Localization of the ATP/phosphatidylinositol 4,5 diphosphate-binding site to a 39-amino acid region of the carboxyl terminus of the ATP-regulated K+ channel Kir1.1

Pharmacological Characterization of a Recombinant Mitochondrial ROMK2 Potassium Channel Expressed in Bacteria and Reconstituted in Planar Lipid Bilayers

In the inner mitochondrial membrane, several potassium channels that play a role in cell life and death have been identified. One of these channels is the ATP-regulated potassium channel (mitoK_ATP). The ROMK2 potassium channel is a potential molecular component of the mitoK_ATP channel. The current study aimed to investigate the pharmacological modulation of the activity of the ROMK2 potassium channel expressed in Escherichia coli bacteria. ROMK2 was solubilized in polymer nanodiscs and incorporated in planar lipid bilayers. The impact of known mitoK_ATP channel modulators on the activity of the ROMK2 was characterized. We found that the ROMK2 channel was activated by the mitoK_ATP channel opener diazoxide and blocked by mitoK_ATP inhibitors such as ATP/Mg²⁺, 5-hydroxydecanoic acid, and antidiabetic sulfonylurea glibenclamide. These results indicate that the ROMK2 potassium protein may be a pore-forming subunit of mitoK_ATP and that the impact of channel modulators is not related to the presence of accessory proteins.

A review of TNP-ATP in protein binding studies: benefits and pitfalls

We review 50 years of use of 2',3'-O-trinitrophenyl (TNP)-ATP, a fluorescently tagged ATP analog. It has been extensively used to detect binding interactions of ATP to proteins and to measure parameters of those interactions such as the dissociation constant, K_d, or inhibitor dissociation constant, K_i. TNP-ATP has also found use in other applications, for example, as a fluorescence marker in microscopy, as a FRET pair, or as an antagonist (e.g., of P2X receptors). However, its use in protein binding studies has limitations because the TNP moiety often enhances binding affinity, and the fluorescence changes that occur with binding can be masked or mimicked in unexpected ways. The goal of this review is to provide a clear perspective of the pros and cons of using TNP-ATP to allow for better experimental design and less ambiguous data in future experiments using TNP-ATP and other TNP nucleotides.

Phosphoinositides regulate ion channels

Phosphoinositides serve as signature motifs for different cellular membranes and often are required for the function of membrane proteins. Here, we summarize clear evidence supporting the concept that many ion channels are regulated by membrane phosphoinositides. We describe tools used to test their dependence on phosphoinositides, especially phosphatidylinositol 4,5-bisphosphate, and consider mechanisms and biological meanings of phosphoinositide regulation of ion channels. This lipid regulation can underlie changes of channel activity and electrical excitability in response to receptors. Since different intracellular membranes have different lipid compositions, the activity of ion channels still in transit towards their final destination membrane may be suppressed until they reach an optimal lipid environment. This article is part of a Special Issue entitled Phosphoinositides.

Phosphoinositide control of membrane protein function: a frontier led by studies on ion channels

Anionic phospholipids are critical constituents of the inner leaflet of the plasma membrane, ensuring appropriate membrane topology of transmembrane proteins. Additionally, in eukaryotes, the negatively charged phosphoinositides serve as key signals not only through their hydrolysis products but also through direct control of transmembrane protein function. Direct phosphoinositide control of the activity of ion channels and transporters has been the most convincing case of the critical importance of phospholipid-protein interactions in the functional control of membrane proteins. Furthermore, second messengers, such as [Ca(2+)]i, or posttranslational modifications, such as phosphorylation, can directly or allosterically fine-tune phospholipid-protein interactions and modulate activity. Recent advances in structure determination of membrane proteins have allowed investigators to obtain complexes of ion channels with phosphoinositides and to use computational and experimental approaches to probe the dynamic mechanisms by which lipid-protein interactions control active and inactive protein states.

Potassium excretion through ROMK potassium channel expressed in gill mitochondrion-rich cells of Mozambique tilapia

Despite recent progress in physiology of fish ion homeostasis, the mechanism of plasma K+ regulation has remained unclear. Using Mozambique tilapia, a euryhaline teleost, we demonstrated that gill mitochondrion-rich (MR) cells were responsible for K+ excretion, using a newly invented technique that insolubilized and visualized K+ excreted from the gills. For a better understanding of the molecular mechanism of K+ excretion in the gills, cDNA sequences of renal outer medullary K+ channel (ROMK), potassium large conductance Ca(2+)-activated channel, subfamily M (Maxi-K), K(+)-Cl(-) cotransporters (KCC1, KCC2, and KCC4) were identified in tilapia as the candidate molecules that are involved in K+ handling. Among the cloned candidate molecules, only ROMK showed marked upregulation of mRNA levels in response to high external K+ concentration. In addition, immunofluorescence microscopy revealed that ROMK was localized in the apical opening of gill MR cells, and that the immunosignals were most intense in the fish acclimated to the environment with high K+ concentration. To confirm K+ excretion via ROMK, K+ insolubilization-visualization technique was applied again in combination with K+ channel blockers. The K+ precipitation was prevented in the presence of Ba2+, indicating that ROMK has a pivotal role in K+ excretion. The present study is the first to demonstrate that the fish excrete K+ from the gill MR cells, and that ROMK expressed in the apical opening of the MR cells is a main molecular pathway responsible for K+ excretion.

Genetic defects in the hotspot of inwardly rectifying K(+) (Kir) channels and their metabolic consequences: a review

Inwardly rectifying potassium (Kir) channels are essential for maintaining normal potassium homeostasis and the resting membrane potential. As a consequence, mutations in Kir channels cause debilitating diseases ranging from cardiac failure to renal, ocular, pancreatic, and neurological abnormalities. Structurally, Kir channels consist of two trans-membrane domains, a pore-forming loop that contains the selectivity filter and two cytoplasmic polar tails. Within the cytoplasmic structure, clusters of amino acid sequences form regulatory domains that interact with cellular metabolites to control the opening and closing of the channel. In this review, we present an overview of Kir channel function and recent progress in the characterization of selected Kir channel mutations that lie in and near a C-terminal cytoplasmic 'hotspot' domain. The resultant molecular mechanisms by which the loss or gain of channel function leads to organ failure provide potential opportunities for targeted therapeutic interventions for this important group of channelopathies.

Channelopathies linked to plasma membrane phosphoinositides

The plasma membrane phosphoinositide phosphatidylinositol 4,5-bisphosphate (PIP2) controls the activity of most ion channels tested thus far through direct electrostatic interactions. Mutations in channel proteins that change their apparent affinity to PIP2 can lead to channelopathies. Given the fundamental role that membrane phosphoinositides play in regulating channel activity, it is surprising that only a small number of channelopathies have been linked to phosphoinositides. This review proposes that for channels whose activity is PIP2-dependent and for which mutations can lead to channelopathies, the possibility that the mutations alter channel-PIP2 interactions ought to be tested. Similarly, diseases that are linked to disorders of the phosphoinositide pathway result in altered PIP2 levels. In such cases, it is proposed that the possibility for a concomitant dysregulation of channel activity also ought to be tested. The ever-growing list of ion channels whose activity depends on interactions with PIP2 promises to provide a mechanism by which defects on either the channel protein or the phosphoinositide levels can lead to disease.

A comprehensive guide to the ROMK potassium channel: form and function in health and disease

The discovery of the renal outer medullary K+ channel (ROMK, K(ir)1.1), the founding member of the inward-rectifying K+ channel (K(ir)) family, by Ho and Hebert in 1993 revolutionized our understanding of potassium channel biology and renal potassium handling. Because of the central role that ROMK plays in the regulation of salt and potassium homeostasis, considerable efforts have been invested in understanding the underlying molecular mechanisms. Here we provide a comprehensive guide to ROMK, spanning from the physiology in the kidney to the organization and regulation by intracellular factors to the structural basis of its function at the atomic level.

Physiologic regulation of the epithelial sodium channel by phosphatidylinositides

PURPOSE OF REVIEW

Epithelial sodium channel (ENaC) activity is limiting for sodium reabsorption in the distal nephron. Humans regulate blood pressure by fine-tuning sodium balance through control of ENaC. ENaC dysfunction causes some hypertensive and renal salt wasting diseases. Thus, it is critical to understand the cellular mechanisms controlling ENaC activity.

RECENT FINDINGS

ENaC is sensitive to phosphatidylinositol 4,5-bisphosphate (PIP2), the target of phospholipase C-mediated metabolism, and phosphatidylinositiol 3,4,5-trisphosphate (PIP3), the product of phosphatidylinositide 3-OH kinase (PI3-K). PIP2 is permissive for ENaC gating possibly interacting directly with the channel. Activation of distal nephron P2Y receptors tempers ENaC activity by promoting PIP2 metabolism. This is important because gene deletion of P2Y2 receptors causes hypertension associated with hyperactive ENaC. Aldosterone, the final hormone in a negative-feedback cascade activated by decreases in blood pressure, increases ENaC activity. PIP3 sits at a critical bifurcation in the aldosterone-signaling cascade, increasing ENaC open probability and number. PIP3-effectors mediate increases in ENaC number by suppressing channel retrieval. PIP3 binds ENaC, at a site distinct from that important to PIP2 regulation, to modulate directly open probability.

SUMMARY

Phosphoinositides play key roles in physiologic control of ENaC and perhaps dysregulation plays a role in disease associated with abnormal renal sodium handling.

A carboxy-terminal inter-helix linker as the site of phosphatidylinositol 4,5-bisphosphate action on Kv7 (M-type) K+ channels

The regulation of M-type (KCNQ [Kv7]) K⁺ channels by phosphatidylinositol 4,5-bisphosphate (PIP₂) has perhaps the best correspondence to physiological signaling, but the site of action and structural motif of PIP₂ on these channels have not been established. Using single-channel recordings of chimeras of Kv7.3 and 7.4 channels with highly differential PIP₂ sensitivities, we localized a carboxy-terminal inter-helix linker as the primary site of PIP₂ action. Point mutants within this linker in Kv7.2 and Kv7.3 identified a conserved cluster of basic residues that interact with the lipid using electrostatic and hydrogen bonds. Homology modeling of this putative PIP₂-binding linker in Kv7.2 and Kv7.3 using the solved structure of Kir2.1 and Kir3.1 channels as templates predicts a structure of Kv7.2 and 7.3 very similar to the Kir channels, and to the seven-β-sheet barrel motif common to other PIP₂-binding domains. Phosphoinositide-docking simulations predict affinities and interaction energies in accord with the experimental data, and furthermore indicate that the precise identity of residues in the interacting pocket alter channel–PIP₂ interactions not only by altering electrostatic energies, but also by allosterically shifting the structure of the lipid-binding surface. The results are likely to shed light on the general structural mechanisms of phosphoinositide regulation of ion channels.

Molecular determinants of PI(4,5)P2 and PI(3,4,5)P3 regulation of the epithelial Na+ channel

Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) and phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P₃) are physiologically important second messengers. These molecules bind effector proteins to modulate activity. Several types of ion channels, including the epithelial Na⁺ channel (ENaC), are phosphoinositide effectors capable of directly interacting with these signaling molecules. Little, however, is known of the regions within ENaC and other ion channels important to phosphoinositide binding and modulation. Moreover, the molecular mechanism of this regulation, in many instances, remains obscure. Here, we investigate modulation of ENaC by PI(3,4,5)P₃ and PI(4,5)P₂ to begin identifying the molecular determinants of this regulation. We identify intracellular regions near the inner membrane interface just following the second transmembrane domains in β- and γ- but not α-ENaC as necessary for PI(3,4,5)P₂ but not PI(4,5)P₂ modulation. Charge neutralization of conserved basic amino acids within these regions demonstrated that these polar residues are critical to phosphoinositide regulation. Single channel analysis, moreover, reveals that the regions just following the second transmembrane domains in β- and γ-ENaC are critical to PI(3,4,5)P₃ augmentation of ENaC open probability, thus, defining mechanism. Unexpectedly, intracellular domains within the extreme N terminus of β- and γ-ENaC were identified as being critical to down-regulation of ENaC activity and P_o in response to depletion of membrane PI(4,5)P₂. These regions of the channel played no identifiable role in a PI(3,4,5)P₃ response. Again, conserved positive-charged residues within these domains were particularly important, being necessary for exogenous PI(4,5)P₂ to increase open probability. We conclude that β and γ subunits bestow phosphoinositide sensitivity to ENaC with distinct regions of the channel being critical to regulation by PI(3,4,5)P₃ and PI(4,5)P₂. This argues that these phosphoinositides occupy distinct ligand-binding sites within ENaC to modulate open probability.

Ancient intron insertion sites and palindromic genomic duplication evolutionally shapes an elementally functioning membrane protein family

Background

In spite of the recent accumulation of genomic data, the evolutionary pathway in the individual genes of present-day living taxa is still elusive for most genes. Among ion channels, inward K⁺ rectifier (IRK) channels are the fundamental and well-defined protein group. We analyzed the genomic structures of this group and compared them among a phylogenetically wide range with our sequenced Halocynthia roretzi, a tunicate, IRK genomic genes.

Results

A total of 131 IRK genomic genes were analyzed. The phylogenic trees of amino acid sequences revealed a clear diversification of deuterostomic IRKs from protostomic IRKs and suggested that the tunicate IRKs are possibly representatives of the descendants of ancestor forms of three major groups of IRKs in the vertebrate. However, the exon-intron structures of the tunicate IRK genomes showed considerable similarities to those of Caenorhabditis. In the vertebrate clade, the members in each major group increased at least four times those in the tunicate by various types of global gene duplication. The generation of some major groups was inferred to be due to anti-tandem (palindromic) duplication in early history. The intron insertion points greatly decreased during the evolution of the vertebrates, remaining as a unique conservation of an intron insertion site in the portion of protein-protein interaction within the coding regions of all vertebrate G-protein-activated IRK genes.

Conclusion

From the genomic survey of a family of IRK genes, it was suggested that the ancient intron insertion sites and the unique palindromic genomic duplication evolutionally shaped this membrane protein family.

An NH2-terminal multi-basic RKR motif is required for the ATP-dependent regulation of hIK1

We previously demonstrated that the ATP/PKA-dependent activation of the human intermediate conductance, Ca2+-activated K+ channel, hIK1, is dependent upon a C-terminal motif. The NH2-terminus of hIK1 contains a multi-basic 13RRRKR17 motif, known to be important in the trafficking and function of ion channels. While individual mutations within this domain have no effect on channel function, the triple mutation (15RKR17/AAA), as well as additional double mutations, result in a near complete loss of functional channels, as assessed by whole-cell patch-clamp. However, cell-surface immunoprecipitation studies confirmed expression of these mutated channels at the plasma membrane. To elucidate the functional consequences of the (15)RKR(17)/AAA mutation we performed inside-out patch clamp recordings where we observed no difference in Ca2+ affinity between the wild-type and mutated channels. However, in contrast to wild-type hIK1, channels expressing the 15RKR17/AAA mutation exhibited rundown, which could not be reversed by the addition of ATP. Wild-type hIK1 channel activity was reduced by alkaline phosphatase both in the presence and absence of ATP, indicative of a phosphorylation event, whereas the 15RKR17/AAA mutation eliminated this effect of alkaline phosphatase. Further, single channel analysis demonstrated that the 15RKR17/AAA mutation resulted in a four-fold lower channel open probability (P(o)), in the presence of saturating Ca2+ and ATP, compared to wild-type hIK1. In conclusion, these results represent the first demonstration for a role of the NH2-terminus in the second messenger-dependent regulation of hIK1 and, in combination with our previous findings, suggest that this regulation is dependent upon a close NH2/C-terminal association.

Binding and direct activation of the epithelial Na+ channel (ENaC) by phosphatidylinositides

Several distinct types of ion channels bind and directly respond to phosphatidylinositides, including phosphatidylinositol (3,4,5)-trisphosphate (PI(3,4,5)P(3)) and phosphatidylinositol (4,5)-bisphosphate (PI(4,5)P(2)). This regulation is physiologically relevant for its dysfunction, in some instances, causes disease. Recent studies identify the epithelial Na(+) channel (ENaC) as a channel sensitive to phosphatidylinositides. ENaC appears capable of binding both PI(4,5)P(2) and PI(3,4,5)P(3) with binding stabilizing channel gating. The binding sites for these molecules within ENaC are likely to be distinct with the former phosphoinositide interacting with elements in the cytosolic NH(2)-terminus of the beta- and gamma-ENaC subunits and the latter with cytosolic regions immediately following the second transmembrane domains in these two subunits. PI(4,5)P(2) binding to ENaC appears saturated at rest and necessary for channel gating. Thus, decreases in cellular PI(4,5)P(2) levels may serve as a convergence point for inhibitory regulation of ENaC by G-protein coupled receptors and receptor tyrosine kinases. In contrast, apparent PI(3,4,5)P(3) binding to ENaC is not saturated. This enables the channel to respond with gating changes in a rapid and dynamic manner to signalling input that influences cellular PI(3,4,5)P(3) levels.

An andersen-Tawil syndrome mutation in Kir2.1 (V302M) alters the G-loop cytoplasmic K+ conduction pathway

Loss-of-function mutations in the inward rectifier potassium channel, Kir2.1, cause Andersen-Tawil syndrome (ATS-1), an inherited disorder of periodic paralysis and ventricular arrhythmias. Here, we explore the mechanism by which a specific ATS-1 mutation (V302M) alters channel function. Val-302 is located in the G-loop, a structure that is believed to form a flexible barrier for potassium permeation at the apex of the cytoplasmic pore. Consistent with a role in stabilizing the G-loop in an open conformation, we found the V302M mutation specifically renders the channel unable to conduct potassium without altering subunit assembly or attenuating cell surface expression. As predicted by the position of the Val-302 side chain in the crystal structure, amino acid substitution analysis revealed that channel activity and phosphatidylinositol 4,5-bisphosphate (PIP2) sensitivity are profoundly sensitive to alterations in the size, shape, and hydrophobicity of side chains at the Val-302 position. The observations establish that the Val-302 side chain is a critical determinant of potassium conduction through the G-loop. Based on our functional studies and the cytoplasmic domain crystal structure, we suggest that Val-302 may influence PIP2 gating indirectly by translating PIP2 binding to conformational changes in the G-loop pore.

Interplay between PIP3 and calmodulin regulation of olfactory cyclic nucleotide-gated channels

Phosphatidylinositol-3,4,5-trisphosphate (PIP3) has been proposed to modulate the odorant sensitivity of olfactory sensory neurons by inhibiting activation of cyclic nucleotide-gated (CNG) channels in the cilia. When applied to the intracellular face of excised patches, PIP3 has been shown to inhibit activation of heteromeric olfactory CNG channels, composed of CNGA2, CNGA4, and CNGB1b subunits, and homomeric CNGA2 channels. In contrast, we discovered that channels formed by CNGA3 subunits from cone photoreceptors were unaffected by PIP3. Using chimeric channels and a deletion mutant, we determined that residues 61-90 within the N terminus of CNGA2 are necessary for PIP3 regulation, and a biochemical "pulldown" assay suggests that PIP3 directly binds this region. The N terminus of CNGA2 contains a previously identified calcium-calmodulin (Ca2+/CaM)-binding domain (residues 68-81) that mediates Ca2+/CaM inhibition of homomeric CNGA2 channels but is functionally silent in heteromeric channels. We discovered, however, that this region is required for PIP3 regulation of both homomeric and heteromeric channels. Furthermore, PIP3 occluded the action of Ca2+/CaM on both homomeric and heteromeric channels, in part by blocking Ca2+/CaM binding. Our results establish the importance of the CNGA2 N terminus for PIP3 inhibition of olfactory CNG channels and suggest that PIP3 inhibits channel activation by disrupting an autoexcitatory interaction between the N and C termini of adjacent subunits. By dramatically suppressing channel currents, PIP3 may generate a shift in odorant sensitivity that does not require prior channel activity.

Regulation of the epithelial Na+ channel (ENaC) by phosphatidylinositides

The epithelial Na(+) channel (ENaC) is an end-effector of diverse cellular signaling cascades, including those with phosphatidylinositide second messengers. Recent evidence also suggests that in some instances, phospatidylinositides can directly interact with ENaC to increase channel activity by increasing channel open probability and/or membrane localization. We review here findings relevant to regulation of ENaC by phosphatidylinositol 4,5-bisphosphate (PIP(2)) and phosphatidylinositol 3,4,5-triphosphate (PIP(3)). Similar to its actions on other ion channels, PIP(2) is permissive for ENaC openings having a direct effect on gating. The PIP(2) binding site in ENaC involved in this regulation is most likely localized to the NH(2) terminus of beta-ENaC. PIP(3) also affects ENaC gating but, rather than being permissive, augments open probability. The PIP(3) binding site in ENaC involved in this regulation is localized to the proximal region of the COOH terminus of gamma-ENaC just following the second transmembrane domain. In complementary pathways, PIP(3) also impacts ENaC membrane levels through both direct actions on the channel and via a signaling cascade involving phosphoinositide 3-OH kinase (PI3-K) and the aldosterone-induced gene product serum and glucocorticoid-inducible kinase. The putative PIP(3) binding site in ENaC involved in direct regulation of channel membrane levels has not yet been identified.

Identification of a functional phosphatidylinositol 3,4,5-trisphosphate binding site in the epithelial Na+ channel

Membrane phospholipids, such as phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P(3)), are signaling molecules that can directly modulate the activity of ion channels, including the epithelial Na(+) channel (ENaC). Whereas PI(3,4,5)P(3) directly activates ENaC, its binding site within the channel has not been identified. We identify here a region of gamma-mENaC just following the second trans-membrane domain (residues 569-583) important to PI(3,4,5)P(3) binding and regulation. Deletion of this track decreases activity of ENaC heterologously expressed in Chinese hamster ovary cells. K-Ras and its first effector phosphoinositide 3-OH kinase (PI3-K), as well as RhoA and its effector phosphatidylinositol 4-phosphate 5-kinase increase ENaC activity. Whereas the former, via generation of PI(3,4,5)P(3), increases ENaC open probability, the latter increases activity by increasing membrane levels of the channel. Deletion of the region just distal to the second trans-membrane domain disrupted regulation by K-Ras and PI3-K but not RhoA and phosphatidylinositol 4-phosphate 5-kinase. Moreover, PI(3,4,5)P(3) binds ENaC with deletion of the region following the second transmembrane domain disrupting this interaction and disrupting direct activation of the channel by PI(3,4,5)P(3). Mutation analysis revealed the importance of conserved positive and negative charged residues as well as bulky amino acids within this region to modulation of ENaC by PI3-K. The current results identify the region just distal to the second trans-membrane domain within gamma-mENaC as being part of a functional PI(3,4,5)P(3) binding site that directly impacts ENaC activity. Phospholipid binding to this site is probably mediated by the positively charged amino acids within this track, with negatively charged and bulky residues also influencing specificity of interactions.

KATP channel: relation with cell metabolism and role in the cardiovascular system

ATP-sensitive potassium channel (K(ATP)) is one kind of inwardly rectifying channel composed of two kinds of subunits: the pore forming subunits and the regulatory subunits. K(ATP) channels exist in the sarcolemmal, mitochondrial and nuclear membranes of various tissues. Cell metabolism regulates K(ATP) gene expression and metabolism products regulate the channel by direct interactions, while K(ATP) controls membrane potentials and regulate cell activities including energy metabolism, apoptosis and gene expression. K(ATP) channels from different cell organelles are linked by some signal molecules and they can respond to common stimulation in a coordinate way. In the cardiovascular system K(ATP) has important functions. The most prominent is that opening of this channel can protect cardiac myocytes against ischemic injuries. The sarcolemmal K(ATP) may provide a basic protection against ischemia by energy sparing, while both the sarcolemmal K(ATP) and mitochondrial K(ATP) channels are necessary for the ischemia preconditioning. K(ATP) channels also have important functions including homeostasis maintenance and vascular tone regulation under physiological conditions. Further elucidation of the role of K(ATP) in the cardiovascular system will help us to regulate cell metabolism or prevent damage caused by abnormal channel functions.

Novel nucleotide-binding sites in ATP-sensitive potassium channels formed at gating interfaces

The coupling of cell metabolism to membrane electrical activity is a vital process that regulates insulin secretion, cardiac and neuronal excitability and the responses of cells to ischemia. ATP-sensitive potassium channels (K(ATP); Kir6.x) are a major part of this metabolic-electrical coupling system and translate metabolic signals such as the ATP:ADP ratio to changes in the open or closed state (gate) of the channel. The localization of the nucleotide-binding site (NBS) on Kir6.x channels and how nucleotide binding gates these K(ATP) channels remain unclear. Here, we use fluorescent nucleotide binding to purified Kir6.x proteins to define the peptide segments forming the NBS on Kir6.x channels and show that unique N- and C-terminal interactions from adjacent subunits are required for high-affinity nucleotide binding. The short N- and C-terminal segments comprising the novel intermolecular NBS are next to helices that likely move with channel opening/closing, suggesting a lock-and-key model for ligand gating.

Molecular diversity and regulation of renal potassium channels

K(+) channels are widely distributed in both plant and animal cells where they serve many distinct functions. K(+) channels set the membrane potential, generate electrical signals in excitable cells, and regulate cell volume and cell movement. In renal tubule epithelial cells, K(+) channels are not only involved in basic functions such as the generation of the cell-negative potential and the control of cell volume, but also play a uniquely important role in K(+) secretion. Moreover, K(+) channels participate in the regulation of vascular tone in the glomerular circulation, and they are involved in the mechanisms mediating tubuloglomerular feedback. Significant progress has been made in defining the properties of renal K(+) channels, including their location within tubule cells, their biophysical properties, regulation, and molecular structure. Such progress has been made possible by the application of single-channel analysis and the successful cloning of K(+) channels of renal origin.

Do anionic phospholipids serve as cofactors or second messengers for the regulation of activity of cloned ATP-sensitive K+ channels?

The regulation of ion channels by anionic phospholipids is currently very topical. An outstanding issue is whether phosphatidylinositol 4,5-diphosphate and related species act as true second messengers in signaling or behave in a manner analogous to an enzymatic cofactor. This question is especially pertinent regarding ATP-sensitive K+ channels in smooth muscle, for which there is substantial literature supporting inhibitory regulation by hormones. In this study, we have examined regulation of the potential cloned equivalents of the smooth muscle ATP-sensitive K+ channel (SUR2B/Kir6.1 and SUR2B/Kir6.2). We find that both can be inhibited via the Gq/11-coupled muscarinic M3 receptor but that the pathways by which this occurs are different. Our data show that SUR2B/Kir6.1 is inhibited by protein kinase C and binds anionic phospholipids with high affinity, such that potential physiological fluctuations in their levels do not influence channel activity. In contrast, Kir6.2 is not regulated by protein kinase C but binds anionic phospholipids with low affinity. In this case, phosphatidylinositol 4,5-diphosphate and related species have the potential to act as second messengers in signaling. Thus, Kir6.1 and Kir6.2 are regulated by distinct inhibitory mechanisms.