Structure of the cytoplasmic beta subunit-T1 assembly of voltage-dependent K+ channels

Functionally active t1-t1 interfaces revealed by the accessibility of intracellular thiolate groups in kv4 channels

Gating of voltage-dependent K(+) channels involves movements of membrane-spanning regions that control the opening of the pore. Much less is known, however, about the contributions of large intracellular channel domains to the conformational changes that underlie gating. Here, we investigated the functional role of intracellular regions in Kv4 channels by probing relevant cysteines with thiol-specific reagents. We find that reagent application to the intracellular side of inside-out patches results in time-dependent irreversible inhibition of Kv4.1 and Kv4.3 currents. In the absence or presence of Kv4-specific auxiliary subunits, mutational and electrophysiological analyses showed that none of the 14 intracellular cysteines is essential for channel gating. C110, C131, and C132 in the intersubunit interface of the tetramerization domain (T1) are targets responsible for the irreversible inhibition by a methanethiosulfonate derivative (MTSET). This result is surprising because structural studies of Kv4-T1 crystals predicted protection of the targeted thiolate groups by constitutive high-affinity Zn(2+) coordination. Also, added Zn(2+) or a potent Zn(2+) chelator (TPEN) does not significantly modulate the accessibility of MTSET to C110, C131, or C132; and furthermore, when the three critical cysteines remained as possible targets, the MTSET modification rate of the activated state is approximately 200-fold faster than that of the resting state. Biochemical experiments confirmed the chemical modification of the intact alpha-subunit and the purified tetrameric T1 domain by MTS reagents. These results conclusively demonstrate that the T1--T1 interface of Kv4 channels is functionally active and dynamic, and that critical reactive thiolate groups in this interface may not be protected by Zn(2+) binding.

Fluorescence measurements reveal stoichiometry of K+ channels formed by modulatory and delayed rectifier alpha-subunits

Modulatory alpha-subunits, which comprise one-fourth of all voltagegated K(+) channel (Kv) alpha-subunits, do not assemble into homomeric channels, but selectively associate with delayed rectifier Kv2 subunits to form heteromeric channels of unknown stoichiometry. Their distinct expression patterns and unique functional properties have made these channels candidate molecular correlates for a broad set of native K(+) currents. Here, we combine FRET and electrophysiological measurements to determine the stoichiometry and geometry of heteromeric channels composed of the delayed rectifier Kv2.1 subunit and the modulatory Kv9.3 alpha-subunit. Kv channel alpha-subunits were fused with GFP variants, and heteromerization of different combinations of tagged and untagged alpha-subunits was studied. FRET, evaluated by acceptor photobleaching, was only observed upon formation of functional channels. Our results, obtained from two independent experimental paradigms, suggest the formation of heteromeric Kv2.1/Kv9.3 channels of fixed stoichiometry consisting of three Kv2.1 subunits and one Kv9.3 subunit. Strikingly, despite this uneven stoichiometry, we find that heteromeric Kv2.1/Kv9.3 channels maintain a pseudosymmetric arrangement of subunits around the central pore.

Potassium channel structures: do they conform?

Potassium channels are signalling elements vital to vertebrate neurotransmission, and cardiac and renal function. Two inherent qualities equip them for their role in the interconversion of chemical and electrical messages: high selectivity for potassium ions and the ability to open (gate) on cue. The crystal structure of KcsA, published in 1998, explained much about potassium selectivity and high ion flux. The enormous diversity of potassium channels (some hundreds of genes in humans) may have hampered similar progress in understanding gating processes. The recent determination of several representative structures has provided us with a valuable reference for discriminating between features that are utilized in gating across the potassium channel genre and features that determine responsiveness to family-specific gating cues.

Molecular physiology and modulation of somatodendritic A-type potassium channels

The somatodendritic subthreshold A-type K+ current (ISA) in nerve cells is a critical component of the ensemble of voltage-gated ionic currents that determine somatodendritic signal integration. The underlying K+ channel belongs to the Shal subfamily of voltage-gated K+ channels. Most Shal channels across the animal kingdom share a high degree of structural conservation, operate in the subthreshold range of membrane potentials, and exhibit relatively fast inactivation and recovery from inactivation. Mammalian Shal K+ channels (Kv4) undergo preferential closed-state inactivation with features that are generally inconsistent with the classical mechanisms of inactivation typical of Shaker K+ channels. Here, we review (1) the physiological and genetic properties of ISA, 2 the molecular mechanisms of Kv4 inactivation and its remodeling by a family of soluble calcium-binding proteins (KChIPs) and a membrane-bound dipeptidase-like protein (DPPX), and (3) the modulation of Kv4 channels by protein phosphorylation.

Determinants of voltage-gated potassium channel surface expression and localization in Mammalian neurons

Neurons strictly regulate expression of a wide variety of voltage-dependent ion channels in their surface membranes to achieve precise yet dynamic control of intrinsic membrane excitability. Neurons also exhibit extreme morphological complexity that underlies diverse aspects of their function. Most ion channels are preferentially targeted to either the axonal or somatodendritic compartments, where they become further localized to discrete membrane subdomains. This restricted accumulation of ion channels enables local control of membrane signaling events in specific microdomains of a given compartment. Voltage-dependent K+ (Kv) channels act as potent modulators of diverse excitatory events such as action potentials, excitatory synaptic potentials, and Ca2+ influx. Kv channels exhibit diverse patterns of cellular expression, and distinct subtype-specific localization, in mammalian central neurons. Here we review the mechanisms regulating the abundance and distribution of Kv channels in mammalian neurons and discuss how dynamic regulation of these events impacts neuronal signaling.

Homology Models of the Tetramerization Domain of Six Eukaryotic Voltage-gated Potassium Channels Kv1.1-Kv1.6

The homology models of the tetramerization (T1) domain of six eukaryotic potassium channels, Kv1.1-Kv1.6, were constructed based on the crystal structure of the Shaker T1 domain. The results of amino acid sequence alignment indicate that the T1 domains of these K+ channels are highly conserved, with the similarities varying from 77% between Shaker and Kv1.6 to 93% between Kv1.2 and Kv1.3. The homology models reveal that the T1 domains of these Kv channels exhibit similar folds as those of Shaker K+ channel. These models also show that each T1 monomer consists of three distinct layers, with N-terminal layer 1 and C-terminal layer 3 facing the cytoplasm and the membrane, respectively. Layer 2 exhibits the highest structural conservation because it is located around the central hydrophobic core. For each Kv channel, four identical subunits assemble into the homotetramer architecture around a four-fold axis through the hydrogen bonds and salt bridges formed by 15 highly conserved polar residues. The narrowest opening of the pore is formed by the four conserved residues corresponding to R115 of the Shaker T1 domain. The homology models of these Kv T1 domains provide particularly attractive targets for further structure-based studies.

Computational simulations of interactions of scorpion toxins with the voltage-gated potassium ion channel

Based on a homology model of the Kv1.3 potassium channel, the recognitions of the six scorpion toxins, viz. agitoxin2, charybdotoxin, kaliotoxin, margatoxin, noxiustoxin, and Pandinus toxin, to the human Kv1.3 potassium channel have been investigated by using an approach of the Brownian dynamics (BD) simulation integrating molecular dynamics (MD) simulation. Reasonable three-dimensional structures of the toxin-channel complexes have been obtained employing BD simulations and triplet contact analyses. All of the available structures of the six scorpion toxins in the Research Collaboratory for Structural Bioinformatics Protein Data Bank determined by NMR were considered during the simulation, which indicated that the conformations of the toxin significantly affect both the molecular recognition and binding energy between the two proteins. BD simulations predicted that all the six scorpion toxins in this study use their beta-sheets to bind to the extracellular entryway of the Kv1.3 channel, which is in line with the primary clues from the electrostatic interaction calculations and mutagenesis results. Additionally, the electrostatic interaction energies between the toxins and Kv1.3 channel correlate well with the binding affinities (-logK(d)s), R(2) = 0.603, suggesting that the electrostatic interaction is a dominant component for toxin-channel binding specificity. Most importantly, recognition residues and interaction contacts for the binding were identified. Lys-27 or Lys-28, residues Arg-24 or Arg-25 in the separate six toxins, and residues Tyr-400, Asp-402, His-404, Asp-386, and Gly-380 in each subunit of the Kv1.3 potassium channel, are the key residues for the toxin-channel recognitions. This is in agreement with the mutation results. MD simulations lasting 5 ns for the individual proteins and the toxin-channel complexes in a solvated lipid bilayer environment confirmed that the toxins are flexible and the channel is not flexible in the binding. The consistency between the results of the simulations and the experimental data indicated that our three-dimensional models of the toxin-channel complex are reasonable and can be used as a guide for future biological studies, such as the rational design of the blocking agents of the Kv1.3 channel and mutagenesis in both toxins and the Kv1.3 channel. Moreover, the simulation result demonstrates that the electrostatic interaction energies combined with the distribution frequencies from BD simulations might be used as criteria in ranking the binding configuration of a scorpion toxin to the Kv1.3 channel.

Accessory Kvbeta1 subunits differentially modulate the functional expression of voltage-gated K+ channels in mouse ventricular myocytes

Voltage-gated K+ (Kv) channel accessory (beta) subunits associate with pore-forming Kv alpha subunits and modify the properties and/or cell surface expression of Kv channels in heterologous expression systems. There is very little presently known, however, about the functional role(s) of Kv beta subunits in the generation of native cardiac Kv channels. Exploiting mice with a targeted disruption of the Kvbeta1 gene (Kvbeta1-/-), the studies here were undertaken to explore directly the role of Kvbeta1 in the generation of ventricular Kv currents. Action potential waveforms and peak Kv current densities are indistinguishable in myocytes isolated from the left ventricular apex (LVA) of Kvbeta1-/- and wild-type (WT) animals. Analysis of Kv current waveforms, however, revealed that mean+/-SEM I(to,f) density is significantly (P< or =0.01) lower in Kvbeta1-/- (21.0+/-0.9 pA/pF; n=68), than in WT (25.3+/-1.4 pA/pF; n=42), LVA myocytes, and that mean+/-SEM I(K,slow) density is significantly (P< or =0.01) higher in Kvbeta1-/- (19.1+/-0.9 pA/pF; n=68), compared with WT (15.9+/-0.7 pA/pF; n=42), LVA cells. Pharmacological studies demonstrated that the TEA-sensitive component of I(K,slow), I(K,slow2,) is selectively increased in Kvbeta1-/- LVA myocytes. In parallel with the alterations in I(to,f) and I(K,slow2) densities, Kv4.3 expression is decreased and Kv2.1 expression is increased in Kvbeta1-/- ventricles. Taken together, these results demonstrate that Kvbeta1 differentially regulates the functional cell surface expression of myocardial I(to,f) and I(K,slow2) channels.

Coupled Tertiary Folding and Oligomerization of the T1 Domain of Kv Channels

Acquisition of secondary, tertiary, and quaternary structure is critical to the fabrication, assembly, and function of ion channels, yet the relationship between these biogenic events remains unclear. We now address this issue in voltage-gated K(+) channels (Kv) for the T1 domain, an N-terminal Kv recognition domain that is responsible for subfamily-specific, efficient assembly of Kv subunits. This domain forms a 4-fold symmetric tetramer. We have identified residues along the axial T1-T1 interface that are critical for tertiary and quaternary structure, shown that mutations at one end of the axial T1 interface can perturb the crosslinking of an intersubunit cysteine pair at the other end, and demonstrated that tertiary folding and tetramerization of this Kv domain are coupled. A threshold level of tertiary folding is required for monomers to oligomerize. Coupling between tertiary and quaternary structure formation may be a common feature in the biogenesis of multimeric proteins.

Molecular diversity and regulation of renal potassium channels

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

M-ZDOCK: a grid-based approach for Cn symmetric multimer docking

Computational protein docking is a useful technique for gaining insights into protein interactions. We have developed an algorithm M-ZDOCK for predicting the structure of cyclically symmetric (Cn) multimers based on the structure of an unbound (or partially bound) monomer. Using a grid-based Fast Fourier Transform approach, a space of exclusively symmetric multimers is searched for the best structure. This leads to improvements both in accuracy and running time over the alternative, which is to run a binary docking program ZDOCK and filter the results for near-symmetry. The accuracy is improved because fewer false positives are considered in the search, thus hits are not as easily overlooked. By searching four instead of six degrees of freedom, the required amount of computation is reduced. This program has been tested on several known multimer complexes from the Protein DataBank, including four unbound multimers: three trimers and a pentamer. For all of these cases, M-ZDOCK was able to find at least one hit, whereas only two of the four testcases had hits when using ZDOCK and a symmetry filter. In addition, the running times are 30-40% faster for M-ZDOCK.

AVAILABILITY

M-ZDOCK is freely available to academic users at http://zlab.bu.edu/m-zdock/

CONTACT

zhiping@bu.edu

SUPPLEMENTARY INFORMATION

http://zlab.bu.edu/m-zdock.

Structure acquisition of the T1 domain of Kv1.3 during biogenesis

The T1 recognition domains of voltage-gated K(+) (Kv) channel subunits form tetramers and acquire tertiary structure while still attached to their individual ribosomes. Here we ask when and in which compartment secondary and tertiary structures are acquired. We answer this question using biogenic intermediates and recently developed folding and accessibility assays to evaluate the status of the nascent Kv peptide both inside and outside of the ribosome. A compact structure (likely helical) that corresponds to a region of helicity in the mature structure is already manifest in the nascent protein within the ribosomal tunnel. The T1 domain acquires tertiary structure only after emerging from the ribosomal exit tunnel and complete synthesis of the T1-S1 linker. These measurements of ion channel folding within the ribosomal tunnel and its exit port bear on basic principles of protein folding and pave the way for understanding the molecular basis of protein misfolding, a fundamental cause of channelopathies.

Peering into the birth canal during ion channel parturition

Recent studies have provided detailed structures of the N-terminal T1 domain of Kv channel alpha subunits that mediates contranslational subunit assembly. In this issue of Neuron, Kosolapov et al. probe T1 domain structure within the ribosomal tunnel. They find that the T1 domain forms secondary structure within the tunnel, in preparation for its immediate role in governing channel assembly upon exit.

Isolation from cochlea of a novel human intronless gene with predominant fetal expression

We have cloned a novel human gene, designated PFET1 (predominantly fetal expressed T1 domain) (HUGO-approved symbol KCTD12 or C13orf2), by subtractive hybridization and differential screening of human fetal cochlear cDNA clones. Also, we have identified the mouse homolog, designated Pfet1. PFET1/Pfet1 encode a single transcript of approximately 6 kb in human, and three transcripts of approximately 4, 4.5, and 6 kb in mouse with a 70% GC-rich open reading frame (ORF) consisting of 978 bp in human and 984 bp in mouse. Both genes have unusually long 3' untranslated (3' UTR) regions (4996 bp in human PFET1, 3700 bp in mouse Pfet1) containing 12 and 5 putative polyadenylation consensus sequences, respectively. Pfetin, the protein encoded by PFET1/Pfet1, is predicted to have 325 amino acids in human and 327 amino acids in mouse and to contain a voltage-gated potassium (K+) channel tetramerization (T1) domain. Otherwise, to date these genes have no significant homology to any known gene. PFET1 maps to the long arm of human chromosome 13, in band q21 as shown by FISH analysis and STS mapping. Pfet1 maps to mouse chromosome 14 near the markers D14Mit8, D14Mit93, and D14Mit145.1. The human 6 kb transcript is present in a variety of fetal organs, with highest expression levels in the cochlea and brain and, in stark contrast, is detected only at extremely low levels in adult organs, such as brain and lung. Immunohistochemistry with a polyclonal antibody raised against a synthetic peptide to PFET1 sequence (pfetin) reveals immunostaining in a variety of cell types in human, monkey, mouse, and guinea pig cochleas and the vestibular system, including type I vestibular hair cells.

Differential regulation of voltage-gated K+ channels by oxidized and reduced pyridine nucleotide coenzymes

The activity of the voltage-sensitive K+ (Kv) channels varies as a function of the intracellular redox state and metabolism, and several Kv channels act as oxygen sensors. However, the mechanisms underlying the metabolic and redox regulation of these channels remain unclear. In this study we investigated the regulation of Kv channels by pyridine nucleotides. Heterologous expression of Kvalpha1.5 in COS-7 cells led to the appearance of noninactivating currents. Inclusion of 0.1-1 mM NAD+ or 0.03-0.5 mM NADP+ in the internal solution of the patch pipette did not affect Kv currents. However, 0.5 and 1 mM NAD+ and 0.1 and 0.5 mM NADP+ prevented inactivation of Kv currents in cells transfected with Kvalpha1.5 and Kvbeta1.3 and shifted the voltage dependence of activation to depolarized potentials. The Kvbeta-dependent inactivation of Kvalpha currents was also decreased by internal pipette perfusion of the cell with 1 mM NAD+. The Kvalpha1.5-Kvbeta1.3 currents were unaffected by the internal application of 0.1 mM NADPH or 0.1 or 1 mM NADH. Excised inside-out patches from cells expressing Kvalpha1.5-Kvbeta1.3 showed transient single-channel activity. The mean open time and the open probability of these currents were increased by the inclusion of 1 mM NAD+ in the perfusate. These results suggest that NAD(P)+ prevents Kvbeta-mediated inactivation of Kv currents and provide a novel mechanism by which pyridine nucleotides could regulate specific K+ currents as a function of the cellular redox state [NAD(P)H-to-NAD(P)+ ratio].

Structure of a Human A-type Potassium Channel Interacting Protein DPPX, a Member of the Dipeptidyl Aminopeptidase Family

It has recently been reported that dipeptidyl aminopeptidase X (DPPX) interacts with the voltage-gated potassium channel Kv4 and that co-expression of DPPX together with Kv4 pore forming alpha-subunits, and potassium channel interacting proteins (KChIPs), reconstitutes properties of native A-type potassium channels in vitro. Here we report the X-ray crystal structure of the extracellular domain of human DPPX determined at 3.0A resolution. This structure reveals the potential for a surface electrostatic change based on the protonation state of histidine. Subtle changes in extracellular pH might modulate the interaction of DPPX with Kv4.2 and possibly with other proteins. We propose models of DPPX interaction with the voltage-gated potassium channel complex. The dimeric structure of DPPX is highly homologous to the related protein DPP-IV. Comparison of the active sites of DPPX and DPP-IV reveals loss of the catalytic serine residue but the presence of an additional serine near the "active" site. However, the arrangement of residues is inconsistent with that of canonical serine proteases and DPPX is unlikely to function as a protease (dipeptidyl aminopeptidase).

The VGL-Chanome: A Protein Superfamily Specialized for Electrical Signaling and Ionic Homeostasis

Complex multicellular organisms require rapid and accurate transmission of information among cells and tissues and tight coordination of distant functions. Electrical signals and resulting intracellular calcium transients, in vertebrates, control contraction of muscle, secretion of hormones, sensation of the environment, processing of information in the brain, and output from the brain to peripheral tissues. In nonexcitable cells, calcium transients signal many key cellular events, including secretion, gene expression, and cell division. In epithelial cells, huge ion fluxes are conducted across tissue boundaries. All of these physiological processes are mediated in part by members of the voltage-gated ion channel protein superfamily. This protein superfamily of 143 members is one of the largest groups of signal transduction proteins, ranking third after the G protein-coupled receptors and the protein kinases in number. Each member of this superfamily contains a similar pore structure, usually covalently attached to regulatory domains that respond to changes in membrane voltage, intracellular signaling molecules, or both. Eight families are included in this protein superfamily-voltage-gated sodium, calcium, and potassium channels; calcium-activated potassium channels; cyclic nucleotide-modulated ion channels; transient receptor potential (TRP) channels; inwardly rectifying potassium channels; and two-pore potassium channels. This article identifies all of the members of this protein superfamily in the human genome, reviews the molecular and evolutionary relations among these ion channels, and describes their functional roles in cell physiology.

N-type inactivation features of Kv4.2 channel gating

We examined whether the N-terminus of Kv4.2 A-type channels (4.2NT) possesses an autoinhibitory N-terminal peptide domain, which, similar to the one of Shaker, mediates inactivation of the open state. We found that chimeric Kv2.1(4.2NT) channels, where the cytoplasmic Kv2.1 N-terminus had been replaced by corresponding Kv4.2 domains, inactivated relatively fast, with a mean time constant of 120 ms as compared to 3.4 s in Kv2.1 wild-type. Notably, Kv2.1(4.2NT) showed features typically observed for Shaker N-type inactivation: fast inactivation of Kv2.1(4.2NT) channels was slowed by intracellular tetraethylammonium and removed by N-terminal truncation (Delta40). Kv2.1(4.2NT) channels reopened during recovery from inactivation, and recovery was accelerated in high external K+. Moreover, the application of synthetic N-terminal Kv4.2 and ShB peptides to inside-out patches containing slowly inactivating Kv2.1 channels mimicked N-type inactivation. Kv4.2 channels, after fractional inactivation, mediated tail currents with biphasic decay, indicative of passage through the open state during recovery from inactivation. Biphasic tail current kinetics were less prominent in Kv4.2/KChIP2.1 channel complexes and virtually absent in Kv4.2Delta40 channels. N-type inactivation features of Kv4.2 open-state inactivation, which may be suppressed by KChIP association, were also revealed by the finding that application of Kv4.2 N-terminal peptide accelerated the decay kinetics of both Kv4.2Delta40 and Kv4.2/KChIP2.1 patch currents. However, double mutant cycle analysis of N-terminal inactivating and pore domains indicated differences in the energetics and structural determinants between Kv4.2 and Shaker N-type inactivation.

Structure of cation channels, revealed by single particle electron microscopy

A large barrier in the way to obtaining high-resolution structures of eukaryotic ion channels remains the expression and purification of sufficient amounts of channel protein to carry out crystallization trials. In the absence of crystals, the main available source of structural information has been electron microscopy (EM), which is well suited to the visualization of isolated macromolecular complexes and their conformational changes. The recently published EM structures outline native conformations of eukaryotic cation channels that until now have eluded crystallization. According to these results, homo-tetrameric K(+) channels have a distinct two-layer architecture with connectors conjoining the two layers, while the pseudo-tetrameric Ca(2+) or Na(+) channels are more globular and have flexible surface loops, which makes the identification of subunits complicated. Subunits can be identified using atomic structure docking into the EM maps, labeling, or deletion studies.

Metabolic Modulation of Potassium Channels

Recent investigations have shown that a range of small molecules that reflect the metabolic state of a cell regulate the activity of potassium channels. For instance, hydrogen peroxide has been shown to activate adenosine 5'-triphosphate-sensitive potassium channels (K(ATP) channels), whereas heme closes certain calcium-activated potassium channels. Although the exact function of the beta subunit associated with voltage-gated potassium channels is still unclear, its crystal structure suggests that membrane excitability is directly coupled to metabolism.

Two N-terminal domains of Kv4 K(+) channels regulate binding to and modulation by KChIP1

The family of calcium binding proteins called KChIPs associates with Kv4 family K(+) channels and modulates their biophysical properties. Here, using mutagenesis and X-ray crystallography, we explore the interaction between Kv4 subunits and KChIP1. Two regions in the Kv4.2 N terminus, residues 7-11 and 71-90, are necessary for KChIP1 modulation and interaction with Kv4.2. When inserted into the Kv1.2 N terminus, residues 71-90 of Kv4.2 are also sufficient to confer association with KChIP1. To provide a structural framework for these data, we solved the crystal structures of Kv4.3N and KChIP1 individually. Taken together with the mutagenesis data, the individual structures suggest that that the Kv4 N terminus is required for stable association with KChIP1, perhaps through a hydrophobic surface interaction, and that residues 71-90 in Kv4 subunits form a contact loop that mediates the specific association of KChIPs with Kv4 subunits.

Three-dimensional structure of I(to); Kv4.2-KChIP2 ion channels by electron microscopy at 21 Angstrom resolution

Regulatory KChIP2 subunits assemble with pore-forming Kv4.2 subunits in 4:4 complexes to produce native voltage-gated potassium (Kv) channels like cardiac I(to) and neuronal I(A) subtypes. Here, negative stain electron microscopy (EM) and single particle averaging reveal KChIP2 to create a novel approximately 35 x 115 x 115 Angstrom, intracellular fenestrated rotunda: four peripheral columns that extend down from the membrane-embedded portion of the channel to enclose the Kv4.2 "hanging gondola" (a platform held beneath the transmembrane conduction pore by four internal columns). To reach the pore from the cytosol, ions traverse one of four external fenestrae to enter the rotundal vestibule and then cross one of four internal windows in the gondola.

The dipole moments of membrane proteins: potassium channel proteins. II. T1 assembly. Search for the voltage sensor

The dipole moments of potassium channel protein (Kcsa) and beta-subunits were discussed in the previous paper of this series [Takashima, Biophys. Chem. 94 (2001) 209-218]. While the dipole moment of beta-subunits was found to be very large, the dipole moment of Kcsa turned out to be somewhat smaller than beta-subunits. As the continuation of this work, the discussion of the present paper is focussed on the dipole moment of T1 assembly, another component of the K-channel. As discussed later, the calculation using the X-ray crystallographic data by MacKinnon et al., revealed an astoundingly large dipole moment for T1 assembly. The dipole moment of T1 assembly combined with the likewise large dipole moment of beta-subunits amounts to a sufficient value to play an essential role as a voltage sensor of potassium channel.

Potassium Channels in T Lymphocytes

Human peripheral blood T lymphocytes possess two types of K(+) channels: the voltage-gated Kv1.3 and the calcium-activated IKCa1 channels. The use of peptidyl inhibitors of Kv1.3 and IKCa1 indicated that these channels are involved in the maintenance of membrane potential and that they play a crucial role in Ca(2+) signaling during T-cell activation. Thus, in vitro blockade of Kv1.3 and IKCa1 leads to inhibition of cytokine production and lymphocyte proliferation. These observations prompted several groups of investigators in academia and pharmaceutical companies to characterize the expression of Kv1.3 and IKCa1 in different subsets of human T lymphocytes and to evaluate their potential as novel targets for immunosuppression. Recent in vivo studies showed that chronically activated T lymphocytes involved in the pathogenesis of multiple sclerosis present unusually high expression of Kv1.3 channels and that the treatment with selective Kv1.3 inhibitors can either prevent or ameliorate the symptoms of the disease. In this model of multiple sclerosis, blockade of IKCa1 channels had no effect alone, but improved the response to Kv1.3 inhibitors. In addition, the expression of Kv1.3 and IKCa1 channels in human cells is very restricted, which makes them attractive targets for a more cell-specific and less harmful action than what is typically obtained with classical immunosuppressants. Studies using high-throughput toxin displacement, (86)Rb-efflux screening or membrane potential assays led to the identification of non-peptidyl small molecules with high affinity for Kv1.3 or IKCa1 channels. Analysis of structure-function relationships in Kv1.3 and IKCa1 channels helped define the binding sites for channel blockers, allowing the design of a new generation of small molecules with selectivity for either Kv1.3 or IKCa1, which could help the development of new drugs for safer treatment of auto-immune diseases.

Arranging the elements of the potassium channel: the T1 domain occludes the cytoplasmic face of the channel

The voltage-gated potassium channel is currently one of the few membrane proteins where functional roles have been mapped onto specific segments of sequence. Although high-resolution structures of the transmembrane portions of three bacterial potassium channels, the tetramerization domain and the cytoplasmic "ball" are available, their relative spatial arrangement in mammalian channels remains a matter of ongoing debate. Cryo-electron microscopic images of the six transmembrane voltage-gated Kv channel have been reconstructed at up to 18 A resolution, revealing that the T1 domain tetramerizes and is suspended below the transmembrane segments. However, the resolution of these images is insufficient to reveal the location of the third piece of the puzzle, the inactivating ball domain. We have used the aberrant interactions observed in a series of chimaeric channels to establish that an assembled T1 domain restricts access to the cytoplasmic face of the channel, suggesting that the N-terminal "ball and chain" may be confined in the space between the T1 domain and the transmembrane portion of the channel.

The birth of a channel

An ion channel protein begins life as a nascent peptide inside a ribosome, moves to the endoplasmic reticulum where it becomes integrated into the lipid bilayer, and ultimately forms a functional unit that conducts ions in a well-regulated fashion. Here, I discuss the nascent peptide and its tasks as it wends its way through ribosomal tunnels and exit ports, through translocons, and into the bilayer. We are just beginning to explore the sequence of these events, mechanisms of ion channel structure formation, when biogenic decisions are made, and by which participants. These decisions include when to exit the endoplasmic reticulum and with whom to associate. Such issues govern the expression of ion channels at the cell surface and thus the electrical activity of a cell.

Ito channels are octomeric complexes with four subunits of each Kv4.2 and K+ channel-interacting protein 2

Mammalian voltage-gated K+ channels are assemblies of pore-forming alpha-subunits and modulating beta-subunits. To operate correctly, Kv4 alpha-subunits in the heart and central nervous system require recently identified beta-subunits of the neuronal calcium sensing protein family called K+ channel-interacting proteins (KChIPs). Here, Kv4.2.KChIP2 channels are purified, integrity of isolated complexes confirmed, molar ratio of the subunits determined, and subunit valence established. A complex has 4 subunits of each type, a stoichiometry expected for other channels employing neuronal calcium sensing beta-subunits.

Structural characterisation of neuronal voltage-sensitive K+ channels heterologously expressed in Pichia pastoris

Neuronal voltage-dependent K(+) channels of the delayed rectifier type consist of multiple Kv alpha subunit variants, which assemble as hetero- or homotetramers, together with four Kv beta auxiliary subunits. Direct structural information on these proteins has not been forthcoming due to the difficulty in isolating the native K(+) channels. We have overexpressed the subunit genes in the yeast Pichia pastoris. The Kv1.2 subunit expressed alone is shown to fold into a native conformation as determined by high-affinity binding of 125I-labelled alpha-dendrotoxin, while co-expressed Kv1.2 and Kv beta 2 subunits co-assembled to form native-like oligomers. Sites of post-translational modifications causing apparent heterogeneity on SDS-PAGE were identified by site-directed mutagenesis. Engineering to include affinity tags and scale-up of production by fermentation allowed routine purification of milligram quantities of homo- and heteroligomeric channels. Single-particle electron microscopy of the purified channels was used to generate a 3D volume to 2.1 nm resolution. Protein domains were assigned by fitting crystal structures of related bacterial proteins. Addition of exogenous lipid followed by detergent dialysis produced well-ordered 2D crystals that exhibited mostly p12(1) symmetry. Projection maps of negatively stained crystals show the constituent molecules to be 4-fold symmetric, as expected for the octameric K(+) channel complex.

Conformational changes in the C terminus of Shaker K+ channel bound to the rat Kvbeta2-subunit

We studied the structure of the C terminus of the Shaker potassium channel. The 3D structures of the full-length and a C-terminal deletion (Delta C) mutant of Shaker were determined by electron microscopy and single-particle analysis. The difference map between the full-length and the truncated channels clearly shows a compact density, located on the sides of the T1 domain, that corresponds to a large part of the C terminus. We also expressed and purified both WT and Delta C Shaker, assembled with the rat KvBeta2-subunit. By using a difference map between the full-length and truncated Shaker alpha-beta complexes, a conformational change was identified that shifts a large part of the C terminus away from the membrane domain and into close contact with the Beta-subunit. This conformational change, induced by the binding of the KvBeta2-subunit, suggests a possible mechanism for the modulation of the K+ voltage-gated channel function by its Beta-subunit.

A conserved domain in axonal targeting of Kv1 (Shaker) voltage-gated potassium channels

Axonal voltage-gated potassium (Kv1) channels regulate action-potential invasion and hence transmitter release. Although evolutionarily conserved, what mediates their axonal targeting is not known. We found that Kv1 axonal targeting required its T1 tetramerization domain. When fused to unpolarized CD4 or dendritic transferrin receptor, T1 promoted their axonal surface expression. Moreover, T1 mutations eliminating Kvbeta association compromised axonal targeting, but not surface expression, of CD4-T1 fusion proteins. Thus, proper association of Kvbeta with the Kv1 T1 domain is essential for axonal targeting.

Differential association of the auxiliary subunit Kvbeta2 with Kv1.4 and Kv4.3 K+ channels

Kvbeta2 subunits associate with Kv1 and Kv4 K+ channels, but the basis of preferential association is not understood. For example, detergent resistance suggests stronger auxiliary subunit association with Kv4.2 than with Kv1.2, but Kvbeta2 preferentially localizes with the latter channels in brain. Here we examine the interaction of Kvbeta2 with two native binding partners in brain: Kv4.3 and Kv1.4. We show that the auxiliary subunit binds more efficiently to Kv1.4 than to Kv4.3 in mammalian cells. However, preexisting Kvbeta2 complexes with Kv1.4 and Kv4.3 have similar detergent sensitivity. Thus, preferential steady state binding may reflect a difference in initial association rather than stability. We also find that that the cytoplasmic C-terminus of Kv4.3 inhibits Kvbeta2 association. Apparently, a region proximal to the Kv4.3 pore contributes to the inefficient auxiliary subunit interaction that produces preferential binding of Kvbeta2 to Kv1 channels.

Probing the volume changes during voltage gating of Porin 31BM channel with nonelectrolyte polymers

To probe the volume changes of the voltage-dependent anion-selective channel (VDAC), the nonelectrolyte exclusion technique was taken because it is one of the few existing methods that may define quite accurately the rough geometry of lumen of ion channels (in membranes) for which there is no structural data.Here, we corroborate the data from our previous study [FEBS Lett. 416 (1997) 187] that the gross structural features of VDAC in its highest conductance state are asymmetric with respect to the plane of the membrane, and state that this asymmetry is not dependent on sign of voltage applied. Hence, the plasticity of VDAC does not play a role in the determination of lumen geometry at this state and the asymmetry is an internal property of the channel. We also show that the apparent diameter of the cis segment of the pore decreases slightly from 2 to 1.8 nm when the channel's conductance decreases from its high to low state. However, the trans funnel segment undergoes a more marked change in polymer accessible volume. Specifically, its larger diameter decreases from approximately 4 to 2.4 nm. Supposing the channel's total length is 4.6 nm, the apparent change in channel volume during this transition is estimated to be about 10 nm(3), i.e. about 40% of the channel's volume in the high conductance state.

The Roles of N- and C-terminal determinants in the activation of the Kv2.1 potassium channel

The human and rat forms of the Kv2.1 channel have identical amino acids over the membrane-spanning regions and differ only in the N- and C-terminal intracellular regions. Rat Kv2.1 activates much faster than human Kv2.1. Here we have studied the role of the N- and C-terminal residues that determine this difference in activation kinetics between the two channels. For this, we constructed mutants and chimeras between the two channels, expressed them in oocytes, and recorded currents by two-electrode voltage clamping. In the N-terminal region, mutation Q67E in the rat channel displayed a slowing of activation relative to rat wild type, whereas mutation D75E in the human channel showed faster activation than human wild type. In the C-terminal region, we found that some residues within the region of amino acids 740-853 ("CTA" domain) were also involved in determining activation kinetics. The electrophysiological data also suggested interactions between the N and C termini. Such an interaction was confirmed directly by using a glutathione S-transferase (GST) fusion protein with the N terminus of Kv2.1, which we showed to bind to the C terminus of Kv2.1. Taken together, these data suggest that exposed residues in the T1 domain of the N terminus, as well as the CTA domain in the C terminus, are important in determining channel activation kinetics and that these N- and C-terminal regions interact.

Meeting of the minds: metalloneurochemistry

Metalloneurochemistry is the study of metal ion function in the brain and nervous system at the molecular level. Research in this area is exemplified through discussion of several forefront areas where significant progress has been made in recent years. The structure and function of ion channels have been elucidated through high-resolution x-ray structural work on the bacterial K(+) ion channel. Selection of potassium over sodium ions is achieved by taking advantage of key principles of coordination chemistry. The role of calcium ions in neuronal signal transduction is effected by several Ca(2+)-binding protein such as calmodulin, calcineurin, and synaptotagmin. Structural changes in response to calcium ion concentrations allow these proteins to function in memory formation and other neurochemical roles. Metallochaperones help to achieve metal ion homeostasis and thus prevent neurological diseases because of metal ion imbalance. Much detailed chemical information about these systems has become available recently. Zinc is another important metal ion in neuroscience. Its concentration in brain is in part controlled by metallothionein, and zinc is released in the hippocampus at glutamatergic synapses. New fluorescent sensors have become available to help track such zinc release.

An overexpression of chalcone reductase of Pueraria montana var. lobata alters biosynthesis of anthocyanin and 5'-deoxyflavonoids in transgenic tobacco

We isolated the chalcone reductase (pl-chr) gene of Pueraria montana var. lobata by using a PCR strategy from cDNA pools of storage roots. A high level of expression of RNA was found in both stems and roots. The genomic Southern blot result suggests that pl-chr exists as a member of a small gene family. By introducing a pl-chr gene under the control of the 35S CaMV promoter into the pink-flowering Xanthi line of Nicotiana tabacum, the flower color was changed from pink to white-to-pink. The contents of anthocyanin in the flowers of the transgenic lines were dramatically decreased by 40%, but the total UV absorption compounds remained unchanged. The production of liquiritigenin in pl-chr overexpressed transgenic tobacco lines was confirmed by HPLC and MS analysis. The introduction of pl-chr gene provides a method to redirect the flavonoid pathway into 5'-deoxyflavonoid production in non-legume crops, in order to manipulate the phenylpropanoid pathway for isoflavonoid production.

Regulation of the K channels by cytoplasmic domains

Regulation of intracellular potassium levels is one of the basic functions of all cells, controlling cellular osmolarity and transmitting information. In higher organisms, elaborate control of transmembrane potassium flux has evolved to endow nervous systems with the remarkable ability to transmit electrical signals between cells. Multiple genes, gene splicing, mRNA editing, and selective tetrameric assembly of K channel genes provide the basis for creating distinct electrophysiological properties at varying developmental and cellular stages. This assembly mechanism, primarily governed by the T1 domain, is under the control of intracellular signals. Atomic structures of the isolated T1 domains of Shaker and Shaw subfamilies provided us with valuable structural insights into understanding both channel assembly and functional regulation of the entire channel molecule through conformational changes.

Voltage-gated K+ channel from mammalian brain: 3D structure at 18A of the complete (alpha)4(beta)4 complex

Voltage-sensitive K(+) channels (Kv) serve numerous important roles, e.g. in the control of neuron excitability and the patterns of synaptic activity. Here, we use electron microscopy (EM) and single particle analysis to obtain the first, complete structure of Kv1 channels, purified from rat brain, which contain four transmembrane channel-forming alpha-subunits and four cytoplasmically-associated beta-subunits. The 18A resolution structure reveals an asymmetric, dumb-bell-shaped complex with 4-fold symmetry, a length of 140A and variable width. By fitting published X-ray data for recombinant components to our EM map, the modulatory (beta)(4) was assigned to the innermost 105A end, the N-terminal (T1)(4) domain of the alpha-subunit to the central 50A moiety and the pore-containing portion to the 125A membrane part. At this resolution, the selectivity filter could not be localised. Direct contact of the membrane component with the central (T1)(4) domain occurs only via peripheral connectors, permitting communication between the channel and beta-subunits for coupling of responses to changes in excitability and metabolic status of neurons.

The high resolution crystal structure of rat liver AKR7A1: understanding the substrate specificites of the AKR7 family

The structure of the rat liver aflatoxin dialdehyde reductase (AKR7A1) has been solved to 1.38 A resolution. The crystal structure reveals details of the ternary complex as one subunit of the dimer contains NADP+ and the inhibitor citrate. The underlying catalytic mechanism appears similar to other aldo-keto reductases (AKR), whilst the substrate-binding pocket contains several positively charged amino acids (Arg-231 and Arg-327) which distinguishes it from the well characterised AKR1 family of enzymes. These differences account for the substrate specificity for 4-carbon acid-aldehydes such as succinic semialdehyde (SSA) and 2-carboxybenzaldehyde, as well as for the idiosyncratic substrate aflatoxin B1 dialdehyde of this subfamily of enzymes. The AKR7 enzymes seem to be subdivided into two subgroups based on their sequence and kinetic properties. Modelling of the rat AKR7A4 highlights important structural differences localised within the active site of the two isoenzymes.

Structural basis of inward rectification: cytoplasmic pore of the G protein-gated inward rectifier GIRK1 at 1.8 A resolution

Inward rectifier K(+) channels govern the resting membrane voltage in many cells. Regulation of these ion channels via G protein-coupled receptor signaling underlies the control of heart rate and the actions of neurotransmitters in the central nervous system. We have determined the protein structure formed by the intracellular N- and C termini of the G protein-gated inward rectifier K(+) channel GIRK1 at 1.8 A resolution. A cytoplasmic pore, conserved among inward rectifier K(+) channels, extends the ion pathway to 60 A, nearly twice the length of a canonical transmembrane K(+) channel. The cytoplasmic pore is lined by acidic and hydrophobic amino acids, creating a favorable environment for polyamines, which block the pore. These results explain in structural and chemical terms the basis of inward rectification, and they also have implications for G protein regulation of GIRK channels.

Zinc mediates assembly of the T1 domain of the voltage-gated K channel 4.2

An intermolecular Zn(2+)-binding site was identified in the structure of the T1 domain of the Shaw-type potassium channels (aKv3.1). T1 is a BTB/POZ-type domain responsible for the ordered assembly of voltage-gated potassium channels and interactions with other macromolecules. In this structure, a Zn(2+) ion was found to be coordinated between each of the four assembly interfaces of the T1 tetramer by three Cys and one His encoded in the sequence motif (HX(5)CX(20)CC) of the T1 domain. This sequence motif is conserved among all non-Shaker-type voltage-dependent potassium (Kv) channels, but not in Shaker-type channels. The presence of this conserved Zn(2+)-binding site is a primary molecular determinant that distinguishes the tetrameric assembly of non-Shaker Kv channel subunits from that of Shaker channels. We report here that tetramerization of the Shal (rKv4.2) T1 in solution requires the presence of Zn(2+), and the addition/removal of Zn(2+) reversibly switches the protein between a stable tetrameric or monomeric state. We further show that the conversion from tetramers to monomers is profoundly pH-dependent: as the solution pH gets lower, the dissociation rate increases significantly. The unfolding energy of the T1 tetramer as a measure of the conformational stability of the structure is also pH-dependent. Surprisingly, at a lower pH we observe a distinctly altered conformational state of the T1 tetramer trapped during the process of unfolding of the T1 tetramer in the presence of Zn(2+). The conformational alteration may be responsible for increased rate of dissociation at lower pH by allowing Zn(2+) to be removed more effectively by EDTA. The ability of the T1 domain to adopt stable alternative conformations may be essential to its function as a protein-protein interaction/signaling domain to modulate the ion conduction properties of intact full-length Kv channels.

SUR-dependent modulation of KATP channels by an N-terminal KIR6.2 peptide. Defining intersubunit gating interactions

Ntp and Ctp, synthetic peptides based on the N- and C-terminal sequences of K(IR)6.0, respectively, were used to probe gating of K(IR)6.0/SUR K(ATP) channels. Micromolar Ntp dose-dependently increased the mean open channel probability in ligand-free solution (P(O(max))) and attenuated the ATP inhibition of K(IR)6.2/SUR1, but had no effect on homomeric K(IR)6.2 channels. Ntp (up to approximately 10(-4) m) did not affect significantly the mean open or "fast," K(+) driving force-dependent, intraburst closed times, verifying that Ntp selectively modulates the ratio of mean burst to interburst times. Ctp and Rnp, a randomized Ntp, had no effect, indicating that the effects of Ntp are structure specific. Ntp opened K(IR)6.1/SUR1 channels normally silent in the absence of stimulatory Mg(-) nucleotide(s) and attenuated the coupling of high-affinity sulfonylurea binding with K(ATP) pore closure. These effects resemble those seen with N-terminal deletions (DeltaN) of K(IR)6.0, and application of Ntp to DeltaNK(ATP) channels decreased their P(O(max)) and apparent IC(50) for ATP in the absence of Mg(2+). The results are consistent with a competition between Ntp and the endogenous N terminus for a site of interaction on the cytoplasmic face of the channel or with partial replacement of the deleted N terminus by Ntp, respectively. The K(IR) N terminus and the TMD0-L0 segment of SUR1 are known to control the P(O(max)). The L0 linker has been reported to be required for glibenclamide binding, and DeltaNK(IR)6.2/SUR1 channels exhibit reduced labeling of K(IR) with (125)I-azidoglibenclamide, implying that the K(IR) N terminus and L0 of SUR1 are in proximity. We hypothesize that L0 interacts with the K(IR) N terminus in ligand-inhibited K(ATP) channels and put forward a model, based on the architecture of BtuCD, MsbA, and the KcsA channel, in which TMD0-L0 links the MDR-like core of SUR with the K(IR) pore.

Electron tomography of frozen-hydrated isolated triad junctions

Cryoelectron microscopy and tomography have been applied for the first time to isolated, frozen-hydrated skeletal muscle triad junctions (triads) and terminal cisternae (TC) vesicles derived from sarcoplasmic reticulum. Isolated triads were selected on the basis of their appearance as two spherical TC vesicles attached to opposite sides of a flattened vesicle derived from a transverse tubule (TT). Foot structures (ryanodine receptors) were resolved within the gap between the TC vesicles and TT vesicles, and some residual ordering of the receptors into arrays was apparent. Organized dense layers, apparently containing the calcium-binding protein calsequestrin, were found in the lumen of TC vesicles underlying the foot structures. The lamellar regions did not directly contact the sarcoplasmic reticulum membrane, thereby creating an approximately 5-nm-thick zone that potentially constitutes a subcompartment for achieving locally elevated [Ca(2+) ] in the immediate vicinity of the Ca(2+)-conducting ryanodine receptors. The lumen of the TT vesicles contained globular mass densities of unknown origin, some of which form cross-bridges that may be responsible for the flattened appearance of the transverse tubules when viewed in cross-section. The spatial relationships among the TT membrane, ryanodine receptors, and calsequestrin-containing assemblage are revealed under conditions that do not use dehydration, heavy-metal staining, or chemical fixation, thus exemplifying the potential of cryoelectron microscopy and tomography to reveal structural detail of complex subcellular structures.

Potassium channels: structures, models, simulations

Potassium channels have been studied intensively in terms of the relationship between molecular structure and physiological function. They provide an opportunity to integrate structural and computational studies in order to arrive at an atomic resolution description of mechanism. We review recent progress in K channel structural studies, focussing on the bacterial channel KcsA. Structural studies can be extended via use of computational (i.e. molecular simulation) approaches in order to provide a perspective on aspects of channel function such as permeation, selectivity, block and gating. Results from molecular dynamics simulations are shown to be in good agreement with recent structural studies of KcsA in terms of the interactions of K(+) ions with binding sites within the selectivity filter of the channel, and in revealing the importance of filter flexibility in channel function. We discuss how the KcsA structure may be used as a template for developing structural models of other families of K channels. Progress in this area is explored via two examples: inward rectifier (Kir) and voltage-gated (Kv) potassium channels. A brief account of structural studies of ancillary domains and subunits of K channels is provided.

Novel homodimeric and heterodimeric rat gamma-hydroxybutyrate synthases that associate with the Golgi apparatus define a distinct subclass of aldo-keto reductase 7 family proteins

The aldo-keto reductase (AKR) 7 family is composed of the dimeric aflatoxin B(1) aldehyde reductase (AFAR) isoenzymes. In the rat, two AFAR subunits exist, designated rAFAR1 and rAFAR2. Herein, we report the molecular cloning of rAFAR2, showing that it shares 76% sequence identity with rAFAR1. By contrast with rAFAR1, which comprises 327 amino acids, rAFAR2 contains 367 amino acids. The 40 extra residues in rAFAR2 are located at the N-terminus of the polypeptide as an Arg-rich domain that may form an amphipathic alpha-helical structure. Protein purification and Western blotting have shown that the two AFAR subunits are found in rat liver extracts as both homodimers and as a heterodimer. Reductase activity in rat liver towards 2-carboxybenzaldehyde (CBA) was resolved by anion-exchange chromatography into three peaks containing rAFAR1-1, rAFAR1-2 and rAFAR2-2 dimers. These isoenzymes are functionally distinct; with NADPH as cofactor, rAFAR1-1 has a low K(m) and high activity with CBA, whereas rAFAR2-2 exhibits a low K(m) and high activity towards succinic semialdehyde. These data suggest that rAFAR1-1 is a detoxication enzyme, while rAFAR2-2 serves to synthesize the endogenous neuromodulator gamma-hydroxybutyrate (GHB). Subcellular fractionation of liver extracts showed that rAFAR1-1 was recovered in the cytosol whereas rAFAR2-2 was associated with the Golgi apparatus. The distinct subcellular localization of the rAFAR1 and rAFAR2 subunits was confirmed by immunocytochemistry in H4IIE cells. Association of rAFAR2-2 with the Golgi apparatus presumably facilitates secretion of GHB, and the novel N-terminal domain may either determine the targeting of the enzyme to the Golgi or regulate the secretory process. A murine AKR protein of 367 residues has been identified in expressed sequence tag databases that shares 91% sequence identity with rAFAR2 and contains the Arg-rich extended N-terminus of 40 amino acids. Further bioinformatic evidence is presented that full-length human AKR7A2 is composed of 359 amino acids and also possesses an additional N-terminal domain. On the basis of these observations, we conclude that AKR7 proteins can be divided into two subfamilies, one of which is a Golgi-associated GHB synthase with a unique, previously unrecognized, N-terminal domain that is absent from other AKR proteins.

The voltage-gated potassium channels and their relatives

The voltage-gated potassium channels are the prototypical members of a family of membrane signalling proteins. These protein-based machines have pores that pass millions of ions per second across the membrane with astonishing selectivity, and their gates snap open and shut in milliseconds as they sense changes in voltage or ligand concentration. The architectural modules and functional components of these sophisticated signalling molecules are becoming clear, but some important links remain to be elucidated.

Intracellular chloride modulates A-type potassium currents in astrocytes

Application of the GABA(A) receptor agonist muscimol to astrocytes in situ or in vitro results in a receptor-mediated Cl(-) current with a concomitant block of outward K(+) currents. The effect on K(+) current is largely selective for the inactivating A-type current. Parallel experiments with various Cl(-) pipette concentrations show a significant reduction in A-type current under low Cl(-) conditions with minimal effect on delayed current. In addition, lower Cl(-) conditions caused a depolarizing shift of steady-state inactivation (V(1/2), -68 to -57 mV) and activation (V(1/2), -5.8 to 34 mV) kinetics of A-type current only. Cl(-) had no effect on the time course of inactivation or reactivation kinetics, suggesting the Cl(-)-mediated effect is largely on activation kinetics, indirectly affecting steady-state inactivation. Muscimol application to astrocytes under perforated patch control (gramicidin) displayed a similar block of A-type current to that of conventional whole cell patch at 40 or 20 mM pipette Cl(-) concentrations. With barium application under perforated patch conditions, the study of muscimol-mediated Cl(-) current in isolation of the effect on K(+) currents was possible. This allowed estimation of intracellular Cl(-) concentration from receptor current reversal information. The average intracellular Cl(-) concentration was found to be 29 +/- 3.2 mM. The effect on activation kinetics and lack of effect on time course of inactivation or reactivation suggest that intracellular anion concentrations have an effect on the K(+) channel voltage sensor region. Cl(-) may modulate K(+) currents by altering membrane field potentials surrounding K(+) channel proteins.