The cation selectivity filter of the bacterial sodium channel, NaChBac

Calcium signaling through CatSper channels in mammalian fertilization

The molecular mechanisms underlying Ca(2+) entry into sperm are now much more well defined thanks to direct recordings of mature sperm cells. This article reviews the function of a sperm-specific ion channel, CatSper. CatSpers have a clearly defined function in sperm's hyperactivated motility and are essential for male fertility. We propose that physiological stimuli such as zona pellucida and cyclic nucleotides induce Ca(2+) entry through CatSper channels instead of acting on Ca(V) and CNG channels as previously thought.

Calcium channels in photosynthetic eukaryotes: implications for evolution of calcium-based signalling

Much of our current knowledge on the mechanisms by which Ca(2+) signals are generated in photosynthetic eukaryotes comes from studies of a relatively small number of model species, particularly green plants and algae, revealing some common features and notable differences between 'plant' and 'animal' systems. Physiological studies from a broad range of algal cell types have revealed the occurrence of animal-like signalling properties, including fast action potentials and fast propagating cytosolic Ca(2+) waves. Genomic studies are beginning to reveal the widespread occurrence of conserved channel types likely to be involved in Ca(2+) signalling. However, certain widespread 'ancient' channel types appear to have been lost by certain groups, such as the embryophytes. More recent channel gene loss is also evident from comparisons of more closely related algal species. The underlying processes that have given rise to the current distributions of Ca(2+) channel types include widespread retention of ancient Ca(2+) channel genes, horizontal gene transfer (including symbiotic gene transfer and acquisition of bacterial genes), gene loss and gene expansion within taxa. The assessment of the roles of Ca(2+) channel genes in diverse physiological, developmental and life history processes represents a major challenge for future studies.

Structural and regulatory evolution of cellular electrophysiological systems

Cellular electrophysiological systems, like developmental systems, appear to evolve primarily by means of regulatory evolution. It is suggested that electrophysiological systems share two key features with developmental systems that account for this dependence on regulatory evolution. For both systems, structural evolution has the potential to create significant problems of pleiotropy and both systems are predominantly computational in nature. It is concluded that the relative balance of physical and computational tasks that a biological system has to perform, combined with the probability that these tasks may have to change significantly during the course of evolution, will be major factors in determining the relative mix of regulatory and structural evolution that is observed for a given system. Physiological systems that directly interface with the environment will almost always perform some low-level physical task. In the majority of cases this will require evolution of protein function in order for the tasks themselves to evolve. For complex physiological systems a large fraction of their function will be devoted to high-level control functions that are predominantly computational in nature. In most cases regulatory evolution will be sufficient in order for these computational tasks to evolve.

Modulation of TRPM2 by acidic pH and the underlying mechanisms for pH sensitivity

TRPM2 is a Ca²⁺-permeable nonselective cation channel that plays important roles in oxidative stress–mediated cell death and inflammation processes. However, how TRPM2 is regulated under physiological and pathological conditions is not fully understood. Here, we report that both intracellular and extracellular protons block TRPM2 by inhibiting channel gating. We demonstrate that external protons block TRPM2 with an IC₅₀ of pH_o = 5.3, whereas internal protons inhibit TRPM2 with an IC₅₀ of pH_i = 6.7. Extracellular protons inhibit TRPM2 by decreasing single-channel conductance. We identify three titratable residues, H958, D964, and E994, at the outer vestibule of the channel pore that are responsible for pH_o sensitivity. Mutations of these residues reduce single-channel conductance, decrease external Ca²⁺ ([Ca²⁺]_o) affinity, and inhibit [Ca²⁺]_o-mediated TRPM2 gating. These results support the following model: titration of H958, D964, and E994 by external protons inhibits TRPM2 gating by causing conformation change of the channel, and/or by decreasing local Ca²⁺ concentration at the outer vestibule, therefore reducing [Ca²⁺]_o permeation and inhibiting [Ca²⁺]_o-mediated TRPM2 gating. We find that intracellular protons inhibit TRPM2 by inducing channel closure without changing channel conductance. We identify that D933 located at the C terminus of the S4-S5 linker is responsible for intracellular pH sensitivity. Replacement of Asp⁹³³ by Asn⁹³³ changes the IC₅₀ from pH_i = 6.7 to pH_i = 5.5. Moreover, substitution of Asp⁹³³ with various residues produces marked changes in proton sensitivity, intracellular ADP ribose/Ca²⁺ sensitivity, and gating profiles of TRPM2. These results indicate that D933 is not only essential for intracellular pH sensitivity, but it is also crucial for TRPM2 channel gating. Collectively, our findings provide a novel mechanism for TRPM2 modulation as well as molecular determinants for pH regulation of TRPM2. Inhibition of TRPM2 by acidic pH may represent an endogenous mechanism governing TRPM2 gating and its physiological/pathological functions.

A novel TRPC6 mutation that causes childhood FSGS

BACKGROUND

TRPC6, encoding a member of the transient receptor potential (TRP) superfamily of ion channels, is a calcium-permeable cation channel, which mediates capacitive calcium entry into the cell. Until today, seven different mutations in TRPC6 have been identified as a cause of autosomal-dominant focal segmental glomerulosclerosis (FSGS) in adults.

METHODOLOGY/PRINCIPAL FINDINGS

Here we report a novel TRPC6 mutation that leads to early onset FSGS. We identified one family in whom disease segregated with a novel TRPC6 mutation (M132T), that also affected pediatric individuals as early as nine years of age. Twenty-one pedigrees compatible with an autosomal-dominant mode of inheritance and biopsy-proven FSGS were selected from a worldwide cohort of 550 families with steroid resistant nephrotic syndrome (SRNS). Whole cell current recordings of the mutant TRPC6 channel, compared to the wild-type channel, showed a 3 to 5-fold increase in the average out- and inward TRPC6 current amplitude. The mean inward calcium current of M132T was 10-fold larger than that of wild-type TRPC6. Interestingly, M132T mutants also lacked time-dependent inactivation. Generation of a novel double mutant M132T/N143S did not further augment TRPC6 channel activity.

CONCLUSIONS

In summary, our data shows that TRPC6 mediated FSGS can also be found in children. The large increase in channel currents and impaired channel inactivation caused by the M132T mutant leads to an aggressive phenotype that underlines the importance of calcium dose channeled through TRPC6.

Antibacterial activity of six novel peptides from Tityus discrepans scorpion venom. A fluorescent probe study of microbial membrane Na+ permeability changes

Six novel peptides (named bactridines) were isolated from Tityus discrepans scorpion venom. From mass spectrometry molecular masses were 6916, 7362, 7226, 7011, 7101 and 7173 Da (bactridines 1-6). Bactridines 1 and 2 were sequenced by Edman degradation. The sequences and in silico analysis, indicated that they are positively charged polypeptides comprised of 61 and 64 amino acids (AA), respectively, bactridine 1 and bactridine 2 containing 4 disulfide bridges. Bactridine 1 was only toxic to cockroaches and crabs, and bactridine 2-6 were only toxic to mice. Bactridine 1 has a 78% sequence identity with ardiscretin. Ardisctretin is an insect specific sodium toxin which also produces a small depolarization and induces repetitive firing in squid axons resembling those of DDT [1,10(pchlorobenzyl) 2-trichloretane] in its ability to slow down action potential, to induce repetitive firing. Measured as the minimal inhibitory concentration, bactridines had high antibacterial activity against a wide range of gram positive and gram negative bacteria. Complete bacterial growth inhibition occurred at concentrations from 20 to 80 microM depending on the bacteria and peptide tested. Effects on membrane Na(+) permeability induced by bactridines were observed on Yersinia enterocolitica loaded with 1 microM CoroNa Red. CoroNa Red fluorescence leakage from bacteria was observed after exposure to 0.3 microM of any bactridine tested, indicating that they modified Na(+) membrane permeability. This effect was blocked by 10 microM amiloride and by 25 microM mibefradil drugs that affect Na(+) and Ca(2+) channels respectively. We found no evidence of changes of K(+) or Ca(2+) concentrations neither inside nor outside the bacteria in experiments using the fluorescent dyes Fluo 4AM (10 microM) and PBFI (20 microM).

Ion channels in microbes

Studies of ion channels have for long been dominated by the animalcentric, if not anthropocentric, view of physiology. The structures and activities of ion channels had, however, evolved long before the appearance of complex multicellular organisms on earth. The diversity of ion channels existing in cellular membranes of prokaryotes is a good example. Although at first it may appear as a paradox that most of what we know about the structure of eukaryotic ion channels is based on the structure of bacterial channels, this should not be surprising given the evolutionary relatedness of all living organisms and suitability of microbial cells for structural studies of biological macromolecules in a laboratory environment. Genome sequences of the human as well as various microbial, plant, and animal organisms unambiguously established the evolutionary links, whereas crystallographic studies of the structures of major types of ion channels published over the last decade clearly demonstrated the advantage of using microbes as experimental organisms. The purpose of this review is not only to provide an account of acquired knowledge on microbial ion channels but also to show that the study of microbes and their ion channels may also hold a key to solving unresolved molecular mysteries in the future.

Models of the structure and gating mechanisms of the pore domain of the NaChBac ion channel

The NaChBac prokaryotic sodium channel appears to be a descendent of an evolutionary link between voltage-gated K(V) and Ca(V) channels. Like K(V) channels, four identical six-transmembrane subunits comprise the NaChBac channel, but its selectivity filter possesses a signature sequence of eukaryotic Ca(V) channels. We developed structural models of the NaChBac channel in closed and open conformations, using K(+)-channel crystal structures as initial templates. Our models were also consistent with numerous experimental results and modeling criteria. This study concerns the pore domain. The major differences between our models and K(+) crystal structures involve the latter portion of the selectivity filter and the bend region in S6 of the open conformation. These NaChBac models may serve as a stepping stone between K(+) channels of known structure and Na(V), Ca(V), and TRP channels of unknown structure.

Tetrameric bacterial sodium channels: characterization of structure, stability, and drug binding

NaChBac from Bacillus halodurans is a bacterial homologue of mammalian voltage-gated sodium channels. It has been proposed that a NaChBac monomer corresponds to a single domain of the mammalian sodium channel and that, like potassium channels, four monomers form a tetrameric channel. However, to date, although NaChBac has been well-characterized for functional properties by electrophysiological measurements on protein expressed in tissue culture, little information about its structural properties exists because of the difficulties in expressing the protein in large quantities. In this study, we present studies on the overexpression of NaChBac in Escherichia coli, purification of the functional detergent-solubilized channel, its identification as a tetramer, and characterization of its secondary structure, drug binding, and thermal stability. These studies are correlated with a model produced for the protein and provide new insights into the structure-function relationships of this sodium channel.

Steric selectivity in Na channels arising from protein polarization and mobile side chains

Monte Carlo simulations of equilibrium selectivity of Na channels with a DEKA locus are performed over a range of radius R and protein dielectric coefficient epsilon(p). Selectivity arises from the balance of electrostatic forces and steric repulsion by excluded volume of ions and side chains of the channel protein in the highly concentrated and charged (approximately 30 M) selectivity filter resembling an ionic liquid. Ions and structural side chains are described as mobile charged hard spheres that assume positions of minimal free energy. Water is a dielectric continuum. Size selectivity (ratio of Na+ occupancy to K+ occupancy) and charge selectivity (Na+ to Ca2+) are computed in concentrations as low as 10(-5) M Ca2+. In general, small R reduces ion occupancy and favors Na+ over K+ because of steric repulsion. Small epsilon(p) increases occupancy and favors Na+ over Ca2+ because protein polarization amplifies the pore's net charge. Size selectivity depends on R and is independent of epsilon(p); charge selectivity depends on both R and epsilon(p). Thus, small R and epsilon(p) make an efficient Na channel that excludes K+ and Ca2+ while maximizing Na+ occupancy. Selectivity properties depend on interactions that cannot be described by qualitative or verbal models or by quantitative models with a fixed free energy landscape.

Molecular determinants of Mg2+ and Ca2+ permeability and pH sensitivity in TRPM6 and TRPM7

The channel kinases TRPM6 and TRPM7 have recently been discovered to play important roles in Mg2+ and Ca2+ homeostasis, which is critical to both human health and cell viability. However, the molecular basis underlying these channels' unique Mg2+ and Ca2+ permeability and pH sensitivity remains unknown. Here we have created a series of amino acid substitutions in the putative pore of TRPM7 to evaluate the origin of the permeability of the channel and its regulation by pH. Two mutants of TRPM7, E1047Q and E1052Q, produced dramatic changes in channel properties. The I-V relations of E1052Q and E1047Q were significantly different from WT TRPM7, with the inward currents of 8- and 12-fold larger than TRPM7, respectively. The binding affinity of Ca2+ and Mg2+ was decreased by 50- to 140-fold in E1052Q and E1047Q, respectively. Ca2+ and Mg2+ currents in E1052Q were 70% smaller than those of TRPM7. Strikingly, E1047Q largely abolished Ca2+ and Mg2+ permeation, rendering TRPM7 a monovalent selective channel. In addition, the ability of protons to potentiate inward currents was lost in E1047Q, indicating that E1047 is critical to Ca2+ and Mg2+ permeability of TRPM7, and its pH sensitivity. Mutation of the corresponding residues in the pore of TRPM6, E1024Q and E1029Q, produced nearly identical changes to the channel properties of TRPM6. Our results indicate that these two glutamates are key determinants of both channels' divalent selectivity and pH sensitivity. These findings reveal the molecular mechanisms underpinning physiological/pathological functions of TRPM6 and TRPM7, and will extend our understanding of the pore structures of TRPM channels.

Acidic residues on the voltage-sensor domain determine the activation of the NaChBac sodium channel

The voltage-sensing domain of voltage-gated ion channels is characterized by specific, conserved, charged residues. Positively charged residues on segment S4 are the main contributors to voltage-sensing and negatively charged residues on the S2 and S3 segments are believed to participate to the process. However, their function in the voltage sensor is not well understood. To probe the role of three acidic residues in NaChBac (D-58 and E-68 in S2, and D-91 in S3), we employed site-directed mutagenesis to substitute native acidic residues with cysteine (neutral), lysine (positive charge), or either aspartate or glutamate (negative charge). We used a combination of the patch-clamp technique to record Na+ currents and molecular modeling to visualize interacting amino acid residues. We suggest that the acidic residues on the S2 and S3 segments form specific interactions with adjacent amino acids in the voltage-sensor domain. The main interactions in NaChBac are D-58 (S2) with A-97-G-98 (S3) and R-120 (S4), E-68 (S2) with R-129 (L4-5), and D-91 (S3) with R-72 (S2). Changing these acidic residues modified the interactions, which in turn altered the sensitivity of the voltage sensor.

Functional characterization of homo- and heteromeric channel kinases TRPM6 and TRPM7

TRPM6 and TRPM7 are two known channel kinases that play important roles in various physiological processes, including Mg²⁺ homeostasis. Mutations in TRPM6 cause hereditary hypomagnesemia and secondary hypocalcemia (HSH). However, whether TRPM6 encodes functional channels is controversial. Here we demonstrate several signature features of TRPM6 that distinguish TRPM6 from TRPM7 and TRPM6/7 channels. We show that heterologous expression of TRPM6 but not the mutant TRPM6^S141L produces functional channels with divalent cation permeability profile and pH sensitivity distinctive from those of TRPM7 channels and TRPM6/7 complexes. TRPM6 exhibits unique unitary conductance that is 2- and 1.5-fold bigger than that of TRPM7 and TRPM6/7. Moreover, micromolar levels of 2-aminoethoxydiphenyl borate (2-APB) maximally increase TRPM6 but significantly inhibit TRPM7 channel activities; whereas millimolar concentrations of 2-APB potentiate TRPM6/7 and TRPM7 channel activities. Furthermore, Mg²⁺ and Ca²⁺ entry through TRPM6 is enhanced three- to fourfold by 2-APB. Collectively, these results indicate that TRPM6 forms functional homomeric channels as well as heteromeric TRPM6/7 complexes. The unique characteristics of these three channel types, TRPM6, TRPM7, and TRPM6/7, suggest that they may play different roles in vivo.

Voltage-gated sodium and calcium channels in nerve, muscle, and heart

Voltage-gated ion channels are membrane proteins which underlie rapid electrical signals among neurons and the spread of excitation in skeletal muscle and heart. We outline some recent advances in the study of voltage-sensitive sodium and calcium channels. Investigations are providing insight into the changes in molecular conformation associated with open-closed gating of the channels, the mechanisms by which they allow only specific ion species to pass through and carry an electric current, and the pathological consequences of small perturbations in channel structure which result from genetic mutations. Determination of three-dimensional structures, coupled with molecular manipulations by site-directed mutagenesis, and parallel electrophysiological analyses of currents through the ion channels, are providing an understanding of the roles and function of these channels at an unprecedented level of molecular detail. Crucial to these advances are studies of bacterial homologues of ion channels from man and other eukaryotes, and the use of naturally occurring peptide toxins which target different ion channel types with exquisite specificity.

Potentiation of TRPM7 inward currents by protons

TRPM7 is unique in being both an ion channel and a protein kinase. It conducts a large outward current at +100 mV but a small inward current at voltages ranging from −100 to −40 mV under physiological ionic conditions. Here we show that the small inward current of TRPM7 was dramatically enhanced by a decrease in extracellular pH, with an ∼10-fold increase at pH 4.0 and 1–2-fold increase at pH 6.0. Several lines of evidence suggest that protons enhance TRPM7 inward currents by competing with Ca²⁺ and Mg²⁺ for binding sites, thereby releasing blockade of divalent cations on inward monovalent currents. First, extracellular protons significantly increased monovalent cation permeability. Second, higher proton concentrations were required to induce 50% of maximal increase in TRPM7 currents when the external Ca²⁺ and Mg²⁺ concentrations were increased. Third, the apparent affinity for Ca²⁺ and Mg²⁺ was significantly diminished at elevated external H⁺ concentrations. Fourth, the anomalous-mole fraction behavior of H⁺ permeation further suggests that protons compete with divalent cations for binding sites in the TRPM7 pore. Taken together, it appears that at physiological pH (7.4), Ca²⁺ and Mg²⁺ bind to TRPM7 and inhibit the monovalent cationic currents; whereas at high H⁺ concentrations, the affinity of TRPM7 for Ca²⁺ and Mg²⁺ is decreased, thereby allowing monovalent cations to pass through TRPM7. Furthermore, we showed that the endogenous TRPM7-like current, which is known as Mg²⁺-inhibitable cation current (MIC) or Mg nucleotide–regulated metal ion current (MagNuM) in rat basophilic leukemia (RBL) cells was also significantly potentiated by acidic pH, suggesting that MIC/MagNuM is encoded by TRPM7. The pH sensitivity represents a novel feature of TRPM7 and implies that TRPM7 may play a role under acidic pathological conditions.

The pore, not cytoplasmic domains, underlies inactivation in a prokaryotic sodium channel

Kinetics and voltage dependence of inactivation of a prokaryotic voltage-gated sodium channel (NaChBac) were investigated in an effort to understand its molecular mechanism. NaChBac inactivation kinetics show strong, bell-shaped voltage dependence with characteristic time constants ranging from approximately 50 ms at depolarized voltages to a maximum of approximately 100 s at the inactivation midpoint. Activation and inactivation parameters for four different covalently linked tandem dimer or tandem tetramer constructs were indistinguishable from those of the wild-type channel. Point mutations in the outer part of the pore revealed an important influence of the S195 residue on the process of inactivation. For two mutants (S195D and S195E), the maximal and minimal rates of inactivation observed were increased by approximately 2.5-fold, and the midpoint of the steady-state inactivation curve was shifted approximately 20 mV in the hyperpolarizing direction, compared to the wild-type channel. Our data suggest that pore vestibule structure is an important determinant of NaChBac inactivation, whereas the inactivation mechanism is independent of the number of free cytoplasmic N- and C-termini in the functional channel. In these respects, NaChBac inactivation resembles C-type or slow inactivation modes observed in other voltage-gated K and Na channels.

Channels in microbes: so many holes to fill

Among players in neurobiology, ion channels are the demigods that underlie all our senses, behaviour and intelligence. In animals, these 'gated pores' detect ligands, voltage, heat or stretch forces and emit electric or ionic signals. Patch clamp and genome sequencing now show that nearly all microbes also have these 'smart' molecules. Microbial channel proteins have yielded crystal structures so dear to neuroscientists. However, their natural roles in microbial physiology remain largely unknown. The intellectual and technical schisms between 'neuro' and 'micro' biology must be bridged before we know how we became so smart, and whether microbes are just as smart.

Gating of the bacterial sodium channel, NaChBac: voltage-dependent charge movement and gating currents

The bacterial sodium channel, NaChBac, from Bacillus halodurans provides an excellent model to study structure–function relationships of voltage-gated ion channels. It can be expressed in mammalian cells for functional studies as well as in bacterial cultures as starting material for protein purification for fine biochemical and biophysical studies. Macroscopic functional properties of NaChBac have been described previously (Ren, D., B. Navarro, H. Xu, L. Yue, Q. Shi, and D.E. Clapham. 2001. Science. 294:2372–2375). In this study, we report gating current properties of NaChBac expressed in COS-1 cells. Upon depolarization of the membrane, gating currents appeared as upward inflections preceding the ionic currents. Gating currents were detectable at −90 mV while holding at −150 mV. Charge–voltage (Q–V) curves showed sigmoidal dependence on voltage with gating charge saturating at −10 mV. Charge movement was shifted by −22 mV relative to the conductance–voltage curve, indicating the presence of more than one closed state. Consistent with this was the Cole-Moore shift of 533 μs observed for a change in preconditioning voltage from −160 to −80 mV. The total gating charge was estimated to be 16 elementary charges per channel. Charge immobilization caused by prolonged depolarization was also observed; Q–V curves were shifted by approximately −60 mV to hyperpolarized potentials when cells were held at 0 mV. The kinetic properties of NaChBac were simulated by simultaneous fit of sodium currents at various voltages to a sequential kinetic model. Gating current kinetics predicted from ionic current experiments resembled the experimental data, indicating that gating currents are coupled to activation of NaChBac and confirming the assertion that this channel undergoes several transitions between closed states before channel opening. The results indicate that NaChBac has several closed states with voltage-dependent transitions between them realized by translocation of gating charge that causes activation of the channel.

Permeation properties of an engineered bacterial OmpF porin containing the EEEE-locus of Ca2+ channels

The selectivity filter of the bacterial porin OmpF carries a small net charge close to -1 e and is therefore only slightly cation-selective. Calcium channels, on the other hand, contain four negatively charged glutamates, the EEEE-locus, and are among the most selective cation channels known. We aimed to turn the essentially nonselective OmpF into a Ca2+-selective channel. To that end, two additional glutamates (R42E and R132E) were introduced in the OmpF constriction zone that already contains D113 and E117. Mutant OmpF containing this DEEE-locus has a high Ca2+ over Cl- selectivity and a Na+ current with a strongly increased sensitivity to 1 mM Ca2+. The charge/space competition model, initially applied to the L-type Ca2+ channel, identifies the fixed charge and filter volume as key determinants of ion selectivity, with the precise atomic arrangement having only second-order effects. By implication, the reproduction of fixed charge and filter volume should transform two channels into channels of similar selectivity, even if the two belong to entirely different ion channel families, as is the case for OmpF and the L-type Ca2+ channel. The results presented here fit quite well in the framework of charge/space competition theory.

The voltage-gated Na+ channel NaVBP has a role in motility, chemotaxis, and pH homeostasis of an alkaliphilic Bacillus

The prokaryotic voltage-gated Na(+) channel, NaChBac, is one of a growing channel superfamily of unknown function. Here we show that Na(V)BP, the NaChBac homologue encoded by ncbA in alkaliphilic Bacillus pseudofirmus OF4, is a voltage-gated Na(+) channel potentiated by alkaline pH. Na(V)BP has roles in motility, chemotaxis, and pH homeostasis at high pH. Reduced motility of bacteria lacking functional Na(V)BP was reversed by restoration of the native channel but not by a mutant Na(V)BP engineered to be Ca(2+)-selective. Motile ncbA mutant cells and wild-type cells treated with a channel inhibitor exhibited behavior opposite to the wild type in response to chemoeffectors. Mutants lacking functional Na(V)BP were also defective in pH homeostasis in response to a sudden alkaline shift in external pH under conditions in which cytoplasmic [Na(+)] is limiting for this crucial process. The defect was exacerbated by mutation of motPS, the motility channel genes. We hypothesize that activation of Na(V)BP at high pH supports diverse physiological processes by a combination of direct and indirect effects on the Na(+) cycle and the chemotaxis system.

A Voltage-Gated Calcium-Selective Channel Encoded by a Sodium Channel-like Gene

BSC1, which was originally identified by its sequence similarity to voltage-gated Na(+) channels, encodes a functional voltage-gated cation channel whose properties differ significantly from Na(+) channels. BSC1 has slower kinetics of activation and inactivation than Na(+) channels, it is more selective for Ba(2+) than for Na(+), it is blocked by Cd(2+), and Na(+) currents through BSC1 are blocked by low concentrations of Ca(2+). All of these properties are more similar to voltage-gated Ca(2+) channels than to voltage-gated Na(+) channels. The selectivity for Ba(2+) is partially due to the presence of a glutamate in the pore-forming region of domain III, since replacing that residue with lysine (normally present in voltage-gated Na(+) channels) makes the channel more selective for Na(+). BSC1 appears to be the prototype of a novel family of invertebrate voltage-dependent cation channels with a close structural and evolutionary relationship to voltage-gated Na(+) and Ca(2+) channels.

A superfamily of voltage-gated sodium channels in bacteria

NaChBac, a six-alpha-helical transmembrane-spanning protein cloned from Bacillus halodurans, is the first functionally characterized bacterial voltage-gated Na(+)-selective channel. As a highly expressing ion channel protein, NaChBac is an ideal candidate for high resolution structural determination and structure-function studies. The biological role of NaChBac, however, is still unknown. In this report, another 11 structurally related bacterial proteins are described. Two of these functionally expressed as voltage-dependent Na(+) channels (Na(V)PZ from Paracoccus zeaxanthinifaciens and Na(V)SP from Silicibacter pomeroyi). Na(V)PZ and Na(V)SP share approximately 40% amino acid sequence identity with NaChBac. When expressed in mammalian cell lines, both Na(V)PZ and Na(V)SP were Na(+)-selective and voltage-dependent. However, their kinetics and voltage dependence differ significantly. These single six-alpha-helical transmembrane-spanning subunits constitute a widely distributed superfamily (Na(V)Bac) of channels in bacteria, implying a fundamental prokaryotic function. The degree of sequence homology (22-54%) is optimal for future comparisons of Na(V)Bac structure and function of similarity and dissimilarity among Na(V)Bac proteins. Thus, the Na(V)Bac superfamily is fertile ground for crystallographic, electrophysiological, and microbiological studies.

Redesigning the monovalent cation specificity of an enzyme

Monovalent-cation-activated enzymes are abundantly represented in plants and in the animal world. Most of these enzymes are specifically activated by K+, whereas a few of them show preferential activation by Na+. The monovalent cation specificity of these enzymes remains elusive in molecular terms and has not been reengineered by site-directed mutagenesis. Here we demonstrate that thrombin, a Na+-activated allosteric enzyme involved in vertebrate blood clotting, can be converted into a K+-specific enzyme by redesigning a loop that shapes the entrance to the cation-binding site. The conversion, however, does not result into a K+-activated enzyme.