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

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

A human homolog of yeast pre-mRNA splicing gene, PRP31, underlies autosomal dominant retinitis pigmentosa on chromosome 19q13.4 (RP11)

We report mutations in a gene (PRPF31) homologous to Saccharomyces cerevisiae pre-mRNA splicing gene PRP31 in families with autosomal dominant retinitis pigmentosa linked to chromosome 19q13.4 (RP11; MIM 600138). A positional cloning approach supported by bioinformatics identified PRPF31 comprising 14 exons and encoding a protein of 499 amino acids. The level of sequence identity to the yeast PRP31 gene indicates that PRPF31 is also likely to be involved in pre-mRNA splicing. Mutations that include missense substitutions, deletions, and insertions have been identified in four RP11-linked families and three sporadic RP cases. The identification of mutations in a pre-mRNA splicing gene implicates defects in the splicing process as a novel mechanism of photoreceptor degeneration.

Interactions within the coiled-coil domain of RetGC-1 guanylyl cyclase are optimized for regulation rather than for high affinity

RetGC-1, a member of the membrane guanylyl cyclase family of proteins, is regulated in photoreceptor cells by a Ca(2+)-binding protein known as GCAP-1. Proper regulation of RetGC-1 is essential in photoreceptor cells for normal light adaptation and recovery to the dark state. In this study we show that cGMP synthesis by RetGC-1 requires dimerization, because critical functions in the catalytic site must be provided by each of the two polypeptide chains of the dimer. We also show that an intact alpha-helical coiled-coil structure is required to provide dimerization strength for the catalytic domain of RetGC-1. However, the dimerization strength of this domain must be precisely optimized for proper regulation by GCAP-1. We found that Arg(838) within the dimerization domain establishes the Ca(2+) sensitivity of RetGC-1 by determining the strength of the coiled-coil interaction. Arg(838) substitutions dominantly enhance cGMP synthesis even at the highest Ca(2+) concentrations that occur in normal dark-adapted photoreceptor cells. Molecular dynamics simulations suggest that Arg(838) substitutions disrupt a small network of salt bridges to allow an abnormal extension of coiled-coil structure. Substitutions at Arg(838) were first identified by linkage to the retinal degenerative disease, autosomal dominant cone rod dystrophy (adCORD). Consistent with the characteristics of this disease, the Arg(838)-substituted RetGC-1 mutants exhibit a dominant biochemical phenotype. We propose that accelerated cGMP synthesis in humans with adCORD is the primary cause of cone-rod degeneration.

Patello-femoral tracking in the weight-bearing knee: a study of asymptomatic volunteers utilising dynamic magnetic resonance imaging: a preliminary report

Normal patello-femoral tracking is not well defined, and conventional radiological techniques do not allow imaging in the physiological, weight-bearing stance. A vertical-access open configuration magnetic resonance scanner allows imaging of patello-femoral tracking during weight-bearing and through a wide range of knee flexion. We imaged 40 asymptomatic knees in this way, producing axial scans which were analysed qualitatively and quantitatively using sulcus angle, congruence angle, lateral patello-femoral angle and patellar centralisation, to assess patellar tilt and displacement. Mild lateral tilting in hyperextension with the quadriceps relaxed was seen, but quantitative assessment of this was impeded by internal rotation of the femur in extension. One-half of the knees were slightly laterally displaced in hyper-extension, becoming central during the first 30 degrees of knee flexion. During passive flexion of the knee in a seated position, fewer knees were laterally tilted or displaced, and no consistent change was seen during flexion. These results indicate that mild lateral tilting and displacement can be normal phenomena in the weight-bearing knee in early flexion and should not necessarily be taken as evidence of abnormal tracking in symptomatic patients. Lateral to medial movement of the patella occurs during normal knee flexion. In addition, imaging in the weight-bearing knee can provide valuable information not gained by imaging during passive knee flexion.

Seeing rainbows

Mammals generally have colour vision inferior to that of other vertebrates. This is partially redressed in primates, but the underlying mechanisms differ in Old World and New World primate groups. How has this occurred and what were the key events in the molecular evolution of primate colour vision?

Spectrum of dominant mutations in the desmosomal cadherin desmoglein 1, causing the skin disease striate palmoplantar keratoderma

The adhesive proteins of the desmosome type of cell junction consist of two types of cadherin found exclusively in that structure, the desmogleins and desmocollins, coded by two closely linked loci on human chromosome 18q12.1. Recently we have identified a mutation in the DSG1 gene coding for desmoglein 1 as the cause of the autosomal dominant skin disease striate palmoplantar keratoderma (SPPK) in which affected individuals have marked hyperkeratotic bands on the palms and soles. In the present study we present the complete exon-intron structure of the DSG1 gene, which occupies approximately 43 kb, and intron primers sufficient to amplify all the exons. Using these we have analysed the mutational changes in this gene in five further cases of SPPK. All were heterozygotic mutations in the extracellular domain leading to a truncated protein, due either to an addition or deletion of a single base, or a base change resulting in a stop codon. Three mutations were in exon 9 and one in exon 11, both of which code for part of the third and fourth extracellular domains, and one was in exon 2 coding for part of the prosequence of this processed protein. This latter mutation thus results in the mutant allele synthesising only 25 amino acid residues of the prosequence of the protein so that this is effectively a null mutation implying that dominance in the case of this mutation was caused by haploinsufficiency. The most severe consequences of SPPK mutations are in regions of the body where pressure and abrasion are greatest and where desmosome function is most necessary. SPPK therefore provides a very sensitive measure of desmosomal function.

Characterization of a novel human opsin gene with wide tissue expression and identification of embedded and flanking genes on chromosome 1q43

As part of an ongoing search to identify novel mammalian photopigments that may mediate nonvisual tasks such as circadian entrainment and acute suppression of pineal melatonin levels, a number of recently cloned nonvisual opsin sequences were used to search dbEST. panopsin (OPN3) was one of the clones identified using this approach. Expression analysis detects two transcripts of approximately 2.1 and 2.5 kb, in a wide range of tissues including brain, liver, and retina, which encode a predicted protein of 403 amino acids. The gene was localized to the region of chromosome 1q43 also encompassing the kynurenine monooxygenase (KMO) and choroideremia-like Rab escort protein 2 (CHML) genes. KMO and panopsin overlap at their 3' ends but are transcribed in opposite directions. CHML, an intronless gene, lies in intron 1 of panopsin.

The destabilization of human GCAP1 by a proline to leucine mutation might cause cone-rod dystrophy

Guanylate cyclase activating protein-1 (GCAP1) is required for activation of retinal guanylate cyclase-1 (RetGC1), which is essential for recovery of photoreceptor cells to the dark state. In this paper, experimentally derived observations are reported that help in explaining why a proline-->leucine mutation at position 50 of human GCAP1 results in cone-rod dystrophy in a family carrying this mutation. The primary amino acid sequence of wild-type GCAP1 was mutated using site-directed mutagenesis to give a leucine at position 50. In addition, serine replaced a glutamic acid residue at position 6 to promote N-terminal myristoylation, yielding the construct GCAP1 E6S/P50L. The enzyme was over-expressed in Escherichia coli cells, isolated and purified before being used in assays with RetGC1, characterized by circular dichroism (CD) spectroscopy, and investigated for protease resistance and thermal stability. Assays of cyclic guanosine monophosphate (cGMP) synthesis from guanosine triphosphate by RetGC1 in the presence of E6S/P50L showed that E6S/P50L could activate RetGC1 and displayed similar calcium sensitivity to wild-type GCAP1. In addition, E6S/P50L and wild-type GCAP1 possess similar CD spectra. However, there was a marked increase in the susceptibility to protease degradation and also a reduction in the thermal stability of E6S/P50L as observed by both the cGMP assay and CD spectroscopy. It is therefore suggested that although GCAP1 E6S/P50L has a similar activity and calcium dependency profile to the wild-type GCAP1, its lower stability could reduce its cellular concentration, which would in turn alter [Ca2+] and result in death of cells.

Functional characterization of missense mutations at codon 838 in retinal guanylate cyclase correlates with disease severity in patients with autosomal dominant cone-rod dystrophy

Three different mutations in codon 838 of GUCY2D, the gene for retinal guanylate cyclase 1, have been linked to autosomal dominant cone-rod dystrophy at the CORD6 locus. To examine the relationship between enzyme activity and disease severity, the three disease-causing substitutions (R838C, R838H and R838S) and four artificial mutations (R838A, R838E, R838L and R838K) were generated. Assay of GCAP1-stimulated cyclase activity in vitro shows that, compared with wild-type, R838E, R838L and R838K possess only low activity, whereas R838A, R838C, R838H and R838S have activity equal or superior to wild-type at low Ca(2+) concentrations. These four latter mutants showed a higher apparent affinity for GCAP1 than did wild-type. The Ca(2+) sensitivity of the GCAP1 activation was also altered with marked residual activity at high Ca(2+), the effect increasing: wild-type < R838C < R838H << R838A < R838S. Within the photoreceptor, this would result in a failure to inactivate cyclase activity at high physiological Ca(2+ )concentrations. Amongst the three disease-associated mutations, the effect correlates directly with disease severity. The wild-type and R838H mutant displayed a difference in pH sensitivity, with the mutant showing a higher specific activity with pH > 6.0. Site 838 is in the dimerization domain that forms a coiled-coil in the active protein. A computer-aided structure prediction of this region indicates that R838 in the wild-type breaks the structure at four helical turns, and there is an increasing tendency for the structure to continue for further turns in the order R838C < R838H,S,K << R838E < R838A < R838L.

Cloning, sequencing and analysis of the enterocin biosynthesis gene cluster from the marine isolate 'Streptomyces maritimus': evidence for the derailment of an aromatic polyketide synthase

BACKGROUND

Polycyclic aromatic polyketides, such as the tetracyclines and anthracyclines, are synthesized by bacterial aromatic polyketide synthases (PKSs). Such PKSs contain a single set of iteratively used individual proteins for the construction of a highly labile poly-beta-carbonyl intermediate that is cyclized by associated enzymes to the core aromatic polyketide. A unique polyketide biosynthetic pathway recently identified in the marine strain 'Streptomyces maritimus' deviates from the normal aromatic PKS model in the generation of a diverse series of chiral, non-aromatic polyketides.

RESULTS

A 21.3 kb gene cluster encoding the biosynthesis of the enterocin and wailupemycin family of polyketides from 'S. maritimus' has been cloned and sequenced. The biosynthesis of these structurally diverse polyketides is encoded on a 20 open reading frames gene set containing a centrally located aromatic PKS. The architecture of this novel type II gene set differs from all other aromatic PKS clusters by the absence of cyclase and aromatase encoding genes and the presence of genes encoding the biosynthesis and attachment of the unique benzoyl-CoA starter unit. In addition to the previously reported heterologous expression of the gene set, in vitro and in vivo expression studies with the cytochrome P-450 EncR and the ketoreductase EncD, respectively, support the involvement of the cloned genes in enterocin biosynthesis.

CONCLUSIONS

The enterocin biosynthesis gene cluster represents the most versatile type II PKS system investigated to date. A large series of divergent metabolites are naturally generated from the single biochemical pathway, which has several metabolic options for creating structural diversity. The absence of cyclase and aromatase gene products and the involvement of an oxygenase-catalyzed Favorskii-like rearrangement provide insight into the observed spontaneity of this pathway. This system provides the foundation for engineering hybrid expression sets in the generation of structurally novel compounds for use in drug discovery.

Visual responses of ganglion cells of a New-World primate, the capuchin monkey, Cebus apella

1. The genetic basis of colour vision in New-World primates differs from that in humans and other Old-World primates. Most New-World primate species show a polymorphism; all males are dichromats and most females trichromats. 2. In the retina of Old-World primates such as the macaque, the physiological correlates of trichromacy are well established. Comparison of the retinae in New- and Old-World species may help constrain hypotheses as to the evolution of colour vision and the pathways associated with it. 3. Ganglion cell behaviour was recorded from trichromatic and dichromatic members of a New-World species (the capuchin monkey, Cebus apella) and compared with macaque data. Despite some differences in quantitative detail (such as a temporal response extended to higher frequencies), results from trichromatic animals strongly resembled those from the macaque. 4. In particular, cells of the parvocellular (PC) pathway showed characteristic frequency-dependent changes in responsivity to luminance and chromatic modulation, cells of the magnocellular (MC) pathway showed frequency-doubled responses to chromatic modulation, and the surround of MC cells received a chromatic input revealed on changing the phase of heterochromatically modulated lights. 5. Ganglion cells of dichromats were colour-blind versions of those of trichromats. 6. This strong physiological homology is consistent with a common origin of trichromacy in New- and Old-World monkeys; in the New-World primate the presence of two pigments in the middle-to-long wavelength range permits full expression of the retinal mechanisms of trichromatic vision.

Genomic organization and amplification of the human desmosomal cadherin genes DSC1 and DSC3, encoding desmocollin types 1 and 3

The desmosomal cadherins comprise the desmocollins and desmogleins and are involved in epithelial cell-cell adhesion. There are three desmocollins (DSC 1-3) and three desmogleins (DSG 1-3) that are expressed in a tissue- and development-specific manner. Desmosomal proteins have been implicated in a number of disorders characterized by loss of cell-cell adhesion and trauma-induced skin fragility. Therefore, the desmocollins are potential candidates for genodermatoses involving epithelial tissues. In order to screen the entire DSC1 and DSC3 genes, we have characterized their intron-exon organization. The DSC1 gene comprises 17 exons spanning approximately 33 kb on 18q12.1, and the DSC3 gene comprises 17 exons spanning approximately 49 kb on 18q12.1. We have also developed a comprehensive PCR-based mutation detection strategy for desmocollins 1, 2, and 3 using primers placed on flanking introns followed by direct sequencing of the PCR products.

Clinical outcome of congenital talipes equinovarus diagnosed antenatally by ultrasound

Congenital talipes equinovarus is a common anomaly which can now be diagnosed prenatally on a routine ultrasound scan at 20 weeks of gestation. Prenatal counselling is increasingly offered to parents with affected fetuses, but it is difficult to counsel parents if there is a chance that the fetus may not have talipes. Our study correlates the prenatal ultrasound findings of 14 infants diagnosed as having unilateral or bilateral talipes during their routine 20-week ultrasound scan with their clinical findings at birth and the treatment received. No feet diagnosed as talipes on the ultrasound scan were completely normal at birth and therefore there were no true false-positive results. One foot graded as normal at 20 weeks was found to have a mild grade-1 talipes at birth, but did not require treatment other than simple stretches. A total of 32% of feet required no treatment and so could be considered functional false-positive results on the scan. Serial casting was required by 13% of feet and surgical treatment by 55%. The severity of the talipes is difficult to establish before birth. A number of patients are likely to need surgical treatment, but a proportion will have talipes so mildly that no treatment will be required. In counselling parents at 20 weeks, orthopaedic surgeons need to know whether or not there is a small chance that the ultrasound diagnosis could be wrong and also that the talipes may be so mild that the foot will not require treatment.

Spectral tuning of avian violet- and ultraviolet-sensitive visual pigments

The violet- and ultraviolet-sensitive visual pigments of birds belong to the same class of pigments as the violet-sensitive (so-called blue) pigments of mammals. However, unlike the pigments from mammals and other vertebrate taxa which, depending on species, have lambda(max) values of either around 430 nm or around 370 nm, avian pigments are found with lambda(max) values spread across this range. In this paper, we present the sequences of two pigments isolated from Humbolt penguin and pigeon with intermediate lambda(max) values of 403 and 409 nm, respectively. By comparing the amino acid sequences of these pigments with the true UV pigments of budgerigar and canary and with chicken violet with a lambda(max) value of 420 nm, we have been able to identify five amino acid sites that show a pattern of substitution between species that is consistent with differences in lambda(max). Each of these substitutions has been introduced into budgerigar cDNA and expressed in vitro in COS-7 cells. Only three resulted in spectral shifts in the regenerated pigment; two had relatively small effects and may account for the spectral shifts between penguin, pigeon, and chicken whereas one, the replacement of Ser by Cys at site 90 in the UV pigments, produced a 35 nm shortwave shift that could account for the spectral shift from 403 nm in penguin to around 370 nm in budgerigar and canary.

Restoration of photoreceptor ultrastructure and function in retinal degeneration slow mice by gene therapy

The gene Prph2 encodes a photoreceptor-specific membrane glycoprotein, peripherin-2 (also known as peripherin/rds), which is inserted into the rims of photoreceptor outer segment discs in a complex with rom-1 (ref. 2). The complex is necessary for the stabilization of the discs, which are renewed constantly throughout life, and which contain the visual pigments necessary for photon capture. Mutations in Prph2 have been shown to result in a variety of photoreceptor dystrophies, including autosomal dominant retinitis pigmentosa and macular dystrophy. A common feature of these diseases is the loss of photoreceptor function, also seen in the retinal degeneration slow (rds or Prph2 Rd2/Rd2) mouse, which is homozygous for a null mutation in Prph2. It is characterized by a complete failure to develop photoreceptor discs and outer segments, downregulation of rhodopsin and apoptotic loss of photoreceptor cells. The electroretinograms (ERGs) of Prph2Rd2/Rd2 mice have greatly diminished a-wave and b-wave amplitudes, which decline to virtually undetectable concentrations by two months. Subretinal injection of recombinant adeno-associated virus (AAV) encoding a Prph2 transgene results in stable generation of outer segment structures and formation of new stacks of discs containing both perpherin-2 and rhodopsin, which in many cases are morphologically similar to normal outer segments. Moreover, the re-establishment of the structural integrity of the photoreceptor layer also results in electrophysiological correction. These studies demonstrate for the first time that a complex ultrastructural cell defect can be corrected both morphologically and functionally by in vivo gene transfer.

The orthopaedic management of peripheral ischaemia in meningococcal septicaemia in children

Between March 1993 and February 1999, 14 children aged from eight months to 14.75 years were admitted to the paediatric intensive-care unit with meningococcal septicaemia in association with severe peripheral ischaemia. Of these, 13 were operated upon, eight of whom had early fasciotomies. Five children died. Of the nine survivors, one had no amputations while in the other eight 14 limb segments were amputated. We review the case histories and propose a protocol for the early management of these children.

A cluster of single nucleotide polymorphisms in the 5'-leader of the human dopamine D3 receptor gene (DRD3) and its relationship to schizophrenia

The association between schizophrenia and the Ser9Gly variant of the dopamine D3 receptor gene (DRD3) has been the subject of numerous studies. Under meta-analysis this site, or one or more in linkage disequilibrium with it, appears to contribute a small increase to the relative risk of schizophrenia. In this study, 768 bp of the 5'-leader region of DRD3 mRNA was screened for polymorphisms to assess their contribution to the association of DRD3 with schizophrenia. A cluster of three single nucleotide polymorphisms (SNPs) was identified in tight linkage disequilibrium with each other and with the Ser9Gly polymorphism. One of the 5'-leader SNPs encodes a Lys9Glu variant within a 36 amino acid residue stretch of an upstream open reading frame (uORF). Two common haplotypes are found in the population examined; one is linked to the Ser9 coding variant and the other to the Gly9 variant. A panel of 73 schizophrenic patients and 56 matched controls recruited from the East Anglia region of the United Kingdom was screened for disease association at these sites. Since the 5'-leader and coding sites are in tight disequilibrium, the combined genotype of all 4 sites was scored for each patient. A significant association was seen between disease and the frequency distribution of these genotypes (chi2 = 13.19, d.f. = 3, P = 0.0042; Cochran method for sparse cells applied). A 20% excess of one of the heterozygous genotypes, in which the sequences differ at three of the four SNPs, including Ser9/Gly9 in the receptor and Lys9/Glu9 in the uORF, was found in the patient group. An absence of association of disease with the Ser9Gly polymorphism had previously been reported for this panel. This suggests that these SNPs and the corresponding coding changes may exert a combined or synergistic effect on susceptibility to schizophrenia.

Autosomal dominant cone-rod retinal dystrophy (CORD6) from heterozygous mutation of GUCY2D, which encodes retinal guanylate cyclase

OBJECTIVE

To describe the clinical features of autosomal dominant cone-rod retinal dystrophy (CRD) in a British family mapping to chromosome 17p12-p13 (CORD6), with a heterozygous mutation (Glu837Asp/ Arg838Ser) of GUCY2D.

DESIGN

A prospective, clinical family survey.

PATIENTS

Ten affected members of a family with autosomal dominant CRD.

METHODS

Full clinical examinations were undertaken. Selected affected family members underwent electrophysiologic evaluation, scotopic static perimetry, dark adaptometry, and color vision assessment.

MAIN OUTCOME MEASURES

Clinical appearance and electroretinographic responses.

RESULTS

Typical clinical and electroretinographic features of childhood-onset CRD were recorded. In addition, moderate myopia and pendular nystagmus were seen in affected individuals. Color vision assessment in the youngest affected individual showed no color discrimination on a tritan axis, but retention of significant red-green discrimination. Electronegative electroretinogram responses were seen on electrophysiology in the only young family member examined.

CONCLUSIONS

The phenotype associated with GUCY2D CRD is clinically distinct from that associated with other dominant CRD loci. Unusual electroretinographic responses may indicate that this mutation of GUCY2D is associated with early defects in photoreceptor synaptic transmission to second-order neurons.

Clustered cadherin genes: a sequence-ready contig for the desmosomal cadherin locus on human chromosome 18

We describe the assembly of a cosmid and PAC contig of approximately 700 kb on human chromosome 18q12 spanning the DSC and DSG genes coding for the desmocollins and desmogleins. These are members of the cadherin superfamily of calcium-dependent cell adhesion proteins present in the desmosome type of cell junction found especially in epithelial cells. They provide the strong cell-cell adhesion generated by this type of cell junction for which expression of both a desmocollin and a desmoglein is required. In the autoimmune skin diseases pemphigus foliaceous and pemphigus vulgaris (PV), where the autoantigens are, respectively, encoded by the DSG1 and DSG3 genes, severe areas of acantholysis (cell separation), potentially life-threatening in the case of PV, are evident. Dominant mutations in the DSG1 gene causing striate palmoplantar keratoderma result in hyperkeratosis of the skin on the parts of the body where pressure and abrasion are greatest, viz., on the palms and soles. These genes are also candidate tumor suppressor genes in squamous cell carcinomas and other epithelial cancers. We have screened two chromosome 18-specific cosmid libraries by hybridization with previously isolated YAC clones and DSC and DSG cDNAs, and a whole genome PAC library, both by hybridization with the YACs and by screening by PCR using cDNA sequences and YAC end sequence. The contigs were extended by further PCR screens using STSs generated by vectorette walking from the ends of the cosmids and PACs, together with sequence from PAC ends. Despite screening of two libraries, the cosmid contig still had four gaps. The PAC contig filled these gaps and in fact covered the whole locus. The positions of 45 STSs covering the whole of this region are presented. The desmocollin and desmoglein genes, which are about 30-35 kb in size, are quite well separated at approximately 20-30 kb apart and are arranged in two clusters, one DSC cluster and one DSG cluster, which are transcribed outward from the interlocus region. The order of the genes is correlated with the spatial order of gene expression in the developing mouse embryo, and this, and previous transgenic experiments, suggests that long-range genetic elements that coordinate expression of these genes may be present. The complete bacterial clone contig described in this paper is thus a resource not only for future sequencing but also for investigations into the control of expression of these clustered genes.

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.

Visual pigments and oil droplets in the retina of a passerine bird, the canary Serinus canaria: microspectrophotometry and opsin sequences

The visual receptors of the passeriform bird Serinus canaria, the canary, have been examined microspectrophotometrically and the sequences of the opsins determined. Rods have a maximum absorbance (lambda max) at 506 nm. Four spectral classes of single cone are present: long-wave-sensitive (LWS) containing a photopigment with lambda max at 569 nm, middle-wave-sensitive (MWS) with lambda max at 505 nm, short-wave-sensitive (SWS) with lambda max at 442 nm, and ultraviolet-sensitive (UVS) with lambda max at about 366 nm. Double cones possess the 569-nm pigment in both members. Typical combinations of photopigment and oil droplet occur in most cone classes. An ambiguity exists in the oil droplet of the single LWS cones. In some birds, LWS cones are paired with an R-type droplet, whereas in the majority of canaries the LWS pigment is paired with a droplet similar to the P-type of double cones. Mechanisms of spectral tuning within each opsin class are discussed.

Enhanced retinal longwave sensitivity using a chlorophyll-derived photosensitiser in Malacosteus niger, a deep-sea dragon fish with far red bioluminescence

Through partial bleaching of both visual pigment extracts and cell suspensions we show that the deep-sea stomiid Malacosteus niger, which produces far red bioluminescence, has two visual pigments within its retina which form a rhodopsin/porphyropsin pigment pair with lambda max values around 520 and 540 nm, but lacks the very longwave sensitive visual pigments (lambda max > 550 nm) observed in two other red light producing stomiids. The presence of only a single opsin gene in the M. niger genome was confirmed by molecular and cladistic analysis. To compensate for its apparently reduced longwave sensitivity compared to related species, the outer segments of M. niger contain additional pigments, which we identify as a mixture of defarnesylated and demetallated derivatives of bacteriochlorophylls c and d, that are used as a photosensitiser to enhance its sensitivity to longwave radiation.