampus. The protein was therefore designated hippocalcin.

Isolation of human retinal genes: recoverin cDNA and gene

A human retina cDNA library enriched for retina-specific clones was prepared by subtraction with a non-retina population of cDNA in combination with polymerase chain reaction (PCR) amplifications. A highly retina-specific cDNA clone (1190 bp) was obtained through this library encoding a 200 amino acid protein with three calcium binding sites and 87% homology to the bovine photoreceptor protein, recoverin, which has been shown to mediate the recovery of the dark current after photoactivation, and 58% homology to the calcium-binding chick cone protein, visinin. Analysis of the gene indicated a 9-10 kb single-copy gene with at least three exons and two introns. The three exons contained the entire coding sequence, and all of the calcium-binding EF-hand regions were in putative exon 1. The recoverin gene was mapped to human chromosome 17 by hybridization to a panel of human-rodent hybrid DNAs.

Purification and characterization of S-modulin, a calcium-dependent regulator on cGMP phosphodiesterase in frog rod photoreceptors

S-modulin is a 26 kDa protein that regulates light sensitivity of cGMP phosphodiesterase in a Ca(2+)-dependent manner in frog rod outer segments (ROSs). In the present study, we purified S-modulin by taking advantage of a hydrophobic interaction between Phenyl Sepharose and S-modulin at high Ca2+ concentrations. The yield was greater than 90%. 45Ca(2+)-binding experiment showed that S-modulin is a Ca(2+)-binding protein. At high Ca2+ concentrations, S-modulin binds to ROS membranes. The binding target of the Ca2+/S-modulin complex is possibly a ROS membrane lipid(s), but it was difficult to identify. The binding was observed mainly at greater than 1 microM Ca2+. The amino acid sequence deduced from proteolytic fragments of S-modulin was approximately 80% and 60% identical to those of recovering and visinin, respectively.

Cloning, expression, and crystallization of recoverin, a calcium sensor in vision

Recoverin, a recently discovered 23-kDa calcium-binding protein, activates retinal rod guanylate cyclase when the calcium level is lowered in the submicromolar range. We report here the cloning and sequencing of a cDNA for recoverin from a bovine retinal expression library. The recoverin coding sequence was inserted into a pET-11a expression vector under control of the T7 phage promoter. A second expression system, in which the coding sequence was placed under control of the lambda phage PR promoter, gave 10-fold higher yields (10 mg of purified recoverin per liter of Escherichia coli culture). The finding that retinal recoverin is myristoylated at its amino terminus led us to coexpress the recombinant protein and N-myristoyltransferase (EC 2.3.1.97). Myristoylated recombinant recoverin formed in this way in E. coli is like retinal recoverin in exhibiting a large calcium-induced shift in its tryptophan fluorescence emission spectrum. The availability of abundant protein enabled us to crystallize unmyristoylated recombinant recoverin and initiate x-ray studies. The space group of tetragonal crystals obtained from 75% saturation ammonium sulfate is I4 with unit cell dimensions a = 85.1 A and c = 59.8 A. These crystals of the calcium-bound form of the protein diffracted to a resolution of 2.2 A. The expression systems described here open the door to high-resolution x-ray crystallographic and nuclear magnetic resonance studies of this new member of the EF-hand superfamily and to the elucidation of its precise mode of action as a calcium switch.

Visinin: biochemical and molecular comparisons in normal and rd chick retina

Western, northern and DNA sequence analyses were used to determine if the retinal protein, visinin, is defective in the chicken retinal degeneration mutant, rd. A 22kDa band, corresponding to purified visinin, was stained with equal intensity on Western blots of +/+, +/rd and rd/rd retinal protein probed with a visinin polyclonal antibody. Hybridization of a northern blot of +/+, +/rd and rd/rd poly(A)+ RNA with a random-primer labelled visinin cDNA probe showed a single, equally labelled 1-Kbp band in each of the samples. Finally, no differences were found between the nucleic acid sequences of the 579 bp cDNAs encoding +/+ and rd/rd visinin. We did, however, find one significant difference between our visinin DNA sequence and the previously published chick visinin DNA sequence. We consistently observed a C at position 118 rather than the published T which changes the amino acid residue at position 40 from serine to proline. Based on the results of this study, we conclude that visinin is not defective in the rd chick model of hereditary retinal degeneration.

Isolation and characterization of recoverin-like Ca(2+)-binding protein from rat brain

Rat brain was found, by immunoblot analysis, to have a protein of Mr 23,000 (P23k) that was clearly different from recoverin and was labeled with an antiserum raised against the NH2-terminus of recoverin. P23k could not be detected by an antiserum raised against the COOH-terminus of recoverin. Blots with 45Ca demonstrated that P23k bound Ca2+. This calciprotein was further purified by Ca(2+)-dependent hydrophobic interaction and ion-exchange chromatography. In SDS polyacrylamide gel electrophoresis, P23k had an apparent Mr of 21,000 in the presence of 10 microM Ca2+ and 23,000 in the absence of Ca2+ (0.1 mM EGTA). The isoelectric point of P23k was 5.6. Ca(2+)-binding analysis indicated that P23k bound 2 moles of Ca2+ per mole of protein and had two binding sites with dissociation constants of 13 microM and 0.2 microM. Purified P23k bound to the crude membrane fractions from the cerebellum, cerebrum and retina in a Ca(2+)-dependent manner. Partial amino acid sequence analysis of proteolytic fragments of P23k revealed the sequence homology between P23k and recoverin. These results suggested that P23k may act as a Ca(2+)-sensitive regulator by forming a complex with its target on the membrane.