cDNA cloning and functional expression of the dolphin retinal rod Na-Ca+K exchanger NCKX1: comparison with the functionally silent bovine NCKX1

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.

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.

Functional and structural properties of the NCKX2 Na(+)-Ca (2+)/K (+) exchanger: a comparison with the NCX1 Na (+)/Ca (2+) exchanger

Na(+)/Ca(2+)-K(+) exchangers (NCKX), alongside the more widely known Na(+)/Ca(2+) exchangers (NCX), are important players in the cellular Ca(2+) toolkit. But, unlike NCX, much less is known about the physiological roles of NCKX, while emergent evidence indicates that NCKX has highly specialized functions in cells and tissues where it is expressed. As their name implies, there are functional similarities in the properties of the two Ca(2+) exchanger families, but there are specific differences as well. Here, we compare and contrast their key functional properties of ionic dependence and affinities, as well as report on the effects of KB-R7943 - a compound that is widely used to differentiate the two exchangers. We also review structural similarities and differences between the two exchangers. The aim is to draw attention to key differences that will aid in differentiating the two exchangers in physiological contexts where both exist but perhaps play distinct roles.

Structure-function relationships of the NCKX2 Na+/Ca2+-K+ exchanger

K+-dependent Na+/Ca2+ exchangers (NCKX) have been shown to play important roles in physiological processes as diverse as phototransduction in rod photoreceptors, motor learning and memory in mice, and skin pigmentation in humans. Most structure-function studies on NCKX proteins have been carried out on the NCKX2 isoform, but sequence similarity suggests that the results obtained with the NCKX2 isoform are likely to apply to all NCKX1-5 members of the human SLC24 gene family. Here we review our recent work on the NCKX2 protein concerning the topological arrangement of transmembrane segments carrying out cation transport, and concerning residues important for transport function and cation binding.

Na+ /Ca2+ exchanger contributes to asterosap-induced elevation of intracellular Ca2+ concentration in starfish spermatozoa

Asterosap, a group of equally active isoforms of sperm-activating peptides from the egg jelly of the starfish Asterias amurensis, functions as a chemotactic factor for sperm. It transiently increases the intracellular cGMP level of sperm, which in turn induces a transient elevation of intracellular Ca(2+) concentration ([Ca(2+)](i)). Using a fluorescent Ca(2+)-sensitive dye, Fluo-4 AM, we measured the changes in sperm [Ca(2+)](i) in response to asterosap. KB-R7943 (KB), a selective inhibitor of Na(+)/Ca(2+) exchanger (NCX), significantly inhibited the asterosap-induced transient elevation of [Ca(2+)](i), suggesting that asterosap influences [Ca(2+)](i) through activation of a K+-dependent NCX (NCKX). An NCKX activity of starfish sperm also shows K(+) dependency like other NCKXs. Therefore, we cloned an NCKX from the starfish testes and predicted that it codes for a 616 amino acid protein that is a member of the NCKX family. Pharmacological evidence suggests that this exchanger participates in the asterosap-induced Ca(2+) entry into sperm.

Functional characterization and molecular cloning of the K+-dependent Na+/Ca2+ exchanger in intact retinal cone photoreceptors

Light-dependent changes in cytoplasmic free Ca(2+) are much faster in the outer segment of cone than rod photoreceptors in the vertebrate retina. In the limit, this rate is determined by the activity of an electrogenic Na(+)/Ca(2+) exchanger located in the outer segment plasma membrane. We investigate the functional properties of the exchanger activity in intact, single cone photoreceptors isolated from striped bass retina. Exchanger function is characterized through analysis both of the electrogenic exchanger current and cytoplasmic free Ca(2+) measured with optical probes. The exchanger in cones is K(+) dependent and operates both in forward and reverse modes. In the reverse mode, the K(+) dependence of the exchanger is described by binding to a single site with K(1/2) about 3.6 mM. From the retina of the fish we cloned exchanger molecules bassNCKX1 and bassNCKX2. BassNCKX1 is a single class of molecules, homologous to exchangers previously cloned from mammalian rods. BassNCKX2 exists in four splice variants that differ from each other by small sequence differences in the single, large cytoplasmic loop characteristic of these molecules. We used RT-PCR (reverse transcriptase polymerase chain reaction) of individual cells to identify the exchanger molecule specifically expressed in bass single and twin cone photoreceptors. Each and every one of the four bassNCKX2 splice variants is expressed in both single and twin cones indistinguishably. BassNCKX1 is not expressed in cones and, by exclusion, it is likely to be an exchanger expressed in rods.

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.

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.

The Na/Ca-K exchanger gene family

Ca(2+) extrusion driven by both the inward Na(+) gradient as well as the outward K(+) gradient is essential for visual transduction in retinal rod and cone photoreceptors because it removes Ca(2+) that enters photoreceptors via the cGMP-gated and light-sensitive channels. We have cloned rod and cone Na/Ca-K exchanger (NCKX) cDNAs from several species, and we have cloned NCKX cDNAs from lower organisms that lack vertebrate-type vision. Although in situ NCKX physiology has only been documented for vertebrate photoreceptors, it is now clear that NCKX gene products have a much broader distribution pattern. Here, we review some of the structural and functional features that have emerged from our studies on different members of the NCKX gene family.

Potassium-dependent sodium-calcium exchange through the eye of the fly

In this review, we describe the characterization of a Drosophila sodium/calcium-potassium exchanger, Nckx30C. Sodium/calcium (-potassium) exchangers (NCX and NCKX) are required for the rapid removal of calcium in excitable cells. The deduced protein topology for NCKX30C is similar to that of mammalian NCKX, with 5 hydrophobic domains in the amino terminus separated from 6 at the carboxy-terminal end by a large intracellular loop. NCKX30C functions as a potassium-dependent sodium-calcium exchanger and is expressed in adult neurons and during ventral nerve cord development in the embryo. Nckx30C is expressed in a dorsal/ventral pattern in the eye-antennal disc, suggesting that large fluxes of calcium may be occurring during imaginal disc development in the larvae. NCKX30C may play a critical role in modulating calcium during development as well as in the removal of calcium and maintenance of calcium homeostasis in adults.

A flagellar K(+)-dependent Na(+)/Ca(2+) exchanger keeps Ca(2+) low in sea urchin spermatozoa

The metabolism, flagellar beating, and acrosome reaction of spermatozoa are regulated by ion flux across the plasma membrane. As is true of most cells, swimming sperm maintain intracellular Ca(2+) concentrations at submicromolar levels. Here we describe a K(+)-dependent Na(+)/Ca(2+) exchanger (suNCKX) from sea urchin sperm. The suNCKX is phylogenetically related to other NCKXs, which use high relative intracellular K(+), and high relative extracellular Na(+), to couple the efflux of 1 Ca(2+) and 1 K(+) to the influx of 4 Na(+). The 652-aa suNCKX shares structural topology with other NCKX proteins, and has two protein kinase A sites and a His-rich region in its cytoplasmic loop. The suNCKX is encoded by a single gene, which is highly expressed in testes. The suNCKX activity of whole sperm shows Na(+) and K(+) dependence, and like other NCKXs can run in reverse exchange mode. An inhibitor blocks the suNCKX activity and sperm motility. suNCKX localizes to the plasma membrane over the sperm flagellum. The suNCKX may play a major role in keeping Ca(2+) low in swimming sperm.

Molecular and functional characterization of a urea transporter from the kidney of a short-finned pilot whale

Cetaceans (whales and dolphins) always excrete urine with an osmolality markedly higher than that of plasma. Although the mechanisms by which cetaceans concentrate urine have not been elucidated, data support a role for medullary urea accumulation in this process, as is the case for terrestrial mammals. Therefore, we hypothesized that facilitated urea transporters are present in the kidney of cetaceans. Using 5'/3'-rapid amplification of cDNA ends, we cloned a 2.7-kb cDNA from the kidney of the short-finned pilot whale Globicephala macrorhynchus. The putative open-reading frame encoded a 397-amino acid protein [pilot whale urea transporter A2 (whUT-A2)] that has 94% amino acid sequence identity to the A2 isoform of the human urea transporter (hUT-A2). Heterologous expression of whUT-A2 cRNA in Xenopus oocytes induced phloretin-inhibitable urea transport. Although Northern analysis and RT-PCR indicated that whUT-A2 was exclusively expressed in kidney, Western blotting using a polyclonal antibody to rat UT-A1/UT-A2 detected various immunoreactive proteins in kidney and other tissues. Furthermore, RT-PCR analysis suggested the presence of alternatively spliced UT-A transcripts in the kidney as well as extrarenal tissues. We conclude that renal urea transporters are highly conserved among mammals inhabiting terrestrial and pelagic environments. A urea-based concentrating mechanism, presumably evolved to meet the demands of an arid terrestrial environment, may have contributed a fortuitous preadaptation that enabled the ancestors of cetaceans to reinvade the sea.

Electrophysiological characterization and ionic stoichiometry of the rat brain K(+)-dependent NA(+)/CA(2+) exchanger, NCKX2

We have recently described a novel K(+)-dependent Na(+)/Ca(2+) exchanger, NCKX2, that is abundantly expressed in brain neurons (Tsoi, M., Rhee, K.-H., Bungard, D., Li, X.-F., Lee, S.-L., Auer, R. N., and Lytton, J. (1998) J. Biol. Chem. 273, 4115--4162). The precise role for NCKX2 in neuronal Ca(2+) homeostasis is not yet clearly understood but will depend upon the functional properties of the molecule. Here, we have performed whole-cell patch clamp analysis to characterize cation dependences and ion stoichiometry for rat brain NCKX2, heterologously expressed in HEK293 cells. Outward currents generated by reverse NCKX2 exchange depended on external Ca(2+) with a K(12) of 1.4 or 101 microm without or with 1 mm Mg(2+), and on external K(+) with a K(1/2) of about 12 or 36 mm with choline or Li(+) as counter ion, respectively. Na(+) inhibited outward currents with a K(1/2) of about 60 mm. Inward currents generated by forward NCKX2 exchange depended upon external Na(+) with a K(1/2) of 30 mm and a Hill coefficient of 2.8. K(+) inhibited the inward currents by a maximum of 40%, with a K(1/2) of 2 mm or less, depending upon the conditions. The transport stoichiometry of NCKX2 was determined by observing the change in reversal potential as individual ion gradients were altered. Our data support a stoichiometry for rat brain NCKX2 of 4 Na(+):(1 Ca(2+) + 1 K(+)). These findings provide the first electrophysiological characterization of rat brain NCKX2, and the first evidence that a single recombinantly expressed NCKX polypeptide encodes a K(+)-transporting Na(+)/Ca(2+) exchanger with a transport stoichiometry of 4 Na(+):(1 Ca(2+) + 1 K(+)).

Molecular cloning of a third member of the potassium-dependent sodium-calcium exchanger gene family, NCKX3

We describe here the identification and characterization of a novel member of the family of K(+)-dependent Na(+)/Ca(2+) exchangers, NCKX3 (gene SLC24A3). Human NCKX3 encodes a protein of 644 amino acids that displayed a high level of sequence identity to the other family members, rod NCKX1 and cone/neuronal NCKX2, in the hydrophobic regions surrounding the "alpha -repeat" sequences thought to form the ion-binding pocket for transport. Outside of these regions NCKX3 showed no significant identity to other known proteins. As anticipated from this sequence similarity, NCKX3 displayed K(+)-dependent Na(+)/Ca(2+) exchanger activity when assayed in heterologous expression systems, using digital imaging of fura-2 fluorescence, electrophysiology, or radioactive (45)Ca(2+) uptake. The N-terminal region of NCKX3, although not essential for expression, increased functional activity at least 10-fold and may represent a cleavable signal sequence. NCKX3 transcripts were most abundant in brain, with highest levels found in selected thalamic nuclei, in hippocampal CA1 neurons, and in layer IV of the cerebral cortex. Many other tissues also expressed NCKX3 at lower levels, especially aorta, uterus, and intestine, which are rich in smooth muscle. The discovery of NCKX3 thus expands the K(+)-dependent Na(+)/Ca(2+) exchanger family and suggests this class of transporter has a more widespread role in cellular Ca(2+) handling than previously appreciated.

Na(+)-Ca(2+)-K(+) currents measured in insect cells transfected with the retinal cone or rod Na(+)-Ca(2+)-K(+) exchanger cDNA

The recently cloned retinal cone Na(+)-Ca(2+)-K(+) exchanger (NCKX) was expressed in cultured insect cells, and whole-cell patch clamp was used to measure transmembrane currents generated by this transcript and compare them with currents generated by retinal rod NCKX or by a deletion mutant rod NCKX from which the two large hydrophilic loops were removed. We have characterized the ionic currents generated by both the forward (Ca(2+) extrusion) and reverse (Ca(2+) influx) modes of all three NCKX proteins. Reverse NCKX exchange generated outward current that required the simultaneous presence of both external Ca(2+) and external K(+). Forward NCKX exchange carried inward current with Na(+), but not with Li(+) in the bath solution. The cation dependencies of the three NCKX tested (external K(+), external Na(+), internal Ca(2+)) were very similar to each other and to those reported previously for the in situ rod NCKX. These findings provide the first electrophysiological characterization of cone NCKX and the first electrophysiological characterization of potassium-dependent Na(+)-Ca(+) exchangers in heterologous systems. Our results demonstrate the feasibility of combining heterologous expression and biophysical measurements for detailed NCKX structure/function studies.

The N-terminal portion of the main cytosolic loop mediates K+ sensitivity in the retinal rod Na+/Ca2+-K+-exchanger

Two types of Na+/Ca2+-exchangers have been characterized in the literature: The first is the cardiac, skeletal muscle and brain type, which exchanges 1 Ca2+ for 3 Na+, the second, found in retinal photosensor cells, transports 1 Ca2+ and 1 K+ in exchange for 4 Na+. The present work describes the properties of chimeric constructs of the two exchanger types. Ca2+ gel overlay experiments have identified a high affinity (Kd in the 1 microM range) Ca2+-binding domain between Glu601 and Asp733 in the main cytosolic loop of the retinal protein, just after transmembrane domain 5. Insertion of the retinal Ca2+-binding domain in the cytosolic loop of the cardiac exchanger conferred K+-dependence to the Ca2+ uptake activity of the chimeric constructs expressed in HeLa cells. The apparent Km of the K+ effect was about 1 mM. Experiments with C-terminally truncated versions of the retinal insert indicated that the sequence between Leu643 and Asp733 was critical in mediating K+ sensitivity of the recombinant chimeras. Thus, the high affinity Ca2+-binding domain in the main cytosolic loop of the retinal exchanger may regulate the activity of the retinal protein by binding Ca2+, and by conferring to it K+ sensitivity.

Alternatively spliced isoforms of the rat eye sodium/calcium+potassium exchanger NCKX1

We have investigated the structure, function, and expression of the rat eye sodium/calcium+potassium exchanger NCKX1. The sequence of independent rat NCKX1 clones and the analysis of rat eye mRNA by RT-PCR revealed a region of alternative splicing that comprised four exons and encoded a stretch of 113 amino acids near the beginning of the large cytosolic loop. In comparison with other NCKX1 molecules and the rat NCKX2 protein, rat NCKX1 was highly conserved within the hydrophobic regions but was quite divergent in the two large hydrophilic loops. The only exception was the region of the cytosolic loop encoded by the second alternatively spliced exon, which was approximately 60% identical. Similar to bovine, but different from human, rat NCKX1 possessed an acidic motif that was repeated 14 times in the cytoplasmic loop. Analysis of NCKX1 expression in different rat tissues by Northern blot revealed a very high level of expression of a 7-kb transcript in the eye but also lower levels of transcripts of various lengths in other tissues. The recombinant rat NCKX1 protein was tagged in the extracellular loop with the FLAG epitope and expressed in HEK-293 cells. Surface delivery and potassium-dependent sodium/calcium exchange activity were observed for each spliced variant.

Molecular cloning and functional expression of the potassium-dependent sodium-calcium exchanger from human and chicken retinal cone photoreceptors

Light causes a rapid lowering of cytosolic free calcium in the outer segments of both retinal rod and cone photoreceptors. This light-induced lowering of calcium is caused by extrusion via a Na-Ca exchanger located in the rod and cone outer segment plasma membrane and plays a key role in the process of light adaptation. The Na-Ca exchanger in retinal rod outer segment was shown earlier to be a novel Na-Ca+K exchanger (NCKX), and its cDNA was obtained by molecular cloning from several mammalian species. On the other hand, the proper identity of the retinal cone Na-Ca exchanger, in terms of both functional characteristics (e.g., requirement for and transport of potassium) and molecular identity, has not yet been elucidated. Here, we report the molecular cloning, intraretinal localization by in situ hybridization, and initial functional characterization of the chicken and human cone-specific Na-Ca exchangers. In addition we report the chicken rod-specific NCKX. We identified NCKX transcripts in both human and chicken cones and observed strong potassium-dependent Na-Ca exchange activity after heterologous expression of human and chicken cone NCKX cDNAs in cultured insect cells. In situ hybridization in chicken retina showed abundant rod NCKX transcripts only in rod photoreceptors, whereas abundant cone NCKX transcripts were found in most, if not all, cone photoreceptors and also in a subpopulation of retinal ganglion cells. A detailed comparison with the previously described retinal rod and brain NCKX cDNAs is presented.

Minimal domain requirement for cation transport by the potassium-dependent Na/Ca-K exchanger. Comparison with an NCKX paralog from Caenorhabditis elegans

The retinal rod Na/Ca-K exchanger (NCKX) is a unique calcium extrusion protein utilizing both inward sodium gradient and outward potassium gradient. Three mammalian rod NCKX cDNAs have been cloned to date, but quantitative analysis of NCKX function in heterologous systems has proven difficult. Here, we describe a simple system for quantitative analysis of NCKX function; stable transformation of cultured insect cells with the novel pEA1/153A vector containing NCKX cDNAs was combined with measurements of potassium-dependent (45)Ca uptake in sodium-loaded cells. We carried out structure-function studies on NCKX with the following results: 1) two-thirds of the full-length sequence of bovine NCKX could be deleted without affecting potassium-dependent calcium transport and without affecting key properties of the potassium binding site; 2) the affinity of NCKX for potassium was about 10-fold greater in choline medium when compared with lithium medium; this shift was observed in rod outer segments or in cells expressing full-length rod NCKX, the above deletion mutant, or a distantly related NCKX paralog cloned from Caenorhabditis elegans. We conclude that the potassium binding site is highly conserved among members of the NCKX family and is formed by residues located within the two sets of transmembrane spanning segments in the NCKX sequence.

The retinal rod Na(+)/Ca(2+),K(+) exchanger contains a noncleaved signal sequence required for translocation of the N terminus

The retinal rod Na(+)/Ca(2+),K(+) exchanger (RodX) is a polytopic membrane protein found in photoreceptor outer segments where it is the principal extruder of Ca(2+) ions during light adaptation. We have examined the role of the N-terminal 65 amino acids in targeting, translocation, and integration of the RodX using an in vitro translation/translocation system. cDNAs encoding human RodX and bovine RodX through the first transmembrane domain were correctly targeted and integrated into microsomal membranes; deletion of the N-terminal 65 amino acids (aa) resulted in a translation product that was not targeted or integrated. Deletion of the first 65 aa had no effect on membrane targeting of full-length RodX, but the N-terminal hydrophilic domain no longer translocated. Chimeric constructs encoding the first 65 aa of bovine RodX fused to globin were translocated across microsomal membranes, demonstrating that the sequence could function heterologously. Studies of fresh bovine retinal extracts demonstrated that the first 65 aa are present in the native protein. These data demonstrate that the first 65 aa of RodX constitute an uncleaved signal sequence required for the efficient membrane targeting and proper membrane integration of RodX.

Cloning and characterization of a potassium-dependent sodium/calcium exchanger in Drosophila

Sodium/calcium(-potassium) exchangers (NCX and NCKX) are critical for the rapid extrusion of calcium, which follows the stimulation of a variety of excitable cells. To further understand the mechanisms of calcium regulation in signaling, we have cloned a Drosophila sodium/calcium-potassium exchanger, Nckx30C. The overall deduced protein topology for NCKX30C is similar to that of mammalian NCKX, having five membrane-spanning domains in the NH(2) terminus separated from six at the COOH-terminal end by a large intracellular loop. We show that NCKX30C functions as a potassium-dependent sodium/calcium exchanger, and is not only expressed in adult neurons as was expected, but is also expressed during ventral nerve cord development in the embryo and in larval imaginal discs. Nckx30C is expressed in a dorsal-ventral pattern in the eye-antennal disc in a pattern that is similar to, but broader than that of wingless, suggesting that large fluxes of calcium may be occurring during imaginal disc development. Nckx30C may not only function in the removal of calcium and maintenance of calcium homeostasis during signaling in the adult, but may also play a critical role in signaling during development.