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R S Molday

Publications and source records attributed to R S Molday.

At least 37 records · Page 2Linked to original sources
PubMed1998

Molecular properties of the cGMP-gated channel of rod photoreceptors.

The cGMP-gated channel of the rod photoreceptor cell plays a key role in phototransduction by controlling the flow of Na+ and Ca2+ into the outer segment in response to light-induced changes in cGMP concentrations. The rod channel is composed of two homologous subunits designated as alpha and beta. Each subunit contains a core region of six putative membrane spanning segments, a cGMP binding domain, a voltage sensor-like motif and a pore region. In addition the beta-subunit contains an extended N-terminal region that is identical in sequence to a previously cloned retinal glutamic acid rich protein called GARP. Three spliced variants of GARP (the GARP part of the beta channel subunit; full length free GARP; and a truncated form of GARP) are expressed in rod cells and localized within the outer segments. Immunoaffinity chromatography has been used to purify the channel from detergent solubilized rod outer segments. A significant fraction of the rod Na+/Ca(2+)-K+ exchanger copurifies with the channel as measured by western blotting suggesting that the channel can interact with the exchanger under certain conditions.

Amino Acids↗
PubMed1997

The Na/Ca-K exchanger of rod photoreceptor exists as dimer in the plasma membrane.

The oligomeric state of the Na/Ca-K exchanger in the plasma membrane of bovine photoreceptors was investigated using chemical cross-linking techniques. In the natural membrane, virtually all Na/Ca-K exchanger could be cross-linked mainly to a complex having an apparent molecular mass of 490 kDa by cupric phenanthroline catalyzed disulfide bonding as evidenced by Western blotting. Stable cross-links of the exchanger were also obtained with the thiol-specific reagent N,N'-p-phenylidenedimaleimide. Neuraminidase treatment reduced the apparent molecular mass of the highly glycosylated Na/Ca-K exchanger and of the 490 kDa cross-link product by 50 and 85 kDa, respectively. DL-1,4-Bismaleimido-2,3-butanediol (BMBD), a novel cleavable dimaleimide, was synthesized in order to produce cross-links that were stable to reductive conditions. Purification of the BMBD cross-linked exchanger followed by two-dimensional SDS polyacrylamide electrophoresis identified the cross-linked homodimers of the exchanger. There was no indication of higher oligomers, suggesting that the exchanger exists as a dimer in the plasma membrane. Hydrodynamic properties of the detergent-solubilized exchanger were determined by velocity sedimentation and gel filtration chromatography. The Triton X-100-solubilized exchanger ran as a single species having a Stokes radius of 10.0 nm, a sedimentation coefficient of 5.4 S, and a partial specific volume of 0.74 mL/g in Triton X-100. Similar results were obtained for the CHAPS-solubilized exchanger. A molecular mass of 236 and 205 kDa was calculated for the exchanger-detergent complex and the detergent-free protein, respectively. Neuraminidase treatment further reduced the molecular mass of the exchanger indicating that glycosylation contributes significantly to the mass of the exchanger. Cross-links of the exchanger were not detected if cross-linking was attempted after solubilization in 10 mM CHAPS. However, after reconstitution of the purified exchanger into soybean phosphatidylcholine vesicles, chemical cross-linking yielded again dimers. On the basis of these cross-linking and hydrodynamic studies, we conclude that the exchanger exists as a homodimer in the rod outer segment plasma membrane but dissociates into a monomer when solubilized in detergent.

Animals↗
PubMed1997

Evidence from normal and degenerating photoreceptors that two outer segment integral membrane proteins have separate transport pathways.

Detachment of the neural retina from the retinal pigment epithelium induces photoreceptor degeneration. We studied the effects of this degeneration on the localization of two photoreceptor outer segment-specific integral membrane proteins, opsin and peripherin/rds, in rod photoreceptors. Results from laser scanning confocal microscopic and electron microscopic immunolocalization demonstrate that these two proteins, normally targeted to the newly-forming discs of the outer segments, accumulate in different sub-cellular compartments during photoreceptor degeneration: opsin immunolabeling increases throughout the photoreceptor cell's plasma membrane, while peripherin/rds immunolabeling occurs within cytoplasmic vesicles. The simplest hypothesis to explain our results is that these proteins are transported in different post-Golgi transport vesicles and separately inserted into the plasma membrane. More complex mechanisms involve having the two co-transported and then opsin finds its way into the plasma membrane but peripherin/rds does not, remaining behind in vesicles. Alternatively, both insert into the plasma membrane but peripherin/rds is recycled into cytoplasmic vesicles. We believe the data most strongly supports the first possibility. Although the transport pathways for these proteins have not been fully characterized, the presence of peripherin/rds-positive vesicles adjacent to the striated rootlet suggests a transport role for this cytoskeletal element. The accumulation of these proteins in photoreceptors with degenerated outer segments may also indicate that their rate of synthesis has exceeded the combined rates of their incorporation into newly forming outer segment disc membranes and their degradation. The accumulation may also provide a mechanism for rapid recovery of the outer segment following retinal reattachment and return of the photoreceptor cell to an environment favorable to outer segment regeneration.

Animals↗
PubMed1997

The 220-kDa rim protein of retinal rod outer segments is a member of the ABC transporter superfamily.

Outer segments of mammalian rod photoreceptor cells contain an abundantly expressed membrane protein that migrates with an apparent molecular mass of 220 kDa by SDS-gel electrophoresis. We have purified the bovine protein by immunoaffinity chromatography, determined its primary structure by cDNA cloning and direct peptide sequence analysis, and mapped its distribution in photoreceptors by immunocytochemical and biochemical methods. The full-length cDNA encodes a 2280-amino acid protein (calculated molecular mass of 257 kDa) consisting of two structurally related, tandem arranged halves. Each half consists of a hydrophobic domain containing six putative transmembrane segments followed by an ATP-binding cassette. A data base homology search showed that the rod outer segment 220-kDa protein is 40-50% identical in amino acid sequence to the ABC1 and ABC2 proteins cloned from a mouse macrophage cell line. Photoaffinity labeling with 8-azido-ATP and nucleotide inhibition studies confirmed that both ATP and GTP bind to this protein with similar affinities. Concanavalin A labeling and endoglycosidase H digestion indicated that the rod outer segment protein contains at least one carbohydrate chain. Immunocytochemical and biochemical studies have revealed that the 220-kDa glycoprotein is distributed along the rim region and incisures of rod outer segment disc membranes. From these studies we conclude that the 220-kDa glycoprotein of bovine rod outer segment disc membranes or Rim ABC protein is a new member of the superfamily of ABC transporters and is the mammalian homolog of the frog photoreceptor rim protein.

ATP-Binding Cassette Transporters↗
PubMed1996

Primary structure and expression of the human beta-subunit and related proteins of the rod photoreceptor cGMP-gated channel.

The full-length cDNA for the beta-subunit of the human rod photoreceptor cyclic nucleotide-gated channel has been shown to encode a 1251-amino acid ( approximately 140 kDa) polypeptide which, like its bovine counterpart, has an unusual bipartite structure. The C-terminal part corresponds to the previously reported "subunit 2" of the human rod channel and contains the structural features of other cyclic nucleotide-gated channel subunits including six putative membrane spanning segments, a cyclic nucleotide binding domain, a voltage-sensor motif, and a pore region. The N-terminal part contains the human homolog of the bovine glutamic acid-rich protein called GARP. Western blots indicate that both the native and heterologously expressed human beta-subunit migrate anomalously as a 220-kDa polypeptide by SDS-gel electrophoresis. Two other GARP variants, full-length GARP (f-GARP) and truncated GARP (t-GARP), are also present in human, bovine, and rat rod outer segments and migrate as 120-140- and 55-62-kDa polypeptides, respectively. The bovine f-GARP and t-GARP cDNAs code for proteins containing 590 amino acids and 299 amino acids, respectively. The first 571 amino acids of f-GARP and the first 291 amino acids of t-GARP are identical to the corresponding N-terminal amino acid sequence of the bovine beta-subunit. The two GARP variants, themselves, are not tightly associated with the rod channel. These results indicate that mammalian rod outer segments contain three alternatively spliced variants of GARP, one of which constitutes the N-terminal part of the rod channel beta-subunit.

Amino Acid Sequence↗
PubMed1996

Defective subunit assembly underlies a digenic form of retinitis pigmentosa linked to mutations in peripherin/rds and rom-1.

Retinitis pigmentosa (RP) is a group of progressive retinal dystrophies that include the most common hereditary degenerative disease affecting the retina. Although most disease phenotypes appear to result from defects at single genetic loci (monogenic), at least one instance of RP appears to require a coinheritance of defects in the unlinked peripherin/rds and rom-1 alleles (digenic), which encode the polypeptide subunits of an oligomeric transmembrane protein complex present at photoreceptor outer segment disc rims. Sedimentation velocity analysis was performed upon the affected gene products expressed heterologously in COS-1 cells to examine the assembly of the subunit polypeptides. The results indicate that the missense peripherin/rds mutant, L185P, which segregates with instance of digenically inherited RP, is conditionally defective with respect to its subunit assembly. Unlike wild-type peripherin/rds, the L185P mutant does not form native-like homotetramers on its own; however, the L185P mutant can assemble with wild-type rom-1 to form a structurally normal heterotetrameric complex. These findings provide a novel molecular-based rationale for the unusual digenic disease inheritance pattern and offer insight into regions of peripherin/rds and rom-1, which contribute to subunit-subunit interactions.

Animals↗
PubMed1996

Subunit composition of the peripherin/rds-rom-1 disk rim complex from rod photoreceptors: hydrodynamic evidence for a tetrameric quaternary structure.

Peripherin/rds and rom-1 are homologous integral membrane protein subunits found as an oligomeric complex at the rim regions of rod and cone photoreceptor outer segment disks. These proteins are essential for the morphogenesis of normal outer segments and have been linked to a variety of human retinal degenerative diseases. Previous studies have suggested that disulfide-linked homodimers of peripherin/rds and rom-1 can associate noncovalently to form higher order structures. We have characterized the hydrodynamic properties of Triton X-100 solubilized peripherin/rds-rom-1 complexes from bovine ROS membranes by gel exclusion chromatography on Sepharose C1-6B and velocity sedimentation through H2O- and D2O-based sucrose gradients. A single hydrodynamic species is observed which has a Stokes radius of 6.2 nm, a sedimentation coefficient (S20,w) of 5.8 S, and a partial specific volume of 0.83 mL/g. From these data the molecular mass of the detergent-peripherin/rds-rom-1 complex is calculated to be 240 kDa. The protein component of this complex is estimated to be 135 kDa, providing direct evidence that the solubilized peripherin/rds-rom-1 complex is a tetramer. The abundance of this complex as measured by competitive ELISA and immunoaffinity purification is approximately 4% of total bovine ROS membrane protein and indicates that peripherin/rds-rom-1 tetramers are present at a relatively high average surface density (ca. 4100/ microns m2) at the rim surfaces of rod outer segment disks.

Animals↗
PubMed1996

Calmodulin regulation of cyclic-nucleotide-gated channels.

Cyclic-nucleotide-gated (CNG) channels play key roles in photoreceptor and olfactory signal-transduction pathways. Recent studies have focused on the molecular characterization of CNG channel subunits and on the identification of the structural domains that contribute to ligand selectivity and affinity, ion gating and permeation, and regulation of channel activity. Calmodulin has been shown to bind directly to the rod and olfactory channels and to modulate their sensitivity to cyclic nucleotides. This Ca2+-dependent regulation of channel activity appears to play a role in the termination of the signal-transduction pathway in olfactory neurons and rod photoreceptor cells. It remains to be determined whether calmodulin also regulates the activity of related channels in other cells.

Animals↗
PubMed1996

Cell-specific expression of amiloride-sensitive, Na(+)-conducting ion channels in the kidney.

Amiloride-sensitive, electrogenic Na+ absorption across the distal nephron plays a vital role in regulating extracellular fluid volume and blood pressure. Recently, two amiloride-sensitive, Na(+)-conducting ion channel cDNAs were cloned. One, an epithelial Na(+)-selective channel (ENaC), is responsible for Na+ absorption throughout the distal nephron. The second, a guanosine 3',5'-cyclic monophosphate (cGMP)-inhibitable cation channel, is conductive to Na+ and Ca2+ and contributes to Na+ absorption across the inner medullary collecting duct (IMCD). As a first step toward understanding the segment-specific contributions(s) of cGMP-gated cation channels and ENaC to Na+ and Ca2+ uptake along the nephron, we used in situ reverse transcription-polymerase chain reaction (RT-PCR) hybridization, solution-phase RT-PCR, and Western blot analysis to examine the nephron and cell-specific expression of these channels in mouse kidney cell lines and/or dissected nephron segments. cGMP-gated cation channel mRNA was detected in proximal tubule, medullary thick ascending limb (mTAL), distal convoluted tubule (DCT), cortical collecting duct (CCD), outer medullary collecting duct (OMCD), and IMCD. cGMP-gated cation channel protein was detected in DCT, CCD, and IMCD cell lines. These observations suggest that hormones that modulate intracellular cGMP levels may regulate Na+, and perhaps Ca2+, uptake throughout the nephron. mRNA for alpha-mENaC, a subunit of the mouse ENaC, was detected in mTAL, DCT, CCD, OMCD, and IMCD. Coexpression of alpha-mENaC and cGMP-gated cation channel mRNAs in mTAL, DCT, CCD, OMCD, and IMCD suggests that both channels may contribute to Na+ absorption in these nephron segments.

Amiloride↗
PubMed1996

Molecular cloning, membrane topology, and localization of bovine rom-1 in rod and cone photoreceptor cells.

PURPOSE: To characterize the molecular properties, cellular distribution, and subcellular distribution of bovine rom-1 and its interaction with peripherin/rds in photoreceptor cells as an important step toward elucidating the role of rom-1 in photoreceptor outer segment structure-function relationships and in inherited retinal degenerative disorders. METHODS: Bovine rom-1 cDNA, including a portion of the promoter region, was cloned, sequenced, and heterologously expressed in CHO-K1 and COS-1 cultured mammalian cells. Monoclonal and polyclonal antibodies to specific regions of bovine rom-1 were generated and used with biochemical and immunocytochemical techniques to study the membrane topology, subcellular distribution, and interaction of rom-1 with peripherin/rds. RESULTS: The primary structure of bovine rom-1 is highly homologous to that of human and mouse rom-1. Proteolytic digestion studies and immunolabeling studies of rom-1 in rod outer segment membranes indicate that the C-terminus of rom-1 is localized to the cytoplasmic side and that a large segment is localized to the lumen side of the disc membrane. Post-embedding and pre-embedding immunogold labeling studies for electron microscopy show that rom-1 is localized to the rim region of bovine rod outer segment disc membranes; rom-1 or a closely related homologue also is present at the rim region of cone outer segment disc membranes. Immunofluorescence studies of mammalian cells expressing rom-1 indicate that rom-1 is not translocated to the plasma membrane but instead is retained in internal cellular membranes. Immunoprecipitation studies indicate that all rom-1 and peripherin/rds from rod outer segments form a tightly associated complex. CONCLUSIONS: Rom-1 and peripherin/rds are two structurally related protein subunits of an integral membrane protein complex found on the rim region of rod outer segment disc membranes. Rom-1 or a homologue also is present in the rim region of cone outer segment disc membranes, where it most likely associates with peripherin/rds to form a membrane protein complex.

Amino Acid Sequence↗
PubMed1995

Heterologous expression of photoreceptor peripherin/rds and Rom-1 in COS-1 cells: assembly, interactions, and localization of multisubunit complexes.

Peripherin/rds is a 39 kDa integral membrane glycoprotein essential for normal photoreceptor cell development in vertebrates. It has been implicated in several human retinal degenerative diseases including retinitis pigmentosa and macular degeneration and is thought to play a structural role at photoreceptor outer segment disk rims, where it forms a tightly-associated complex with rom-1, a nonglycosylated 37 kDa homologue. Western blot analysis of COS-1 cells transiently transfected with full-length cDNA coding for either peripherin/rds or rom-1 indicates that each protein is expressed primarily as a disulfide-linked homodimer; recombinant peripherin/rds is glycosylated while recombinant rom-1 is not--akin to their counterparts in rod photoreceptor disk membranes. Upon cotransfection of the two cDNAs, the specific assembly of a stable peripherin/rds--rom-1 complex is observed. Immunofluorescence microscopy studies demonstrate that both singly and coexpressed peripherin/rds and rom-1 complexes are localized primarily within internal membranes of transfected cells. Velocity sedimentation data indicate that the recombinant complexes (4.9 S) are assembled with a subunit stoichiometry similar to those extracted from ROS membranes (4.5 S) and are most consistent with a tetrameric arrangement of polypeptides. Sedimentation analyses of individually expressed peripherin/rds (5.1 S) and rom-1 (4.3 S) suggest that each polypeptide can also assemble into a tetrameric form in the absence of its homologue partner. Subunit assembly and interactions are discussed in terms of their potential role in hereditary retinal diseases.

Animals↗
PubMed1994

Interaction of calmodulin with the cyclic GMP-gated channel of rod photoreceptor cells. Modulation of activity, affinity purification, and localization.

The cGMP-gated cation channel of rod photoreceptor cells plays a central role in the phototransduction process by controlling the influx of cations into the rod outer segment in response to changes in cGMP levels. Previous studies have shown that the cGMP-gated channel in native rod outer segment membrane vesicles is modulated by calmodulin in a calcium-dependent manner. In this study we report that the immunoaffinity-purified channel consisting of the 63-kDa alpha-subunit and a 240-kDa protein is also modulated by calmodulin when reconstituted into lipid vesicles. In the absence of calmodulin, the purified channel had an apparent Km of 33 microM and a Hill coefficient of 3.3 for cGMP-dependent efflux of Ca2+ from reconstituted lipid vesicles. In the presence of calmodulin, the Km increased to 44 microM without affecting the Hill coefficient or maximum velocity of ion efflux. Calmodulin modulation of the channel is inhibited by the calmodulin antagonist, mastoparan. In the absence of mastoparan, the half-maximum inhibition of channel activity (IC50) occurred at 1.85 +/- 0.25 nM calmodulin at a cGMP concentration of 12.5 microM; in the presence of mastoparan, the IC50 value increased to 20.3 +/- 3.8 nM calmodulin. Based on the strong, selective interaction of calmodulin with the channel, an efficient, general method has been developed to isolate functionally active cGMP-gated channels from mammalian and amphibian photoreceptor membranes. Calmodulin extraction studies, Western blotting, and channel activity measurements indicate that endogenous rod outer segment calmodulin modulates the activity of the channel through its binding to the 240-kDa protein. From these studies we conclude that the 240-kDa protein of the cGMP-gated channel is a major calmodulin target protein of rod outer segment membranes.

Animals↗
PubMed1994

Subunit 2 (or beta) of retinal rod cGMP-gated cation channel is a component of the 240-kDa channel-associated protein and mediates Ca(2+)-calmodulin modulation.

The cGMP-gated cation channel mediating visual transduction in retinal rods was recently found to comprise at least two subunits, 1 and 2 (or alpha and beta). SDS gels of the purified channel show, in addition to a 63-kDa protein band (subunit 1), a 240-kDa protein band that binds Ca(2+)-calmodulin, a modulator of the channel. To examine any connection between subunit 2 and the 240-kDa protein, cGMP-gated channels formed from the expressed cloned subunits in human embryonic kidney (HEK) 293 cells were tested for Ca(2+)-calmodulin effect. Homooligomeric channels formed by subunit 1 alone showed no sensitivity to Ca(2+)-calmodulin, and neither did heterooligomeric channels formed by subunit 1 and the short alternatively spliced form of subunit 2 (2a). By contrast, the cGMP half-activation constant (K1/2) for heterooligomeric channels formed from subunit 1 and the long form of subunit 2 (2b) was increased 1.5- to 2-fold by Ca(2+)-calmodulin, similar to the increase observed for the native channel. In Western blots of rod outer segment membranes, a subunit 2-specific antibody also recognized the 240-kDa protein. Finally, amino acid sequences derived from peptide fragments of the bovine 240-kDa protein showed approximately 80% identity to regions of subunit 2b of the human channel. These results together suggest that subunit 2b of the rod channel is a component of the 240-kDa protein and that it mediates the Ca(2+)-calmodulin modulation of the channel.

Amino Acid Sequence↗
PubMed1994

Glucose metabolism in photoreceptor outer segments. Its role in phototransduction and in NADPH-requiring reactions.

Glucose metabolism in the photoreceptor rod outer segment produces both ATP (GTP) and NADPH to support phototransduction and NADPH-requiring processes in this organelle. Glycolysis in isolated bovine rod outer segments produces 44.0 +/- 6.4 nmol of ATP/min/mg of protein or 5.7 mM ATP/min. This rate of ATP production is more than sufficient to maintain the basal rate of cGMP synthesis (0.86 mM cGMP/min) in the dark requiring 1.7 mM ATP/min. Following photoexcitation, the 4.5-fold increase in the turnover of cGMP requires an ATP synthesis rate of up to 7.7 mM ATP/min (Ames, A., Walseth, T. F., Heyman, R. A., Barad, M., Graeff, R. M., and Goldberg, N. D. (1986) J. Biol. Chem. 261, 13034-13042). Under these conditions the rate of ATP production by glycolysis as measured in isolated rod outer segments is not sufficient for the regeneration of cGMP. Additional energy is most likely provided by the phosphocreatine shuttle which transports high energy phosphate groups in the form of creatine phosphate from the rod inner segment to the rod outer segment for conversion to ATP. The hexose monophosphate pathway in bovine rod outer segments can produce up to 39.8 +/- 2.2 nmol of NADPH/min/mg of protein. This rate of NADPH production is sufficient to support both the reduction of retinal to retinol (1.2 +/- 0.2 nmol of NADPH/min/mg of protein) following the photobleaching of rhodopsin and glutathione reduction (1.1 +/- 0.1 nmol of NADPH/min/mg of protein) for the protection of rod outer segments from oxidative damage. These studies provide insight into the contribution of anaerobic glycolysis and the hexose monophosphate pathway in providing energy and nucleotides for phototransduction and other outer segment processes.

Adenosine Triphosphate↗
PubMed1993

Structural and functional properties of rhodopsin from rod outer segment disk and plasma membrane.

The structural and functional properties of bovine rhodopsin from rod outer segment disk and plasma membranes were compared by high performance liquid chromatography (HPLC), mass spectrometric analyses, and in vitro rhodopsin phosphorylation assays. Disk and plasma membranes separated by a ricin gold-dextran affinity perturbation method were treated with trypsin or cyanogen bromide, and the N-terminal and C-terminal rhodopsin peptides were isolated by immunoaffinity chromatography using antirhodopsin monoclonal antibodies coupled to Sepharose. Reverse phase HPLC chromatograms of the C-terminal and N-terminal peptides from disk and plasma membrane rhodopsin were found to be similar. Mass spectrometric, PicoTag, and hexose analyses of the tryptic 1-16 N-terminal peptides further indicated that the post-translational glycosylation of plasma membrane rhodopsin is identical to that of disk membrane rhodopsin. HPLC analysis of soluble peptides obtained from cyanogen bromide and tryptic digestion of immunoaffinity purified rhodopsin also indicated that no significant differences exist between disk and plasma membrane rhodopsin. Light-induced phosphorylation of rhodopsin in disk and plasma membranes were also compared using in vitro phosphorylation assays. Plasma membrane rhodopsin was found to undergo light-dependent, rhodopsin kinase catalyzed phosphorylation to the same extent as disk membrane rhodopsin. These results indicate that the bulk rhodopsin in rod outer segment plasma membranes appears to be identical to rhodopsin in disk membranes in regard to primary structure, post-translational glycosylation and light-dependent phosphorylation. On this basis, it is unlikely that the sorting of rhodopsin between disk and plasma membranes occurs by a mechanism based on differences in structural properties of rhodopsin.

Animals↗
PubMed1993

Modulation of the cGMP-gated channel of rod photoreceptor cells by calmodulin.

Photobleaching of rhodopsin in rod photoreceptors activates the visual cascade system leading to a decrease in cyclic GMP and the closure of cGMP-gated channels in the rod outer segment plasma membrane. Calcium plays an important role in the recovery of the rod outer segment to its dark state by regulating the resynthesis of cGMP by guanylate cyclase. Here we report that calmodulin, a Ca(2+)-binding protein present in the rod outer segment, increases the apparent Michaelis constant of the channel for cGMP. This results in a decrease in the rate of cation influx into the rod outer segment by two- to sixfold at low cGMP concentrations and has the effect of increasing the sensitivity of the channel to small changes in cGMP levels during phototransduction. Biochemical studies indicate that calcium-calmodulin binds to a protein of M(r) 240K which is tightly associated with the channel. On the basis of these studies, Ca2+ is suggested to play a central role in photorecovery and light adaptation, not only by regulating guanylate cyclase, possibly through recoverin, but also by modulating the cGMP-gated channel through calmodulin interaction with the 240K protein.

Animals↗
PubMed1993

Rod and cone photoreceptor cells express distinct genes for cGMP-gated channels.

Signal transduction in vertebrate rod and cone photoreceptor cells involves ion channels that are directly gated by the internal messenger cGMP. Rods and each type of cones express genetically related yet different forms of photopigments. Enzymes that control the light-stimulated hydrolysis of cGMP in rods and cones are also the product of distinct genes. Two different cDNA clones encoding cGMP-gated channels have been characterized from the chicken retina. Expression of cDNAs in Xenopus oocytes gives rise to cGMP-stimulated channel activity. Antibodies against a synthetic peptide specific for the C-terminal amino acid sequence derived from one clone stain outer segments of cone but not rod photoreceptors. Therefore chicken rod and cone cells each express different forms of cGMP-gated channels that are genetically related to each other. Expression in COS-1 cells produces the complete form of both channel polypeptides, whereas Western blot analysis indicates that channels in outer segment membranes are present in a processed form that is significantly shorter than the full-length polypeptide.

Amino Acid Sequence↗