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

Regulation of K+-Dependent Na+/Ca2+-Exchangers (NCKX)

Potassium-dependent sodium-calcium exchangers (NCKX) have emerged as key determinants of calcium (Ca²⁺) signaling and homeostasis, especially in environments where ion concentrations undergo large changes, such as excitatory cells and transport epithelia. The regulation of NCKX transporters enables them to respond to the changing cellular environment thereby helping to shape the extent and kinetics of Ca²⁺ signals. This review examines the current knowledge of the different ways in which NCKX activity can be modulated. These include (i) cellular and dynamic subcellular location (ii); changes in protein expression mediated at the gene, transcript, or protein level (iii); genetic changes resulting in altered protein structure or expression (iv); regulation via changes in substrate concentration (v); and post-translational modification, partner protein interactions, and allosteric regulation. Detailed mechanistic understanding of NCKX regulation is an emerging area of research with the potential to provide important new insights into transporter function, the control of Ca²⁺ signals, and possible interventions for dysregulated Ca²⁺ homeostasis.

Structure-function relationships of K+-dependent Na+/Ca2+ exchangers (NCKX)

K⁺-dependent Na⁺/Ca²⁺ exchanger proteins (NCKX1-5) of the SLC24 gene family play important roles in a wide range of biological processes including but not limited to rod and cone photoreceptor vision, olfaction, enamel formation and skin pigmentation. NCKX proteins are also widely expressed throughout the brain and NCKX2 and NCKX4 knockouts in mice have specific phenotypes. Here we review our work on structure-function relationships of NCKX proteins. We discuss membrane topology, domains critical to transport function, and residues critical to cation binding and transport function, all in the context of crystal structures that were obtained for the archaeal Na⁺/Ca²⁺ exchanger NCX_Mj.

Evolution of the genes mediating phototransduction in rod and cone photoreceptors

This paper reviews current knowledge of the evolution of the multiple genes encoding proteins that mediate the process of phototransduction in rod and cone photoreceptors of vertebrates. The approach primarily involves molecular phylogenetic analysis of phototransduction protein sequences, combined with analysis of the syntenic arrangement of the genes. At least 35 of these phototransduction genes appear to reside on no more than five paralogons - paralogous regions that each arose from a common ancestral region. Furthermore, it appears that such paralogs arose through quadruplication during the two rounds of genome duplication (2R WGD) that occurred in a chordate ancestor prior to the vertebrate radiation, probably around 600 millions years ago. For several components of the phototransduction cascade, it is shown that distinct isoforms already existed prior to WGD, with the likely implication that separate classes of scotopic and photopic photoreceptor cells had already evolved by that stage. The subsequent quadruplication of the entire genome then permitted the refinement of multiple distinct protein isoforms in rods and cones. A unified picture of the likely pattern and approximate timing of all the important gene duplications is synthesised, and the implications for our understanding of the evolution of rod and cone phototransduction are presented.

Evolution of the calcium feedback steps of vertebrate phototransduction

We examined the genes encoding the proteins that mediate the Ca-feedback regulatory system in vertebrate rod and cone phototransduction. These proteins comprise four families: recoverin/visinin, the guanylyl cyclase activating proteins (GCAPs), the guanylyl cyclases (GCs) and the sodium/calcium-potassium exchangers (NCKXs). We identified a paralogon containing at least 36 phototransduction genes from at least fourteen families, including all four of the families involved in the Ca-feedback loop (recoverin/visinin, GCAPs, GCs and NCKXs). By combining analyses of gene synteny with analyses of the molecular phylogeny for each of these four families of genes for Ca-feedback regulation, we have established the likely pattern of gene duplications and losses underlying the expansion of isoforms, both before and during the two rounds of whole-genome duplication (2R WGD) that occurred in early vertebrate evolution. Furthermore, by combining our results with earlier evidence on the timing of duplication of the visual G-protein receptor kinase genes, we propose that specialization of proto-vertebrate photoreceptor cells for operation at high and low light intensities preceded the emergence of rhodopsin, which occurred during 2R WGD.

The Na+/Ca2+, K+ exchanger NCKX4 is required for efficient cone-mediated vision

Calcium (Ca²⁺) plays an important role in the function and health of neurons. In vertebrate cone photoreceptors, Ca²⁺ controls photoresponse sensitivity, kinetics, and light adaptation. Despite the critical role of Ca²⁺ in supporting the function and survival of cones, the mechanism for its extrusion from cone outer segments is not well understood. Here, we show that the Na⁺/Ca²⁺, K⁺ exchanger NCKX4 is expressed in zebrafish, mouse, and primate cones. Functional analysis of NCKX4-deficient mouse cones revealed that this exchanger is essential for the wide operating range and high temporal resolution of cone-mediated vision. We show that NCKX4 shapes the cone photoresponse together with the cone-specific NCKX2: NCKX4 acts early to limit response amplitude, while NCKX2 acts late to further accelerate response recovery. The regulation of Ca²⁺ by NCKX4 in cones is a novel mechanism that supports their ability to function as daytime photoreceptors and promotes their survival.

DOI:http://dx.doi.org/10.7554/eLife.24550.001

Cells known as photoreceptors sense light in the eye. Light activates signaling pathways inside the photoreceptors that relay visual information to nerve cells, which carry the information to the brain. Photoreceptors called cone cells allow us to distinguish different colors of light and therefore play an important role in daytime vision. Over the course of the day, the overall levels of light in the environment can vary widely and so photoreceptors need to be able to adjust their signaling pathways so that they can still respond to light stimuli.

Calcium ions modulate the signaling pathways inside cone cells to help them adjust to changing light levels. These ions also play other important roles in the health and activity of photoreceptors, so the cells need to carefully control how many calcium ions they contain.

Cone cells contain a structure known as the outer segment, which is responsible for detecting light stimuli. It is believed that cones control the levels of calcium ions in the outer segment by balancing the flow of calcium ions into and out of the segment. The calcium ions enter the outer segment via channels that sit in the membrane surrounding the cell. A transporter protein known as NCKX2, which is only found in cone cells, was thought to pump the calcium ions out of the cell. However, recent data has challenged this idea by demonstrating that NCKX2 only plays a minor role in this process.

Vinberg et al. investigated how calcium ions leave the outer segments of cone cells in several different animals. The experiments show that a transporter protein called NCKX4 – which belongs to the same protein family as NCKX2 – is the main transporter involved in removing calcium ions from the cone cells of mice. Loss of NCKX4 from mouse cones reduced the ability of these cells to respond to fast and rapidly changing light stimuli, and to operate in bright light.

Further experiments show that NCKX4 is also found in the outer segments of zebrafish and monkey cone cells. The next challenges will be to find out if NCKX4 is also present in human cones and whether it plays a role in regulating our daytime vision.

DOI:http://dx.doi.org/10.7554/eLife.24550.002

Cation dependencies and turnover rates of the human K⁺-dependent Na⁺-Ca²⁺ exchangers NCKX1, NCKX2, NCKX3 and NCKX4

The Solute Carrier Family 24 (SLC24) belongs to the CaCA super family of Ca(2+)/cation antiporters and codes for five different K(+)- dependent Na(+)- Ca(2+) exchangers (NCKX1-5). NCKX proteins play a critical role in Ca(2+) homeostasis in a wide variety of biological processes such as vision, olfaction, enamel formation, Melanocortin-4-receptor-dependent satiety and skin pigmentation. NCKX transcripts are widely found throughout the brain. In this study we examine the differences between NCKX1-4 in terms of cation dependencies. We measured changes to Ca(2+) influx via the reverse exchange mode while manipulating external Ca(2+) or K(+) or internal Na(+) concentrations (External Ca(2+) Dependence, External K(+) Dependence and Internal Na(+) Dependence respectively); we also looked at the effect of external Na(+)/Ca(2+) competition and 3' 4'-Dichlorobenzamil on the transport of ions in HEK 293 cell lines. A fluorescence based assay was used to determine differences in transport kinetics of the four membrane spanning exchangers using the Michaelis-Menten constant (Km). Our results show that there are no significant differences between the NCKX isoforms to explain the variation in the specific expression pattern of these exchangers.

Rim formation is not a prerequisite for distribution of cone photoreceptor outer segment proteins

Retinal degeneration slow (RDS/PRPH2) is critical for the formation of the disc/lamella rim in photoreceptor outer segments (OSs), but plays a different role in rods vs. cones. Without RDS, rods fail to form OSs, however, cones lacking RDS (in the rds(-/-)/Nrl(-/-)) exhibit balloon-like OSs devoid of lamellae. We show that distribution of most proteins in the lamella and PM domains is preserved even in the absence of RDS, rim, and lamella structures. However, the rim protein prominin-1 exhibits altered trafficking and OS localization, suggesting that proper targeting and distribution of rim proteins may require RDS. Our ultrastructural studies show that in cones, OS formation is initiated by the growth of opsin-containing membrane with RDS-mediated rim formation as a secondary step. This is directly opposite to rods and significantly advances our understanding of the role of the rim in cone OS morphogenesis. Furthermore, our results suggest that the unique folded lamella architecture of the cone OS may maximize density or proximity of phototransduction proteins, but is not required for OS function or for protein distribution and retention in different membrane domains.

The SLC24 gene family of Na⁺/Ca²⁺-K⁺ exchangers: from sight and smell to memory consolidation and skin pigmentation

Members of the SLC24 gene family encode K(+)-dependent Na(+)/Ca(2+) exchangers (NCKX) that utilize both the inward Na(+) and outward K(+) gradients to extrude Ca(2+) from cells. There are five human SLC24 genes that play a role in biological process as diverse as vision in retinal rod and cone photoreceptors, olfaction, skin pigmentation and at least three of the five genes are also widely expressed in the brain. Here I review the functional, physiological and structural features of NCKX proteins that have emerged in the past few years.

Crystal structure of Ca2+/H+ antiporter protein YfkE reveals the mechanisms of Ca2+ efflux and its pH regulation

Ca(2+) efflux by Ca(2+) cation antiporter (CaCA) proteins is important for maintenance of Ca(2+) homeostasis across the cell membrane. Recently, the monomeric structure of the prokaryotic Na(+)/Ca(2+) exchanger (NCX) antiporter NCX_Mj protein from Methanococcus jannaschii shows an outward-facing conformation suggesting a hypothesis of alternating substrate access for Ca(2+) efflux. To demonstrate conformational changes essential for the CaCA mechanism, we present the crystal structure of the Ca(2+)/H(+) antiporter protein YfkE from Bacillus subtilis at 3.1-Å resolution. YfkE forms a homotrimer, confirmed by disulfide crosslinking. The protonated state of YfkE exhibits an inward-facing conformation with a large hydrophilic cavity opening to the cytoplasm in each protomer and ending in the middle of the membrane at the Ca(2+)-binding site. A hydrophobic "seal" closes its periplasmic exit. Four conserved α-repeat helices assemble in an X-like conformation to form a Ca(2+)/H(+) exchange pathway. In the Ca(2+)-binding site, two essential glutamate residues exhibit different conformations compared with their counterparts in NCX_Mj, whereas several amino acid substitutions occlude the Na(+)-binding sites. The structural differences between the inward-facing YfkE and the outward-facing NCX_Mj suggest that the conformational transition is triggered by the rotation of the kink angles of transmembrane helices 2 and 7 and is mediated by large conformational changes in their adjacent transmembrane helices 1 and 6. Our structural and mutational analyses not only establish structural bases for mechanisms of Ca(2+)/H(+) exchange and its pH regulation but also shed light on the evolutionary adaptation to different energy modes in the CaCA protein family.

Identification of the dimer interface of a bacterial Ca(2+)/H(+) antiporter

Members of the calcium/cation antiporter superfamily, including the cardiac sodium/calcium exchangers, are secondary active transporters that play an essential role in cellular Ca(2+) homeostasis. A notable feature of this group of transporters is the high levels of sequence similarity in relatively short sequences constituting the functionally important α-1 and α-2 regions in contrast to relatively lower degrees of similarity in the extended adjoining sequences. This suggests a similar structure and function of core transport machinery but possible differences in topology and/or oligomerization, a topic that has not been adequately addressed. Here we present the first example of purification of a bacterial member of this superfamily (CAX(CK31)) and analyze its quaternary structure. Purification of CAX(CK31) required the presence of a choline headgroup-containing detergent or lipid to yield stable preparations of the monomeric transporter. H(+)-driven Ca(2+) transport was demonstrated by reconstituting purified CAX(CK31) into liposomes. Dimeric CAX(CK31) could be isolated by manipulation of detergent micelles. Dimer formation was shown to be dependent on micelle composition as well as protein concentration. Furthermore, we establish that CAX(CK31) forms dimers in the membrane by analysis of cross-linked proteins. Using a dimeric homology model derived from the monomeric structure of the archaeal NCX homologue (Protein Data Bank entry 3V5U ), we introduced cysteine residues and through cross-linking experiments established the role of transmembrane helices 2 and 6 in the putative dimer interface.

Ca2+-dependent structural rearrangements within Na+-Ca2+ exchanger dimers

Cytoplasmic Ca(2+) is known to regulate Na(+)-Ca(2+) exchanger (NCX) activity by binding to two adjacent Ca(2+)-binding domains (CBD1 and CBD2) located in the large intracellular loop between transmembrane segments 5 and 6. We investigated Ca(2+)-dependent movements as changes in FRET between exchanger proteins tagged with CFP or YFP at position 266 within the large cytoplasmic loop. Data indicate that the exchanger assembles as a dimer in the plasma membrane. Addition of Ca(2+) decreases the distance between the cytoplasmic loops of NCX pairs. The Ca(2+)-dependent movements detected between paired NCXs were abolished by mutating the Ca(2+) coordination sites in CBD1 (D421A, E451A, and D500V), whereas disruption of the primary Ca(2+) coordination site in CBD2 (E516L) had no effect. Thus, the Ca(2+)-induced conformational changes of NCX dimers arise from the movement of CBD1. FRET studies of CBD1, CBD2, and CBD1-CBD2 peptides displayed Ca(2+)-dependent movements with different apparent affinities. CBD1-CBD2 showed a Ca(2+)-dependent phenotype mirroring full-length NCX but distinct from both CBD1 and CBD2.

Molecular cloning of a sixth member of the K+-dependent Na+/Ca2+ exchanger gene family, NCKX6

Bioinformatic and molecular cloning tools were used to identify and isolate cDNA clones from mouse and human tissues that encode the sixth member of the K(+)-dependent Na+/Ca2+ exchanger family, NCKX6. The mouse NCKX6 protein is 585 amino acids long and shares about 62% sequence similarity with previously identified exchangers in the alpha-repeat regions but has little primary sequence similarity outside these regions. NCKX6 transcripts of 4 kb are abundantly expressed in all tissues examined and are thus more broadly distributed than previously described NC(K)X family members. Two alternatively spliced products of this novel gene were identified that encode proteins of different length. The short isoform differs from the full-length isoform at the C-terminal hydrophobic domain as a result of a shift in the reading frame caused by the deletion of two exons. Both NCKX6 isoforms were expressed in HEK-293 cells. Functional analysis by digital imaging of fura-2 loaded transfected HEK-293 cells demonstrated that the short isoform exhibited K(+)-dependent Na+/Ca2+ exchange activity whereas the full-length isoform did not. The latter was retained within the endoplasmic reticulum, whereas the short isoform was present at the plasma membrane in transfected cells. Immunofluorescence studies examining NCKX6 expression in native tissue using an NCKX6-specific antibody showed intense labeling of the cardiac sarcolemmal membrane. The discovery of NCKX6 therefore reveals a novel member of the Na+/Ca2+ exchanger superfamily whose ubiquitous expression in all tissues suggests an important role for K(+)-dependent Na+/Ca2+ exchange in maintaining cellular Ca2+ homeostasis in diverse tissues and cell types.

Regulation of the rod photoreceptor cyclic nucleotide-gated channel

The cGMP-gated channel of rod photoreceptors plays a key role in phototransduction by controlling the flow of cations into the outer segment in response to light-induced changes in cGMP. The channel is a heterotetramer composed of alpha-subunits required for channel activity and beta-subunits that are important in modulating the activity of channel. Earlier studies have shown that exogenous calmodulin binds to the beta-subunit of the channel and modulates the sensitivity of the channel for cGMP in a calcium dependent manner. In addition unidentified Ca2+-dependent endogenous proteins have been reported to modulate the activity of the frog rod channel. In this paper, we investigated whether endogenous calmodulin and other Ca2+ binding proteins interact with and modulate the cGMP-gated channel in bovine rod outer segments. Using immunoaffinity techniques in conjunction with ion flux assays, we show that endogenous calmodulin, but not other Ca2+ dependent proteins, binds and modulates the rod cGMP-gated channel in bovine rod outer segments. We also show that the beta-subunit of the channel is phosphorylated by endogenous and exogenous casein kinase 2. This posttranslational modification, however, does not alter the sensitivity of the channel for cGMP.

The SLC24 Na+/Ca2+-K+ exchanger family: vision and beyond

Na(+)/Ca(2+)-K(+) exchange (NCKX) was first discovered in the outer segments of vertebrate rod photoreceptors (ROS), where it is the only mechanism for extruding the Ca(2+) that enters ROS via the light-sensitive and cGMP-gated channels. ROS NCKX1 is the only NCKX gene family member studied extensively in situ. ROS NCKX1 cDNAs have been cloned subsequently from a number of species including man and shown to be the first member of a new gene family ( SLCA24). Three further members of the human NCKX gene family have been cloned subsequently ( NCKX2- 4) by homology with NCKX1, while a partial sequence of a fifth human NCKX gene has appeared in the data base. NCKX-related genes have also been identified in lower animals including fruit flies, worms and sea urchins. NCKX2 is expressed in the brain, in retinal cone photoreceptors and in retinal ganglion cells, while NCKX3 and NCKX4 show a broader expression pattern. In situ NCKX1 and heterologously expressed NCKX2 operate at a 4Na(+):1Ca(2+)+1 K(+) stoichiometry; both NCKX1 and NCKX2 are bidirectional transporters normally extruding Ca(2+) from the cell (forward exchange), but also able to carry Ca(2+) into the cell (reverse exchange) when the transmembrane Na(+) gradient is reversed. Sequence changes have been observed for both NCKX1 and NCKX2 in patients with retinal diseases, but a definitive association with retinal disease has not been shown.

A novel topology and redox regulation of the rat brain K+-dependent Na+/Ca2+ exchanger, NCKX2

In this study we have examined the roles of endogenous cysteine residues in the rat brain K(+)-dependent Na(+)/Ca(2+) exchanger protein, NCKX2, by site-directed mutagenesis. We found that mutation of Cys-614 or Cys-666 to Ala inhibited expression of the exchanger protein in HEK-293 cells, but not in an in vitro translation system. We speculated that Cys-614 and Cys-666 might form an extracellular disulfide bond that stabilized protein structure. Such an arrangement would place the C terminus of the exchanger outside the cell, contrary to the original topological model. This hypothesis was tested by adding a hemagglutinin A epitope to the C terminus of the protein. The hemagglutinin A epitope could be recognized with a specific antibody without permeabilization of the cell membrane, supporting an extracellular location for the C terminus. Additionally, the exchanger molecule could be labeled with biotin maleimide only following extracellular application of beta-mercaptoethanol. Surprisingly, mutation of Cys-395, located in the large intracellular loop, to Ala, prevented reduction-dependent labeling of the protein. The activity of wild-type exchanger, but not the Cys-395 --> Ala mutant, was stimulated after application of beta-mercaptoethanol. Co-immunoprecipitation experiments demonstrated self-association between wild-type and FLAG-tagged exchanger proteins that could not be inhibited by Cys-395 --> Ala mutation. These results suggest that NCKX2 associates as a dimer, an interaction that does not require, but may be stabilized by, a disulfide linkage through Cys-395. This linkage, perhaps by limiting protein mobility along the dimer interface, reduces the transport activity of NCKX2.

Binding of the retinal rod Na+/Ca2+-K+ exchanger to the cGMP-gated channel indicates local Ca(2+)-signaling in vertebrate photoreceptors

Ca(2+) ions enter the outer segment of rod or cone photoreceptors exclusively through the cGMP-gated channel and are extruded by the Na(+)/Ca(2+)-K(+) exchanger. Recent evidence indicates that in the plasma membrane, the Na(+)/Ca(2+)-K(+) exchanger is associated with the cGMP-gated channel. In this contribution, the possible physiologic significance of this protein complex is considered. Based on recent experimental evidence, the possibility of a direct functional interaction between the cGMP-gated channel and the Na(+)/Ca(2+)-K(+) exchanger is discussed. Furthermore, a quantitative estimation of the cytoplasmic Ca(2+) diffusion at the cGMP-gated channel indicates that Ca(2+) diffusion is largely confined to the complex of the cGMP-gated channel and the associated Na(+)/Ca(2+)-K(+) exchanger molecules.

Mutual inhibition of the dimerized Na/Ca-K exchanger in rod photoreceptors

In the dark, rod photoreceptors sustain a continuous influx of Na and Ca ions through the cGMP-gated channels of the rod outer segments (ROS). Whereas Na ions are extruded in the inner segment by the Na-pump, Ca ions are extruded already in the ROS by Na/Ca-K exchange. Our previous findings indicate that in the ROS plasma membrane, exchanger and channel form a complex of two exchangers associated per channel. Here, we report evidence of a novel regulatory mechanism of the dimerized exchanger, based on the following findings: (1), thiol-specific cross-linking with dimaleimides resulted in an increase of the Na/Ca-K exchange activity which correlated with the size of the cross-linking reagent, i.e., with increasing separation of the monomers in a dimerized exchanger; (2), partial proteolysis of the exchanger also increased the exchange rate by about a factor of two; (3), disintegration of the channel-exchanger complex by solubilization of the ROS membranes and preparation of proteoliposomes resulted in a twofold enhancement of the exchange rate; however (4), partial proteolysis of proteoliposomes, in which the exchanger molecules exist as monomers, did not result in any enhancement of the exchange rate. These findings suggest an inhibitory protein domain at the contact site of the dimerized exchanger. The physiological implication of this inference will be discussed in terms of a potential allosteric regulation of the exchanger in the channel-exchanger complex.

The cGMP-gated channel and related glutamic acid-rich proteins interact with peripherin-2 at the rim region of rod photoreceptor disc membranes

The rod cGMP-gated channel is localized in the plasma membrane of rod photoreceptor outer segments, where it plays a central role in phototransduction. It consists of alpha- and beta-subunits that assemble into a heterotetrameric protein. Each subunit contains structural features characteristic of nucleotide-gated channels, including a cGMP-binding domain, multiple membrane-spanning segments, and a pore region. In addition, the beta-subunit has a large glutamic acid- and proline-rich region called GARP that is also expressed as two soluble protein variants. Using monoclonal antibodies in conjunction with immunoprecipitation, cross-linking, and electrophoretic techniques, we show that the cGMP-gated channel associates with the Na/Ca-K exchanger in the rod outer segment plasma membrane. This complex and soluble GARP proteins also interact with peripherin-2 oligomers in the rim region of outer segment disc membranes. These results suggest that channel/peripherin protein interactions mediated by the GARP part of the channel beta-subunit play a role in connecting the rim region of discs to the plasma membrane and in anchoring the channel.exchanger complex in the rod outer segment plasma membrane.

Quantitative evaluation of the lengths of homobifunctional protein cross-linking reagents used as molecular rulers

Homobifunctional chemical cross-linking reagents are important tools for functional and structural characterization of proteins. Accurate measures of the lengths of these molecules currently are not available, despite their widespread use. Stochastic dynamics calculations now provide quantitative measures of the lengths, and length dispersions, of 32 widely used molecular rulers. Significant differences from published data have been found.

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Vertebrate photoreceptors

The basis of the duplex theory of vision is examined in view of the dazzling array of data on visual pigment sequences and the pigments they form, on the microspectrophotometry measurements of single photoreceptor cells, on the kinds of photoreceptor cascade enzymes, and on the electrophysiological properties of photoreceptors. The implications of the existence of five distinct visual pigment families are explored, especially with regard to what pigments are in what types of photoreceptors, if there are different phototransduction enzymes associated with different types of photoreceptors, and if there are electrophysiological differences between different types of cones.

Calcineurin controls the transcription of Na+/Ca2+ exchanger isoforms in developing cerebellar neurons

The Na(+)/Ca(2+) exchanger (NCX) and the plasma membrane Ca(2+)-ATPase export Ca(2+) from the cytosol to the extracellular space. Three NCX genes (NCX1, NCX2, and NCX3), encoding proteins with very similar properties, are expressed at different levels in tissues. Essentially, no information is available on the mechanisms that regulate their expression. Specific antibodies have been prepared and used to explore the expression of NCX1 and NCX2 in rat cerebellum. The expression of NCX2 became strongly up-regulated during development, whereas comparatively minor effects were seen for NCX1. This was also observed in cultured granule cells induced to mature in physiological concentrations of potassium. By contrast, higher K(+) concentrations, which induce partial depolarization of the plasma membrane and promote the influx of Ca(2+), caused the complete disappearance of NCX2. Reverse transcription-polymerase chain reaction analysis showed that the process occurred at the transcriptional level and depended on the activation of the Ca(2+) calmodulin-dependent protein phosphatase, calcineurin. The NCX1 and NCX3 genes were also affected by the depolarizing treatment: the transcription of the latter became up-regulated, and the pattern of expression of the splice variants of the former changed. The effects on the NCX1 and NCX3 genes were calcineurin-independent.

Binding of the cGMP-gated channel to the Na/Ca-K exchanger in rod photoreceptors

The intracellular Ca(2+) concentration in rod outer segments of vertebrate photoreceptors is controlled by Ca(2+) influx through cGMP-gated channels and by Ca(2+) efflux driven by Na/Ca-K exchangers. Previously, we suggested that channel and exchanger are associated (Bauer, P. J., and Drechsler, M. (1992) J. Physiol. (Lond. ) 451, 109-131). This suggestion has been thoroughly examined using a variety of biochemical approaches. First, we took advantage of the fact that cGMP-gated channels bind calmodulin (CaM). Using CaM affinity chromatographic purification of the channel in 10 mm CHAPS, a significant fraction of exchanger was co-eluted with the channel indicating a binding affinity between channel and exchanger. Binding of channel and exchanger was examined more directly by cross-linking of proteins in the rod outer segment membranes. Activation of the channel with cyclic 8-bromo-GMP lead to exposure of a cysteine, which allowed cross-linking of the channel to the exchanger with the thiol-specific reagent dl-1,4-bismaleimido-2,3-butanediol. Cleavage of the cross-links and electrophoretic analysis indicated that a cross-link between the alpha-subunit of the channel and the exchanger formed. Furthermore, a cross-link between two adjacent alpha-subunits of the channel was found, suggesting that the alpha-subunits of the native channel are dimerized. Further support for an interaction between alpha-subunit and exchanger was obtained by in vitro experiments. Specific binding of the exchanger to the alpha-subunit but not to the beta-subunit of the channel was observed in Western blots of purified channel incubated with purified exchanger. This study suggests that two exchanger molecules bind to one cGMP-gated channel and, more specifically, that binding of exchanger molecules occurs at the alpha-subunits, which in the native channel are dimerized. The implications of these findings regarding the possibility of local Ca(2+) signaling in vertebrate photoreceptors will be discussed.

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.