Pore mutations affecting tetrameric assembly and functioning of the potassium channel KcsA from Streptomyces lividans

Characterization of hyperactive mutations in the renal potassium channel ROMK uncovers unique effects on channel biogenesis and ion conductance

Hypertension affects one billion people worldwide and is the most common risk factor for cardiovascular disease, yet a comprehensive picture of its underlying genetic factors is incomplete. Amongst regulators of blood pressure is the renal outer medullary potassium (ROMK) channel. While select ROMK mutants are prone to premature degradation and lead to disease, heterozygous carriers of some of these same alleles are protected from hypertension. Therefore, we hypothesized that gain-of-function (GoF) ROMK variants which increase potassium flux may predispose people to hypertension. To begin to test this hypothesis, we employed genetic screens and a candidate-based approach to identify six GoF variants in yeast. Subsequent functional assays in higher cells revealed two variant classes. The first group exhibited greater stability in the endoplasmic reticulum, enhanced channel assembly, and/or increased protein at the cell surface. The second group of variants resided in the PIP2-binding pocket, and computational modeling coupled with patch-clamp studies demonstrated lower free energy for channel opening and slowed current rundown, consistent with an acquired PIP2-activated state. Together, these findings advance our understanding of ROMK structure-function, suggest the existence of hyperactive ROMK alleles in humans, and establish a system to facilitate the development of ROMK-targeted antihypertensives.

Mutational Insight into Allosteric Regulation of Kir Channel Activity

Potassium (K+) channels are regulated in part by allosteric communication between the helical bundle crossing, or inner gate, and the selectivity filter, or outer gate. This network is triggered by gating stimuli. In concert, there is an allosteric network which is a conjugated set of interactions which correlate long-range structural rearrangements necessary for channel function. Inward-rectifier K+ (Kir) channels favor inward K+ conductance, are ligand-gated, and help establish resting membrane potentials. KirBac1.1 is a bacterial Kir (KirBac) channel homologous to human Kir (hKir) channels. Additionally, KirBac1.1 is gated by the anionic phospholipid ligand phosphatidylglycerol (PG). In this study, we use site-directed mutagenesis to investigate residues involved in the KirBac1.1 gating mechanism and allosteric network we previously proposed using detailed solid-state NMR (SSNMR) measurements. Using fluorescence-based K+ and sodium (Na+) flux assays, we identified channel mutants with impaired function that do not alter selectivity of the channel. In tandem, we performed coarse grain molecular dynamics simulations, observing changes in PG-KirBac1.1 interactions correlated with mutant channel activity and contacts between the two transmembrane helices and pore helix tied to this behavior. Lipid affinity is closely tied to the proximity of two tryptophan residues on neighboring subunits which lure anionic lipids to a cationic pocket formed by a cluster of arginine residues. Thus, these simulations establish a structural and functional basis for the role of each mutated site in the proposed allosteric network. The experimental and simulated data provide insight into key functional residues involved in gating and lipid allostery of K+ channels. Our findings also have direct implications on the physiology of hKir channels due to conservation of many of the residues identified in this work from KirBac1.1.

Determinants of trafficking, conduction, and disease within a K+ channel revealed through multiparametric deep mutational scanning

A long-standing goal in protein science and clinical genetics is to develop quantitative models of sequence, structure, and function relationships to understand how mutations cause disease. Deep mutational scanning (DMS) is a promising strategy to map how amino acids contribute to protein structure and function and to advance clinical variant interpretation. Here, we introduce 7429 single-residue missense mutations into the inward rectifier K+ channel Kir2.1 and determine how this affects folding, assembly, and trafficking, as well as regulation by allosteric ligands and ion conduction. Our data provide high-resolution information on a cotranslationally folded biogenic unit, trafficking and quality control signals, and segregated roles of different structural elements in fold stability and function. We show that Kir2.1 surface trafficking mutants are underrepresented in variant effect databases, which has implications for clinical practice. By comparing fitness scores with expert-reviewed variant effects, we can predict the pathogenicity of 'variants of unknown significance' and disease mechanisms of known pathogenic mutations. Our study in Kir2.1 provides a blueprint for how multiparametric DMS can help us understand the mechanistic basis of genetic disorders and the structure-function relationships of proteins.

Folding and misfolding of potassium channel monomers during assembly and tetramerization

The dynamics and folding of potassium channel pore domain monomers are connected to the kinetics of tetramer assembly. In all-atom molecular dynamics simulations of Kv1.2 and KcsA channels, monomers adopt multiple nonnative conformations while the three helices remain folded. Consistent with this picture, NMR studies also find the monomers to be dynamic and structurally heterogeneous. However, a KcsA construct with a disulfide bridge engineered between the two transmembrane helices has an NMR spectrum with well-dispersed peaks, suggesting that the monomer can be locked into a native-like conformation that is similar to that observed in the folded tetramer. During tetramerization, fluoresence resonance energy transfer (FRET) data indicate that monomers rapidly oligomerize upon insertion into liposomes, likely forming a protein-dense region. Folding within this region occurs along separate fast and slow routes, with τfold ∼40 and 1,500 s, respectively. In contrast, constructs bearing the disulfide bond mainly fold via the faster pathway, suggesting that maintaining the transmembrane helices in their native orientation reduces misfolding. Interestingly, folding is concentration independent despite the tetrameric nature of the channel, indicating that the rate-limiting step is unimolecular and occurs after monomer association in the protein-dense region. We propose that the rapid formation of protein-dense regions may help with the assembly of multimeric membrane proteins by bringing together the nascent components prior to assembly. Finally, despite its name, the addition of KcsA's C-terminal "tetramerization" domain does not hasten the kinetics of tetramerization.

Methodological approaches for the analysis of transmembrane domain interactions: A systematic review

The study of protein-protein interactions (PPI) has proven fundamental for the understanding of the most relevant cell processes. Any protein domain can participate in PPI, including transmembrane (TM) segments that can establish interactions with other TM domains (TMDs). However, the hydrophobic nature of TMDs and the environment they occupy complicates the study of intramembrane PPI, which demands the use of specific approaches and techniques. In this review, we will explore some of the strategies available to study intramembrane PPI in vitro, in vivo, and, in silico, focusing on those techniques that could be carried out in a standard molecular biology laboratory regarding its previous experience with membrane proteins.

Inferring functional units in ion channel pores via relative entropy

Coarse-grained protein models approximate the first-principle physical potentials. Among those modeling approaches, the relative entropy framework yields promising and physically sound results, in which a mapping from the target protein structure and dynamics to a model is defined and subsequently adjusted by an entropy minimization of the model parameters. Minimization of the relative entropy is equivalent to maximization of the likelihood of reproduction of (configurational ensemble) observations by the model. In this study, we extend the relative entropy minimization procedure beyond parameter fitting by a second optimization level, which identifies the optimal mapping to a (dimension-reduced) topology. We consider anisotropic network models of a diverse set of ion channels and assess our findings by comparison to experimental results.

H. R, Thamleena A. H, Joseph A, Ben A, V. U. K. Roles of different amino-acid residues towards binding and selective transport of K+ through KcsA K+-ion channel

Each amino acid in the selectivity filter plays a distinct role towards binding and transport of K⁺ ion through KcsA.

Specific interactions between alkali metal cations and the KcsA channel studied using ATR-FTIR spectroscopy

The X-ray structure of KcsA, a eubacterial potassium channel, displays a selectivity filter composed of four parallel peptide strands. The backbone carbonyl oxygen atoms of these strands solvate multiple K(+) ions. KcsA structures show different distributions of ions within the selectivity filter in solutions containing different cations. To assess the interactions of cations with the selectivity filter, we used attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy. Ion-exchange-induced ATR-FTIR difference spectra were obtained by subtracting the spectrum of KcsA soaked in K(+) solution from that obtained in Li(+), Na(+), Rb(+), and Cs(+) solutions. Large spectral changes in the amide-I and -II regions were observed upon replacing K(+) with smaller-sized cations Li(+) and Na(+) but not with larger-sized cations Rb(+) and Cs(+). These results strongly suggest that the selectivity filter carbonyls coordinating Rb(+) or Cs(+) adopt a conformation similar to those coordinating K(+) (cage configuration), but those coordinating Li(+) or Na(+) adopt a conformation (plane configuration) considerably different from those coordinating K(+). We have identified a cation-type sensitive amide-I band at 1681 cm(-1) and an insensitive amide-I band at 1659 cm(-1). The bands at 1650, 1639, and 1627 cm(-1) observed for Na(+)-coordinating carbonyls were almost identical to those observed in Li(+) solution, suggesting that KcsA forms a similar filter structure in Li(+) and Na(+) solutions. Thus, we conclude that the filter structure adopts a collapsed conformation in Li(+) solution that is similar to that in Na(+) solution but is in clear contrast to the X-ray crystal structure of KcsA with Li(+).

Competing Lipid-Protein and Protein-Protein Interactions Determine Clustering and Gating Patterns in the Potassium Channel from Streptomyces lividans (KcsA)

There is increasing evidence to support the notion that membrane proteins, instead of being isolated components floating in a fluid lipid environment, can be assembled into supramolecular complexes that take part in a variety of cooperative cellular functions. The interplay between lipid-protein and protein-protein interactions is expected to be a determinant factor in the assembly and dynamics of such membrane complexes. Here we report on a role of anionic phospholipids in determining the extent of clustering of KcsA, a model potassium channel. Assembly/disassembly of channel clusters occurs, at least partly, as a consequence of competing lipid-protein and protein-protein interactions at nonannular lipid binding sites on the channel surface and brings about profound changes in the gating properties of the channel. Our results suggest that these latter effects of anionic lipids are mediated via the Trp(67)-Glu(71)-Asp(80) inactivation triad within the channel structure and its bearing on the selectivity filter.

Visualizing KcsA conformational changes upon ion binding by infrared spectroscopy and atomistic modeling

The effect of ion binding in the selectivity filter of the potassium channel KcsA is investigated by combining amide I Fourier-transform infrared spectroscopy with structure-based spectral modeling. Experimental difference IR spectra between K(+)-bound KcsA and Na(+)-bound KcsA are in good qualitative agreement with spectra modeled from structural ensembles generated from molecular dynamics simulations. The molecular origins of the vibrational modes contributing to differences in these spectra are determined not only from structural differences in the selectivity filter but also from the pore helices surrounding this region. Furthermore, the coordination of K(+) or Na(+) to carbonyls in the selectivity filter effectively decouples the vibrations of those carbonyls from the rest of the protein, creating local probes of the electrostatic environment. The results suggest that it is necessary to include the influence of the surrounding helices in discussing selectivity and transport in KcsA and, on a more general level, that IR spectroscopy offers a nonperturbative route to studying the structure and dynamics of ion channels.

Assembly of AMPA receptors: mechanisms and regulation

AMPA receptors (AMPARs) play a critical role in excitatory glutamatergic neurotransmission. The number and subunit composition of AMPARs at synapses determines the dynamics of fast glutamatergic signalling. Functional AMPARs on the cell surface are tetramers. Thus tetrameric assembly of AMPARs represents a promising target for modulating AMPAR-mediated signalling in health and disease. Multiple structural domains within the receptor influence AMPAR assembly. In a proposed model for AMPAR assembly, the amino-terminal domain underlies the formation of a dimer pool. The transmembrane domain facilitates the formation and enhances the stability of the tetramer. The ligand-binding domain influences assembly through a process referred to as 'domain swapping'. We propose that this core AMPAR assembly process could be regulated by neuronal signals and speculate on possible mechanisms for such regulation.

ATR-FTIR Spectroscopy Revealing the Different Vibrational Modes of the Selectivity Filter Interacting with K(+) and Na(+) in the Open and Collapsed Conformations of the KcsA Potassium Channel

The potassium channel is highly selective for K(+) over Na(+), and the selectivity filter binds multiple dehydrated K(+) ions upon permeation. Here, we applied attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy to extract ion-binding-induced signals of the KcsA potassium channel at neutral pH. Shifts in the peak of the amide-I signal towards lower vibrational frequencies were observed as K(+) was replaced with Na(+). These ion species-specific shifts deduced the selectivity filter as the source of the signal, which was supported by the spectra of a mutant for the selectivity filter (Y78F). The difference FTIR spectra between the solution containing various concentrations of K(+) and that containing pure Na(+) demonstrated two types of peak shifts of the amide-I vibration in response to the K(+) concentration. These signals represent the binding of K(+) ions to the different sites in the selectivity filter with different dissociation constants (KD = 9 or 18 mM).

Mutations in the K(+)-channel KcsA toward Kir channels alter salt-induced clusterization and blockade by quaternary alkylammonium ions

Protein aggregation is a result of malfunction in protein folding, assembly, and transport, caused by protein mutation and/or changes in the cell environment, thus triggering many human diseases. We have shown that bacterial K(+)-channel KcsA, which acts as a representative model for ion channels, forms salt-induced large conductive complexes in a particular environment. In the present study, we investigated the effects of point mutations in the selectivity filter of KcsA on intrinsic stability, aggregation, and channel blocking behavior. First, we found that a low sodium chloride concentration in potassium-containing media induced fast transfer of single channels to a planar lipid bilayer. Second, increasing the sodium chloride concentration drastically increased the total channel current, indicating enhanced vesicle fusion and transfer of multiple channels to a planar lipid bilayer. However, such complexes exhibited high conductance as well as higher open probability compared to the unmodified KcsA behavior shown previously. Interestingly, the affinity of aggregated complexes for larger symmetric quaternary alkylammonium ions (QAs) was found to be much higher than that for tetraethylammonium, a classical blocker of the K(+) channel. Based on these findings, we propose that mutant channel complexes exhibit larger pore dimensions, thus resembling more the topological properties of voltage-gated and inwardly rectifying K(+) channels.

Separation of heteromeric potassium channel Kcv towards probing subunit composition-regulated ion permeation and gating

The chlorella virus-encoded Kcv can form a homo-tetrameric potassium channel in lipid membranes. This miniature peptide can be synthesized in vitro, and the tetramer purified from the SDS-polyacrylamide gel retains the K(+) channel functionality. Combining this capability with the mass-tagging method, we propose a simple, straightforward approach that can generically manipulate individual subunits in the tetramer, thereby enabling the detection of contribution from individual subunits to the channel functions. Using this approach, we showed that the structural change in the selectivity filter from only one subunit is sufficient to cause permanent channel inactivation ("all-or-none" mechanism), whereas the mutation near the extracellular entrance additively modifies the ion permeation with the number of mutant subunits in the tetramer ("additive" mechanism).

Mg(2+)-dependent gating of bacterial MgtE channel underlies Mg(2+) homeostasis

The MgtE family of Mg(2+) transporters is ubiquitously distributed in all phylogenetic domains. Recent crystal structures of the full-length MgtE and of its cytosolic domain in the presence and absence of Mg(2+) suggested a Mg(2+)-homeostasis mechanism, in which the MgtE cytosolic domain acts as a 'Mg(2+) sensor' to regulate the gating of the ion-conducting pore in response to the intracellular Mg(2+) concentration. However, complementary functional analyses to confirm the proposed model have been lacking. Moreover, the limited resolution of the full-length structure precluded an unambiguous characterization of these regulatory divalent-cation-binding sites. Here, we showed that MgtE is a highly Mg(2+)-selective channel gated by Mg(2+) and elucidated the Mg(2+)-dependent gating mechanism of MgtE, using X-ray crystallographic, genetic, biochemical, and electrophysiological analyses. These structural and functional results have clarified the control of Mg(2+) homeostasis through cooperative Mg(2+) binding to the MgtE cytosolic domain.

Changing Val-76 towards Kir channels drastically influences the folding and gating properties of the bacterial potassium channel KcsA

All K(+)-channels are stabilized by K(+)-ions in the selectivity filter. However, they differ from each other with regard to their selectivity filter. In this study, we changed specific residue Val-76 in the selectivity filter of KcsA to its counterpart Ile in inwardly rectifying K(+)-channels (Kir). The tetramer was exclusively converted into monomers as determined by conventional gel electrophoresis. However, by perfluoro-octanoic acid (PFO) gel electrophoresis mutant channel was mostly detected as tetramer. Tryptophan fluorescence and acrylamide quenching experiments demonstrated significant alteration in channel folding properties via increase in hydrophilicity of local environment. Furthermore, in planar lipid bilayer experiments V76I exhibited drastically lower conductance and decreased channel open time as compared to the unmodified KcsA. These studies suggest that V76I might contribute to determine the stabilizing, folding and channel gating properties in a selective K(+)-channel.

KirBac1.1: it's an inward rectifying potassium channel

KirBac1.1 is a prokaryotic homologue of eukaryotic inward rectifier potassium (Kir) channels. The crystal structure of KirBac1.1 and related KirBac3.1 have now been used extensively to generate in silico models of eukaryotic Kir channels, but functional analysis has been limited to (86)Rb(+) flux experiments and bacteria or yeast complementation screens, and no voltage clamp analysis has been available. We have expressed pure full-length His-tagged KirBac1.1 protein in Escherichia coli and obtained voltage clamp recordings of recombinant channel activity in excised membrane patches from giant liposomes. Macroscopic currents of wild-type KirBac1.1 are K(+) selective and spermine insensitive, but blocked by Ba(2+), similar to "weakly rectifying" eukaryotic Kir1.1 and Kir6.2 channels. The introduction of a negative charge at a pore-lining residue, I138D, generates high spermine sensitivity, similar to that resulting from the introduction of a negative charge at the equivalent position in Kir1.1 or Kir6.2. KirBac1.1 currents are also inhibited by PIP(2), consistent with (86)Rb(+) flux experiments, and reversibly inhibited by short-chain di-c8-PIP(2). At the single-channel level, KirBac1.1 channels show numerous conductance states with two predominant conductances (15 pS and 32 pS at -100 mV) and marked variability in gating kinetics, similar to the behavior of KcsA in recombinant liposomes. The successful patch clamping of KirBac1.1 confirms that this prokaryotic channel behaves as a bona fide Kir channel and opens the way for combined biochemical, structural, and electrophysiological analysis of a tractable model Kir channel, as has been successfully achieved for the archetypal K(+) channel KcsA.

Fluorescence detection of the movement of single KcsA subunits reveals cooperativity

The prokaryotic KcsA channel is gated at the helical bundle crossing by intracellular protons and inactivates at the extracellular selectivity filter. The C-terminal transmembrane helix has to undergo a conformational change for potassium ions to access the central cavity. Whereas a partial opening of the tetrameric channel is suggested to be responsible for subconductance levels of ion channels, including KcsA, a cooperative opening of the 4 subunits is postulated as the final opening step. In this study, we used single-channel fluorescence spectroscopy of KcsA to directly observe the movement of each subunit and the temporal correlation between subunits. Purified KcsA channels labeled at the C terminus near the bundle crossing have been inserted into supported lipid bilayer, and the fluorescence traces analyzed by means of a cooperative or independent Markov model. The analysis revealed that the 4 subunits do not move fully independently but instead showed a certain degree of cooperativity. However, the 4 subunits do not simply open in 1 concerted step.

Intrinsic Free Energy of the Conformational Transition of the KcsA Signature Peptide from Conducting to Nonconducting State

We explore a conformational transition of the TATTVGYG signature peptide of the KcsA ion selectivity filter and its GYG to AYA mutant from the conducting α-strand state into the nonconducting pII-like state using a novel technique for multidimensional optimization of transition path ensembles and free energy calculations. We find that the wild type peptide, unlike the mutant, intrinsically favors the conducting state due to G77 backbone propensities and additional hydrophobic interaction between the V76 and Y78 side chains in water. The molecular mechanical free energy profiles in explicit water are in very good agreement with the corresponding adiabatic energies from the Generalized Born Molecular Volume (GBMV) implicit solvent model. However comparisons of the energies to higher level B3LYP/6-31G(d) Density Functional Theory calculations with Polarizable Continuum Model (PCM) suggest that the nonconducting state might be more favorable than predicted by molecular mechanics simulations. By extrapolating the single peptide results to the tetrameric channel, we propose a novel hypothesis for the ion selectivity mechanism.

N-type inactivation of the potassium channel KcsA by the Shaker B "ball" peptide: mapping the inactivating peptide-binding epitope

The effects of the inactivating peptide from the eukaryotic Shaker BK(+) channel (the ShB peptide) on the prokaryotic KcsA channel have been studied using patch clamp methods. The data show that the peptide induces rapid, N-type inactivation in KcsA through a process that includes functional uncoupling of channel gating. We have also employed saturation transfer difference (STD) NMR methods to map the molecular interactions between the inactivating peptide and its channel target. The results indicate that binding of the ShB peptide to KcsA involves the ortho and meta protons of Tyr(8), which exhibit the strongest STD effects; the C4H in the imidazole ring of His(16); the methyl protons of Val(4), Leu(7), and Leu(10) and the side chain amine protons of one, if not both, the Lys(18) and Lys(19) residues. When a noninactivating ShB-L7E mutant is used in the studies, binding to KcsA is still observed but involves different amino acids. Thus, the strongest STD effects are now seen on the methyl protons of Val(4) and Leu(10), whereas His(16) seems similarly affected as before. Conversely, STD effects on Tyr(8) are strongly diminished, and those on Lys(18) and/or Lys(19) are abolished. Additionally, Fourier transform infrared spectroscopy of KcsA in presence of (13)C-labeled peptide derivatives suggests that the ShB peptide, but not the ShB-L7E mutant, adopts a beta-hairpin structure when bound to the KcsA channel. Indeed, docking such a beta-hairpin structure into an open pore model for K(+) channels to simulate the inactivating peptide/channel complex predicts interactions well in agreement with the experimental observations.

A quantitative description of KcsA gating II: single-channel currents

The kinetic transitions of proton-activated WT KcsA and the noninactivating E71A mutant were studied at the single-channel level in purified, liposome-reconstituted preparations. Single-channel currents were recorded using patch-clamp techniques under nonstationary and steady-state conditions. Maximum-likelihood analyses reveal that the key influence of acidic pH is to increase the frequency of bursting without an effect on the intraburst open and closed dwell times, consistent with the finding from macroscopic currents that protons promote activation without a significant effect on inactivation. However, in steady-conditions of pH, voltage not only alters the burst frequency but also affects their properties, such as the frequency of the flickers and the dwell times of the closed and open states. This is to be expected if voltage modulates pathways connecting open and inactivated states. Upon opening, KcsA can enter at least two closed states that are not part of the activation pathway. The frequency and duration of these closed states was found to be voltage dependent and therefore these are likely to represent short-lived inactivated states. Single-channel recordings of WT KcsA also show varying propensity for the presence of subconductance states. The probability of occurrence of these states did not show clear modulation by voltage or pH and their origin remains unclear and a focus for further investigation. A kinetic model is proposed to describe the gating events in KcsA that recapitulates its macroscopic and single-channel behavior. The model has been constrained by the single-channel analyses presented in this work along with data from macroscopic currents in the preceding paper.

In vivo monitoring of the potassium channel KcsA in Streptomyces lividans hyphae using immuno-electron microscopy and energy-filtering transmission electron microscopy

The previous discovery of theStreptomyces lividans kcsAgene and its overexpression followed by the functional reconstitution of the purified gene product has resulted in new strategies to explore this channel proteinin vitro. KcsA has evolved as a general model to investigate the structure/function relationship of ion channel proteins. Using specific antibodies raised against a domain of KcsA lacking membrane-spanning regions, KcsA has now been localized within numerous separated clusters between the outer face of the cytoplasm and the cell envelope in substrate hyphae of theS. lividanswild-type strain but not in a designed chromosomal disruption mutant ΔK, lacking a functionalkcsAgene. Previous findings had revealed that caesium ions led to a block of KcsA channel activity withinS. lividansprotoplasts fused to giant vesicles. As caesium can be scored by electron energy loss spectroscopy better than potassium, this technique was applied to hyphae that had been briefly exposed to caesium instead of potassium ions. Caesium was found preferentially at the cell envelope. Compared to the ΔK mutant, the relative level of caesium was ≈30 % enhanced in the wild-type. This is attributed to the presence of KcsA channels. Additional visualization by electron spectroscopic imaging supported this conclusion. The data presented are believed to represent the first demonstration ofin vivomonitoring of KcsA in its original host.

Effects of conducting and blocking ions on the structure and stability of the potassium channel KcsA

This article reports on the interaction of conducting (K(+)) and blocking (Na(+)) monovalent metal ions with detergent-solubilized and lipid-reconstituted forms of the K(+) channel KcsA. Monitoring of the protein intrinsic fluorescence reveals that the two ions bind competitively to KcsA with distinct affinities (dissociation constants for the KcsA.K(+) and KcsA.Na(+) complexes of approximately 8 and 190 mm, respectively) and induce different conformations of the ion-bound protein. The differences in binding affinity as well as the higher K(+) concentration bathing the intracellular mouth of the channel, through which the cations gain access to the protein binding sites, should favor that only KcsA.K(+) complexes are formed under physiological-like conditions. Nevertheless, despite such prediction, it was also found that concentrations of Na(+) well below its dissociation constant and even in the presence of higher K(+) concentrations, cause a remarkable decrease in the protein thermal stability and facilitate thermal dissociation into subunits of the tetrameric KcsA, as concluded from the temperature dependence of the protein infrared spectra and from gel electrophoresis, respectively. These latter observations cannot be explained based on the occupancy of the binding sites from above and suggest that there must be additional ion binding sites, whose occupancy could not be detected by fluorescence and in which the affinity for Na(+) must be higher or at least similar to that of K(+). Moreover, cation binding as reported by means of fluorescence does not suffice to explain the large differences in free energy of stabilization involved in the formation of the KcsA.Na(+) and KcsA.K(+) complexes, which for the most part should arise from synergistic effects of the ion-mediated intersubunit interactions.

Clustering and coupled gating modulate the activity in KcsA, a potassium channel model

Different patterns of channel activity have been detected by patch clamping excised membrane patches from reconstituted giant liposomes containing purified KcsA, a potassium channel from prokaryotes. The more frequent pattern has a characteristic low channel opening probability and exhibits many other features reported for KcsA reconstituted into planar lipid bilayers, including a moderate voltage dependence, blockade by Na(+), and a strict dependence on acidic pH for channel opening. The predominant gating event in this low channel opening probability pattern corresponds to the positive coupling of two KcsA channels. However, other activity patterns have been detected as well, which are characterized by a high channel opening probability (HOP patterns), positive coupling of mostly five concerted channels, and profound changes in other KcsA features, including a different voltage dependence, channel opening at neutral pH, and lack of Na(+) blockade. The above functional diversity occurs correlatively to the heterogeneous supramolecular assembly of KcsA into clusters. Clustering of KcsA depends on protein concentration and occurs both in detergent solution and more markedly in reconstituted membranes, including giant liposomes, where some of the clusters are large enough (up to micrometer size) to be observed by confocal microscopy. As in the allosteric conformational spread responses observed in receptor clustering (Bray, D. and Duke, T. (2004) Annu. Rev. Biophys. Biomol. Struct. 33, 53-73) our tenet is that physical clustering of KcsA channels is behind the observed multiple coupled gating and diverse functional responses.

Functional role and affinity of inorganic cations in stabilizing the tetrameric structure of the KcsA K+ channel

Crystal structures of the tetrameric KcsA K+ channel reveal seven distinct binding sites for K+ ions within the central pore formed at the fourfold rotational symmetry axis. Coordination of an individual K+ ion by eight protein oxygen atoms within the selectivity filter suggests that ion-subunit bridging by cation-oxygen interactions contributes to structural stability of the tetramer. To test this hypothesis, we examined the effect of inorganic cations on the temperature dependence of the KcsA tetramer as monitored by SDS-PAGE. Inorganic cations known to permeate or strongly block K+ channels (K+, Rb+, Cs+, Tl+, NH4+, Ba2+, and Sr2+) confer tetramer stability at higher temperatures (T0.5 range = 87 degrees C to >99 degrees C) than impermeant cations and weak blockers (Li+, Na+, Tris+, choline+; T0.5 range = 59 degrees C to 77 degrees C). Titration of K+, Ba2+, and other stabilizing cations protects against rapid loss of KcsA tetramer observed in 100 mM choline Cl at 90 degrees C. Tetramer protection titrations of K+, Rb+, Cs+, Tl+, and NH4+ at 85 degrees C or 90 degrees C exhibit apparent Hill coefficients (N) ranging from 1.7 to 3.3 and affinity constants (K0.5) ranging from 1.1 to 9.6 mM. Ba2+ and Sr2+ titrations exhibit apparent one-site behavior (N congruent with 1) with K0.5 values of 210 nM and 11 microM, respectively. At 95 degrees C in the presence of 5 mM K+, titration of Li+ or Na+ destabilizes the tetramer with K0.5 values of 57 mM and 109 mM, respectively. We conclude that specific binding interactions of inorganic cations with the selectivity filter are an important determinant of tetramer stability of KscA.

Prokaryotic K(+) channels: from crystal structures to diversity

The deep roots and wide branches of the K(+)-channel family are evident from genome surveys and laboratory experimentation. K(+)-channel genes are widespread and found in nearly all the free-living bacteria, archaea and eukarya. The conservation of basic structures and mechanisms such as the K(+) filter, the gate, and some of the gate's regulatory domains have allowed general insights on animal K(+) channels to be gained from crystal structures of prokaryotic channels. Since microbes are the great majority of life's diversity, it is not surprising that microbial genomes reveal structural motifs beyond those found in animals. There are open-reading frames that encode K(+)-channel subunits with unconventional filter sequences, or regulatory domains of different sizes and numbers not previously known. Parasitic or symbiotic bacteria tend not to have K(+) channels, while those showing lifestyle versatility often have more than one K(+)-channel gene. It is speculated that prokaryotic K(+) channels function to allow adaptation to environmental and metabolic changes, although the actual roles of these channels in prokaryotes are not yet known. Unlike enzymes in basic metabolism, K(+) channel, though evolved early, appear to play more diverse roles than revealed by animal research. Finding and sorting out these roles will be the goal and challenge of the near future.

The role of lipids in membrane insertion and translocation of bacterial proteins

Phospholipids are essential building blocks of membranes and maintain the membrane permeability barrier of cells and organelles. They provide not only the bilayer matrix in which the functional membrane proteins reside, but they also can play direct roles in many essential cellular processes. In this review, we give an overview of the lipid involvement in protein translocation across and insertion into the Escherichia coli inner membrane. We describe the key and general roles that lipids play in these processes in conjunction with the protein components involved. We focus on the Sec-mediated insertion of leader peptidase. We describe as well the more direct roles that lipids play in insertion of the small coat proteins Pf3 and M13. Finally, we focus on the role of lipids in membrane assembly of oligomeric membrane proteins, using the potassium channel KcsA as model protein. In all cases, the anionic lipids and lipids with small headgroups play important roles in either determining the efficiency of the insertion and assembly process or contributing to the directionality of the insertion process.

Influence of C-terminal protein domains and protein-lipid interactions on tetramerization and stability of the potassium channel KcsA

KcsA is a prokaryotic potassium channel formed by the assembly of four identical subunits around a central aqueous pore. Although the high-resolution X-ray structure of the transmembrane portion of KcsA is known [Doyle, D. A., Morais, C. J., Pfuetzner, R. A., Kuo, A., Gulbis, J. M., Cohen, S. L., Chait, B. T., and MacKinnon, R. (1998) Science 280, 69-77], the identification of the molecular determinant(s) involved in promoting subunit tetramerization remains to be determined. Here, C-terminal deletion channel mutants, KcsA Delta125-160 and Delta120-160, as well as 1-125 KcsA obtained from chymotrypsin cleavage of full-length 1-160 KcsA, have been used to evaluate the role of the C-terminal segment on the stability and tetrameric assembly of the channel protein. We found that the lack of the cytoplasmic C-terminal domain of KcsA, and most critically the 120-124 sequence stretch, impairs tetrameric assembly of channel subunits in a heterologous E. coli expression system. Molecular modeling of KcsA predicts that, indeed, such sequence stretch provides intersubunit interaction sites by hydrogen bonding to amino acid residues in N- and C-terminal segments of adjacent subunits. However, once the KcsA tetramer is assembled, its remarkable in vitro stability to detergent or to heat-induced dissociation into subunits is not greatly influenced by whether the entire C-terminal domain continues being part of the protein. Finally and most interestingly, it is observed that, even in the absence of the C-terminal domain involved in tetramerization, reconstitution into membrane lipids promotes in vitro KcsA tetramerization very efficiently, an event which is likely mediated by allowing proper hydrophobic interactions involving intramembrane protein domains.

Glycine as a D-amino acid surrogate in the K(+)-selectivity filter

The K(+) channel-selectivity filter consists of two absolutely conserved glycine residues. Crystal structures show that the first glycine in the selectivity filter, Gly-77 in KcsA, is in a left-handed helical conformation. Although the left-handed helical conformation is not favorable for the naturally occurring L-amino acids, it is favorable for the chirally opposite D-amino acids. Here, we demonstrate that Gly-77 can be replaced by D-Ala with almost complete retention of function. In contrast, substitution with an L-amino acid results in a nonfunctional channel. This finding suggests that glycine is used as a surrogate D-amino acid in the selectivity filter. The absolute conservation of glycine in the K(+)-selectivity filter can be explained as a result of glycine being the only natural amino acid that can play this role.

Nonbilayer lipids affect peripheral and integral membrane proteins via changes in the lateral pressure profile

Nonbilayer lipids can be defined as cone-shaped lipids with a preference for nonbilayer structures with a negative curvature, such as the hexagonal phase. All membranes contain these lipids in large amounts. Yet, the lipids in biological membranes are organized in a bilayer. This leads to the question: what is the physiological role of nonbilayer lipids? Different models are discussed in this review, with a focus on the lateral pressure profile within the membrane. Based on this lateral pressure model, predictions can be made for the effect of nonbilayer lipids on peripheral and integral membrane proteins. Recent data on the catalytic domain of Leader Peptidase and the potassium channel KcsA are discussed in relation to these predictions and in relation to the different models on the function of nonbilayer lipids. The data suggest a general mechanism for the interaction between nonbilayer lipids and membrane proteins via the membrane lateral pressure.

Hydrophobic Helical Hairpins: Design and Packing Interactions in Membrane Environments

Helix-helix interactions within membranes are dominated by van der Waals packing motifs and side chain-side chain hydrogen bond formation, which act in tandem to determine the residues that comprise the interface between two given helices. To explore in a systematic manner the tertiary contacts between transmembrane helices, we have designed and expressed in Escherichia coli highly hydrophobic helix-loop-helix constructs of prototypic sequence K(1)KKKKKKFAIAIAIIAWAX(19)AIIAIAIAIKSPGSKIAIAIAIIAZ(44)AWAIIAIAIAFKKKKKKK(62), where "small" (Ala) and "large" (Ile) residues were used to maximize the tertiary contact area. Evidence that the two transmembrane (TM) segments in the AI construct contain an interface conducive for folding into a hairpin structure was obtained from the results that (i) the single TM AI(pep) peptide derived from the AI hairpin forms SDS-resistant dimers on PAGE gels and (ii) the corresponding sequence forms a strong dimer when examined in vivo in TOXCAT assays. Site-directed mutagenesis of AI hairpins was carried out to incorporate each of the 20 commonly occurring amino acids at X positions. Analysis on Western blots using an oligomerization assay in 12% NuPage-sodium dodecyl sulfate (SDS) indicated that mutants with X = E, D, Q, R, N, H, and K largely formed SDS-resistant dimers-which likely correspond to H-bonded four-helix bundles-while all the others (e.g., X = F, W, L, I, M, V, C, Y, A, T, S, G, and P) remained monomeric. Systematic studies of X/Z double mutants indicated that formation of hairpin dimers is the result of the disruption of stabilizing interactions between the antiparallel helices within the AI construct. The overall results suggest that, in situations where hydrophobic van der Waals packing energy between helices is sufficient to prevent significant rotation about the major axes of interacting helices, intrahairpin side chain-side chain H-bond formation will occur mainly when pairs of polar residues are interfacially located and proximal. Knowledge of the relative contributions of these forces should be of value, for example, in clarifying the context--and the structural consequences--of disease-related mutations.

Stability of KcsA Tetramer Depends on Membrane Lateral Pressure

The potassium channel KcsA forms an extremely stable tetramer. Despite this high stability, it has been shown that the membrane-mimicking solvent 2,2,2-trifluoroethanol (TFE) can induce tetramer dissociation [Valiyaveetil, F. I., et al. (2002) Biochemistry 41, 10771-7, and Demmers, J. A. A., et al. (2003) FEBS Lett. 541, 69-77]. Here we have studied the effect of TFE on the structure and oligomeric state of the KcsA tetramer, reconstituted in different lipid systems. It was found that TFE changes the secondary and tertiary structure of KcsA and that it can dissociate the KcsA tetramer in all systems used. The tetramer is stabilized by a lipid bilayer as compared to detergent micelles. The extent of stabilization was found to depend on the nature of the lipids: a strong stabilizing effect of the nonbilayer lipid phosphatidylethanolamine (PE) was observed, but no effect of the charged phoshosphatidylglycerol (PG) as compared to phosphatidylcholine (PC) was found. To understand how lipids stabilize KcsA against TFE-induced tetramer dissociation, we also studied the effect of TFE on the bilayer organization in the various lipid systems, using (31)P and (2)H NMR. The observed lipid dependency was similar as was found for tetramer stabilization: PE increased the bilayer stability as compared to PC, while PG behaved similar to PC. Furthermore, it was found that TFE has a large effect on the acyl chain ordering. The results indicate that TFE inserts primarily in the membrane interface. We suggest that the lipid bilayer stabilizes the KcsA tetramer by the lateral pressure in the acyl chain region and that this stabilizing effect increases when a nonbilayer lipid like PE is present.

Potassium channel, ions, and water: simulation studies based on the high resolution X-ray structure of KcsA

Interactions of Na(+), K(+), Rb(+), and Cs(+) ions within the selectivity filter of a potassium channel have been investigated via multiple molecular dynamics simulations (total simulation time, 48 ns) based on the high resolution structure of KcsA, embedded in a phospholipid bilayer. As in simulations based on a lower resolution structure of KcsA, concerted motions of ions and water within the filter are seen. Despite the use of a higher resolution structure and the inclusion of four buried water molecules thought to stabilize the filter, this region exhibits a significant degree of flexibility. In particular, pronounced distortion of filter occurs if no ions are present within it. The two most readily permeant ions, K(+) and Rb(+), are similar in their interactions with the selectivity filter. In contrast, Na(+) ions tend to distort the filter by binding to a ring of four carbonyl oxygens. The larger Cs(+) ions result in a small degree of expansion of the filter relative to the x-ray structure. Cs(+) ions also appear to interact differently with the gate region of the channel, showing some tendency to bind within a predominantly hydrophobic pocket. The four water molecules buried between the back of the selectivity filter and the remainder of the protein show comparable mobility to the surrounding protein and do not exchange with water molecules within the filter or the central cavity. A preliminary comparison of the use of particle mesh Ewald versus cutoff protocols for the treatment of long-range electrostatics suggests some difference in the kinetics of ion translocation within the filter.

MjK1, a K+ channel from M. jannaschii, mediates K+ uptake and K+ sensitivity in E. coli

The methanogenic and hyperthermophilic deep-sea archaeon Methanococcus jannaschii has three putative K+ channels, MVP (Mj0139), MjK1 (Mj0138.1) and MjK2 (Mj1357). The physiological function of these K+ channels was examined in a viability assay, using the Escherichia coli mutant LB2003 (kup1, DeltakdpABC5, DeltatrkA). While MjK2 expression had no effects on the potassium-dependent phenotype of LB2003, MVP and MjK1 complemented the deficiency at a concentration of 1 mM KCl. In contrast to KcsA, MthK and MVP, MjK1 strongly affected host cell viability at 10 and 100 mM KCl. The toxic effects were less pronounced when growth media were supplemented with the K+ channel blocker BaCl2.

How do helix-helix interactions help determine the folds of membrane proteins? Perspectives from the study of homo-oligomeric helical bundles

The final, structure-determining step in the folding of membrane proteins involves the coalescence of preformed transmembrane helices to form the native tertiary structure. Here, we review recent studies on small peptide and protein systems that are providing quantitative data on the interactions that drive this process. Gel electrophoresis, analytical ultracentrifugation, and fluorescence resonance energy transfer (FRET) are useful methods for examining the assembly of homo-oligomeric transmembrane helical proteins. These methods have been used to study the assembly of the M2 proton channel from influenza A virus, glycophorin, phospholamban, and several designed membrane proteins-all of which have a single transmembrane helix that is sufficient for association into a transmembrane helical bundle. These systems are being studied to determine the relative thermodynamic contributions of van der Waals interactions, conformational entropy, and polar interactions in the stabilization of membrane proteins. Although the database of thermodynamic information is not yet large, a few generalities are beginning to emerge concerning the energetic differences between membrane and water-soluble proteins: the packing of apolar side chains in the interior of helical membrane proteins plays a smaller, but nevertheless significant, role in stabilizing their structure. Polar, hydrogen-bonded interactions occur less frequently, but, nevertheless, they often provide a strong driving force for folding helix-helix pairs in membrane proteins. These studies are laying the groundwork for the design of sequence motifs that dictate the association of membrane helices.

Opening the KcsA K+ channel: tryptophan scanning and complementation analysis lead to mutants with altered gating

The properties of the KcsA channel were investigated using a combination of tryptophan scanning of the two transmembrane helices followed by random mutagenesis at targeted residues. The tryptophan mutants were subjected to two screens: oligomeric stability and ability to complement the K+ uptake deficiency of the TK2420 Escherichia coli strain. Oligomeric stability is affected primarily by mutations at sites that border on and interact with the selectivity filter, while the complementation assays identified residues at the crossing point of the inner helices. Sites identified by the complementation assay in the tryptophan screen were subjected to random mutagenesis and selection by complementation. We have found two mutants, A108S and A108T, which have dramatically increased open probability while retaining the basic property of oligomeric stability.

Influence of lipids on membrane assembly and stability of the potassium channel KcsA

Recently we observed in an in vitro system that newly synthesized KcsA assembles efficiently into a tetramer in lipid vesicles [van Dalen et al. (2002) FEBS Lett. 511, 51-58]. Here we used this system to get insight into the importance of the lipid composition for KcsA membrane association and tetramerization and we compared this to the lipid dependency of the thermo-stability of the KcsA tetramer. It was found that a large amount of phosphatidylethanolamine (>40 mol%) and a lower amount of phosphatidylglycerol (approximately 20-30 mol%) were optimal for efficient KcsA membrane association and tetramerization. Strikingly, vesicles of the abundant and commonly used membrane lipid phosphatidylcholine did not support assembly, further demonstrating the importance of membrane lipid composition for KcsA assembly. The in vitro assembled KcsA tetramer showed similar thermo-stability in biological and pure lipid membranes, demonstrating that both tetramers are alike. In addition, we show that solubilization of the membrane with detergent reduces the thermo-stability of the tetramer. The highest KcsA tetramer stability was observed in intact bilayers in the presence of anionic lipids.

Site-directed glycosylation tagging of functional Kir2.1 reveals that the putative pore-forming segment is extracellular

Inwardly rectifying K+ channels or Kirs are a large gene family and have been predicted to have two transmembrane segments, M1 and M2, intracellular N and C termini, and two extracellular loops, E1 and E2, separated by an intramembranous pore-forming segment, H5. H5 contains a stretch of eight residues that are similar in voltage-dependent K+ channels, Kvs, and this stretch is called the signature sequence of K+ channels. Because mutations in this sequence altered selectivity in Kvs, it has been designated as the selectivity filter. Previously, we used N-glycosylation substitution mutants to map the extracellular topology of a weak inwardly rectifying K+ channel, Kir1.1 or ROMK1, and found that the entire H5 segment was extracellular. We now report utilization of introduced N-glycosylation sites, NX(S/T), at positions Ser(128) in E1, and Gln(140), Ileu(143), and Phe(147) in the H5 sequence of a strong inwardly rectifying K+ channel, Kir2.1. Furthermore, we show that biotinylated channel proteins with N-linked oligosaccharides attached at positions 140 and 143 in the signature sequence are located at the cell surface. Mutant channels were functional as detected by whole-cell and single-channel recordings. Unlike Kir1.1, position Lys(117) was not occupied. We conclude that, for yet another K+ channel, the invariant G(Y/F)G sequence is extracellular rather than intramembranous.

Hierarchical approach to predicting permeation in ion channels

A hierarchical computational strategy combining molecular modeling, electrostatics calculations, molecular dynamics, and Brownian dynamics simulations is developed and implemented to compute electrophysiologically measurable properties of the KcsA potassium channel. Models for a series of channels with different pore sizes are developed from the known x-ray structure, using insights into the gating conformational changes as suggested by a variety of published experiments. Information on the pH dependence of the channel gating is incorporated into the calculation of potential profiles for K(+) ions inside the channel, which are then combined with K(+) ion mobilities inside the channel, as computed by molecular dynamics simulations, to provide inputs into Brownian dynamics simulations for computing ion fluxes. The open model structure has a conductance of approximately 110 pS under symmetric 250 mM K(+) conditions, in reasonable agreement with experiments for the largest conducting substate. The dimensions of this channel are consistent with electrophysiologically determined size dependence of quaternary ammonium ion blocking from the intracellular end of this channel as well as with direct structural evidence that tetrabutylammonium ions can enter into the interior cavity of the channel. Realistic values of Ussing flux ratio exponents, distribution of ions within the channel, and shapes of the current-voltage and current-concentration curves are obtained. The Brownian dynamics calculations suggest passage of ions through the selectivity filter proceeds by a "knock-off" mechanism involving three ions, as has been previously inferred from functional and structural studies of barium ion blocking. These results suggest that the present calculations capture the essential nature of K(+) ion permeation in the KcsA channel and provide a proof-of-concept for the integrated microscopic/mesoscopic multitiered approach for predicting ion channel function from structure, which can be applied to other channel structures.

KcsA: it's a potassium channel

Ion conduction and selectivity properties of KcsA, a bacterial ion channel of known structure, were studied in a planar lipid bilayer system at the single-channel level. Selectivity sequences for permeant ions were determined by symmetrical solution conductance (K(+) > Rb(+), NH(4)(+), Tl(+) >> Cs(+), Na(+), Li(+)) and by reversal potentials under bi-ionic or mixed-ion conditions (Tl(+) > K(+) > Rb(+) > NH(4)(+) >> Na(+), Li(+)). Determination of reversal potentials with submillivolt accuracy shows that K(+) is over 150-fold more permeant than Na(+). Variation of conductance with concentration under symmetrical salt conditions is complex, with at least two ion-binding processes revealing themselves: a high affinity process below 20 mM and a low affinity process over the range 100-1,000 mM. These properties are analogous to those seen in many eukaryotic K(+) channels, and they establish KcsA as a faithful structural model for ion permeation in eukaryotic K(+) channels.

Interchain hydrogen-bonding interactions may facilitate translocation of K+ ions across the potassium channel selectivity filter, as suggested by synthetic modeling chemistry

A 4-fold symmetric arrangement of TVGYG polypeptides forms the selectivity filter of the K+ channel from Streptomyces lividans (KcsA). We report the synthesis and properties of synthetic models for the filter, p-tert-butyl-calix[4]arene-(OCH(2)CO-XOBz)(4) (X = V, VG, VGY), 1-3. The first cation (Na+, K+) binds to the four -[OCH(2)CO]- units, a region devised to mimic the metal-binding site formed by the four T residues in KcsA. NMR studies reveal that cations and valine amide protons compete for the carbonyl oxygen atoms, converting NH(Val)...O=C hydrogen bonds to M+ ...O=C bonds (M+ = Na+ or K+). The strength of these interchain NH(Val)...O=C hydrogen bonds varies in the order 3 > 2 > 1. We propose that such interchain H-bonding may destabilize metal binding in the selectivity filter and thus help create the low energy barrier needed for rapid cation translocation.

Is the molecular composition of K(ATP) channels more complex than originally thought?

ATP-sensitive K+ (K(ATP)) channels are abundantly expressed in the heart and may be involved in the pathogenesis of myocardial ischemia. These channels are heteromultimeric, consisting of four pore-forming subunits (Kir6.1, Kir6.2) and four sulfonylurea receptor (SUR) subunits in an octameric assembly. Conventionally, the molecular composition of K(ATP) channels in cardiomyocytes and pancreatic beta -cells is thought to include the Kir6.2 subunit and either the SUR2A or SUR1 subunits, respectively. However, Kir6.1 mRNA is abundantly expressed in the heart, suggesting that Kir6.1 and Kir6.2 subunits may co-assemble to form functional heteromeric channel complexes. Here we provide two independent lines of evidence that heteromultimerization between Kir6.1 and Kir6.2 subunits is possible in the presence of SUR2A. We generated dominant negative Kir6 subunits by mutating the GFG residues in the channel pore to a series of alanine residues. The Kir6.1-AAA pore mutant subunit suppressed both wt-Kir6.1/SUR2A and wt-Kir6.2/SUR2A currents in transfected HEK293 cells. Similarly, the dominant negative action of Kir6.2-AAA does not discriminate between either of the wild-type subunits, suggesting an interaction between Kir6.1 and Kir6.2 subunits within the same channel complex. Biochemical data support this concept: immunoprecipitation with Kir6.1 antibodies also co-precipitates Kir6.2 subunits and conversely, immunoprecipitation with Kir6.2 antibodies co-precipitates Kir6.1 subunits. Collectively, our data provide direct electrophysiological and biochemical evidence for heteromultimeric assembly between Kir6.1 and Kir6.2. This paradigm has profound implications for understanding the properties of native K(ATP)channels in the heart and other tissues.

The K+ channel signature sequence of murine Kir2.1: mutations that affect microscopic gating but not ionic selectivity

1. We have studied the effects on ionic selectivity and gating of Kir2.1 of replacing Tyr (Y) in the GYG signature sequence with Phe (Y145F), Leu (Y145L), Met (Y145M), Ala (Y145A) or Val (Y145V). 2. The mutant Y145F showed no changes in ionic selectivity (as indicated by the permeability coefficient ratios PNa/PK or PRb/PK), indicating that a hydrogen bond between Tyr and other residues is not essential for K+ selectivity. Y145L, Y145M, Y145A and Y145V did not express as monomers. 3. None of the channels made from covalently linked tandem dimers with wild-type and mutant subunits (WT-mutant) had altered ionic selectivity (PNa/PK or PRb/PK), indicating that 4-fold symmetry is not required. 4. Macroscopic currents activated under hyperpolarization and the time constants for activation were reduced e-fold per 23 mV hyperpolarization in wild-type. This gating, believed to be due to the release of polyamines from the pore, was little affected by mutation of Y14. There was similarly little effect on the relationship between chord conductance (gK) and membrane potential. 5. Unitary conductance (140 mM [K+]o) was also little affected by mutation and was reduced only in channels formed from WT-Y145M, from 22.7 +/- 0.4 pS (n = 5) in wild-type to 17.1 +/- 0.5 pS (n = 4) in WT-Y145M. 6. Steady-state recording of unitary currents showed that channel open times were affected by the residue that replaced Tyr in GYG. Channel openings were particularly brief in WT-Y145V, where the mean open time was reduced from 102 ms at -120 mV in wild-type to 6 ms in WT-Y145V. 7. Thus in Kir2.1, GFG can act as a K+ selectivity filter, as can G(L/M/A/V)G, at least in dimers also containing GYG. Channel open time duration depended on the residue at position 145, consistent with the H5 region helping to determine the dwell time of the channel in the open state.

Efficient membrane assembly of the KcsA potassium channel in Escherichia coli requires the protonmotive force

Very little is known about the biogenesis and assembly of oligomeric membrane proteins. In this study, the biogenesis of KcsA, a prokaryotic homotetrameric potassium channel, is investigated. Using in vivo pulse-chase experiments, both the monomeric and tetrameric form could be identified. The conversion of monomers into a tetramer is found to be a highly efficient process that occurs in the Escherichia coli inner membrane. KcsA does not require ATP hydrolysis by SecA for insertion or tetramerization. The presence of the proton-motive force (pmf) is not necessary for transmembrane insertion of KcsA; however, the pmf proved to be essential for the efficiency of oligomerization. From in vivo and in vitro experiments it is concluded that the electrical component, deltapsi, is the main determinant for this effect. These results demonstrate a new role of the pmf in membrane protein biogenesis.