Structural organization of intercellular channels II. Amino terminal domain of the connexins: sequence, functional roles, and structure.

The amino terminal domain (NT) of the connexins consists of their first 22-23 amino acids. Site-directed mutagenesis studies have demonstrated that NT amino acids are determinants of gap junction channel properties including unitary conductance, permeability/selectivity, and gating in response to transjunctional voltage. The importance of this region has also been emphasized by the identification of multiple disease-associated connexin mutants affecting amino acid residues in the NT region. The first part of the NT is α-helical. The structure of the Cx26 gap junction channel shows that the NT α-helix localizes within the channel, and lines the wall of the pore. Interactions of the amino acid residues in the NT with those in the transmembrane helices may be critical for holding the channel open. The predicted sites of these interactions and the applicability of the Cx26 structure to the NT of other connexins are considered. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.

A molecular model of the inner pore of the Ca channel in its open state.

Structure of the Ca channel open pore is unlikely to be the same as that of the K channel because Ca channels do not contain the hinge residues Gly or Pro. The Ca channel does not have a wide entry into the inner pore, as is found in K channels. First we sought to simulate the open state of the Ca channel by modeling forced opening of the KcsA channel using a procedure of restrained minimization with distance constraints at the level of the α-helical bundle, corresponding to segments Thr-107-Val-115. This produced an intermediate open state, which was populated by amino acid residues of Ca channels and then successively optimized until the opening of the pore reached a diameter of about 10 Å, large enough to allow verapamil to enter and block the Ca channel from inside. Although this approach produced a sterically plausible structure, it was in significant disagreement with the MTSET accessibility data for single cysteine mutations of S6 segments of the P/Q channel(1) that do not fit with an α-helical pattern. Last we explored the idea that the four S6 segments of Ca channels may contain intra-molecular deformations that lead to reorientation of its side chains. After introduction of π-bulges, the model agreed with the MTSET accessibility data. MTSET modification of a cysteine at the C-end of only one S6 could produce physical occlusion and block of the inner pore of the open Ca channel, as observed experimentally, and as expected if the pore opening is narrower than that of K channels.

A molecular switch between the outer and the inner vestibules of the voltage-gated Na+ channel.

Voltage-gated ion channels are transmembrane proteins that undergo complex conformational changes during their gating transitions. Both functional and structural data from K(+) channels suggest that extracellular and intracellular parts of the pore communicate with each other via a trajectory of interacting amino acids. No crystal structures are available for voltage-gated Na(+) channels, but functional data suggest a similar intramolecular communication involving the inner and outer vestibules. However, the mechanism of such communication is unknown. Here, we report that amino acid Ile-1575 in the middle of transmembrane segment 6 of domain IV (DIV-S6) in the adult rat skeletal muscle isoform of the voltage-gated sodium channel (rNa(V)1.4) may act as molecular switch allowing for interaction between outer and inner vestibules. Cysteine scanning mutagenesis of the internal part of DIV-S6 revealed that only mutations at site 1575 rescued the channel from a unique kinetic state ("ultra-slow inactivation," I(US)) produced by the mutation K1237E in the selectivity filter. A similar effect was seen with I1575A. Previously, we reported that conformational changes of both the internal and the external vestibule are involved in the generation of I(US). The fact that mutations at site 1575 modulate I(US) produced by K1237E strongly suggests an interaction between these sites. Our data confirm a previously published molecular model in which Ile-1575 of DIV-S6 is in close proximity to Lys-1237 of the selectivity filter. Furthermore, these functional data define the position of the selectivity filter relative to the adjacent DIV-S6 segment within the ionic permeation pathway.

Sodium channel molecular conformations and antiarrhythmic drug affinity.

Class I cardiac antiarrhythmic drugs, for example, lidocaine, mexiletine, flecainide, quinidine, and procainamide, continue to play an important role in the therapy for cardiac arrhythmias because of the presence of use-dependent block. Lidocaine, as well as related drugs such as mepivacaine, bupivacaine, and cocaine, also belong to the class of medications referred to as local anesthetics. In this review, we will consider lidocaine as the prototypical antiarrhythmic drug because it continues to be widely used both as an antiarrhythmic drug (first used as an antiarrhythmic drug in 1950) as well as a local anesthetic agent. Both of these clinical uses depend upon block of sodium current (I(Na)), but it is the presence of use-dependent I(Na) block, that is, an increasing amount of block at faster heart rates, which enables a local anesthetic agent to be a useful antiarrhythmic drug. Although many early studies investigated the action of antiarrhythmic drugs on Na currents, the availability of site-directed mutant Na channels has enabled for major advances in understanding their mechanisms of action based upon molecular conformations of the Na channel.

Molecular model of anticonvulsant drug binding to the voltage-gated sodium channel inner pore.

The tricyclic anticonvulsant drugs phenytoin, carbamazepine, and lamotrigine block neuronal voltage-gated Na(+) channels, and their binding sites to domain IV-S6 in the channel's inner pore overlap with those of local anesthetic drugs. These anticonvulsants are neutral, in contrast to the mostly positively charged local anesthetics, but their open/inactivated-state blocking affinities are similar. Using a model of the open pore of the Na(+) channel that we developed by homology with the crystal structures of potassium channels, we have docked these three anticonvulsants with residues identified by mutagenesis as important for their binding energy. The three drugs show a common pharmacophore, including an aromatic ring that has an aromatic-aromatic interaction with Tyr-1771 of Na(V)1.2 and a polar amide or imide that interacts with the aromatic ring of Phe-1764 by a low-energy amino-aromatic hydrogen bond. The second aromatic ring is nearly at a right angle to the pharmacophore and fills the pore lumen, probably interacting with the other S6 segments and physically occluding the inner pore to block Na(+) permeation. Hydrophobic interactions with this second aromatic ring may contribute an important component to binding for anticonvulsants, which compensates energetically for the absence of positive charge in their structures. Voltage dependence of block, their important therapeutic property, results from their interaction with Phe-1764, which connects them to the voltage sensors. Their use dependence is modest and this results from being neutral, with a fast drug off-rate after repolarization, allowing a normal action potential rate in the presence of the drugs.

The tetrodotoxin binding site is within the outer vestibule of the sodium channel.

Tetrodotoxin and saxitoxin are small, compact asymmetrical marine toxins that block voltage-gated Na channels with high affinity and specificity. They enter the channel pore's outer vestibule and bind to multiple residues that control permeation. Radiolabeled toxins were key contributors to channel protein purification and subsequent cloning. They also helped identify critical structural elements called P loops. Spacial organization of their mutation-identified interaction sites in molecular models has generated a molecular image of the TTX binding site in the outer vestibule and the critical permeation and selectivity features of this region. One site in the channel's domain I P loop determines affinity differences in mammalian isoforms.

Using lidocaine and benzocaine to link sodium channel molecular conformations to state-dependent antiarrhythmic drug affinity.

RATIONALE: Lidocaine and other antiarrhythmic drugs bind in the inner pore of voltage-gated Na channels and affect gating use-dependently. A phenylalanine in domain IV, S6 (Phe1759 in Na(V)1.5), modeled to face the inner pore just below the selectivity filter, is critical in use-dependent drug block. OBJECTIVE: Measurement of gating currents and concentration-dependent availability curves to determine the role of Phe1759 in coupling of drug binding to the gating changes. METHODS AND RESULTS: The measurements showed that replacement of Phe1759 with a nonaromatic residue permits clear separation of action of lidocaine and benzocaine into 2 components that can be related to channel conformations. One component represents the drug acting as a voltage-independent, low-affinity blocker of closed channels (designated as lipophilic block), and the second represents high-affinity, voltage-dependent block of open/inactivated channels linked to stabilization of the S4s in domains III and IV (designated as voltage-sensor inhibition) by Phe1759. A homology model for how lidocaine and benzocaine bind in the closed and open/inactivated channel conformation is proposed. CONCLUSIONS: These 2 components, lipophilic block and voltage-sensor inhibition, can explain the differences in estimates between tonic and open-state/inactivated-state affinities, and they identify how differences in affinity for the 2 binding conformations can control use-dependence, the hallmark of successful antiarrhythmic drugs.

Mutations in the insulin gene can cause MODY and autoantibody-negative type 1 diabetes.

OBJECTIVE: Mutations in the insulin (INS) gene can cause neonatal diabetes. We hypothesized that mutations in INS could also cause maturity-onset diabetes of the young (MODY) and autoantibody-negative type 1 diabetes. RESEARCH DESIGN AND METHODS: We screened INS in 62 probands with MODY, 30 probands with suspected MODY, and 223 subjects from the Norwegian Childhood Diabetes Registry selected on the basis of autoantibody negativity or family history of diabetes. RESULTS: Among the MODY patients, we identified the INS mutation c.137G>A (R46Q) in a proband, his diabetic father, and a paternal aunt. They were diagnosed with diabetes at 20, 18, and 17 years of age, respectively, and are treated with small doses of insulin or diet only. In type 1 diabetic patients, we found the INS mutation c.163C>T (R55C) in a girl who at 10 years of age presented with ketoacidosis and insulin-dependent, GAD, and insulinoma-associated antigen-2 (IA-2) antibody-negative diabetes. Her mother had a de novo R55C mutation and was diagnosed with ketoacidosis and insulin-dependent diabetes at 13 years of age. Both had residual beta-cell function. The R46Q substitution changes an invariant arginine residue in position B22, which forms a hydrogen bond with the glutamate at A17, stabilizing the insulin molecule. The R55C substitution involves the first of the two arginine residues localized at the site of proteolytic processing between the B-chain and the C-peptide. CONCLUSIONS: Our findings extend the phenotype of INS mutation carriers and suggest that INS screening is warranted not only in neonatal diabetes, but also in MODY and in selected cases of type 1 diabetes.

Insulin gene mutations as a cause of permanent neonatal diabetes.

We report 10 heterozygous mutations in the human insulin gene in 16 probands with neonatal diabetes. A combination of linkage and a candidate gene approach in a family with four diabetic members led to the identification of the initial INS gene mutation. The mutations are inherited in an autosomal dominant manner in this and two other small families whereas the mutations in the other 13 patients are de novo. Diabetes presented in probands at a median age of 9 weeks, usually with diabetic ketoacidosis or marked hyperglycemia, was not associated with beta cell autoantibodies, and was treated from diagnosis with insulin. The mutations are in critical regions of the preproinsulin molecule, and we predict that they prevent normal folding and progression of proinsulin in the insulin secretory pathway. The abnormally folded proinsulin molecule may induce the unfolded protein response and undergo degradation in the endoplasmic reticulum, leading to severe endoplasmic reticulum stress and potentially beta cell death by apoptosis. This process has been described in both the Akita and Munich mouse models that have dominant-acting missense mutations in the Ins2 gene, leading to loss of beta cell function and mass. One of the human mutations we report here is identical to that in the Akita mouse. The identification of insulin mutations as a cause of neonatal diabetes will facilitate the diagnosis and possibly, in time, treatment of this disorder.

Charge at the lidocaine binding site residue Phe-1759 affects permeation in human cardiac voltage-gated sodium channels.

Our homology molecular model of the open/inactivated state of the Na(+) channel pore predicts, based on extensive mutagenesis data, that the local anaesthetic lidocaine docks eccentrically below the selectivity filter, such that physical occlusion is incomplete. Electrostatic field calculations suggest that the drug's positively charged amine produces an electrostatic barrier to permeation. To test the effect of charge at this pore level on permeation in hNa(V)1.5 we replaced Phe-1759 of domain IVS6, the putative binding site for lidocaine's alkylamino end, with positively and negatively charged residues as well as the neutral cysteine and alanine. These mutations eliminated use-dependent lidocaine block with no effect on tonic/rested state block. Mutant whole cell currents were kinetically similar to wild type (WT). Single channel conductance (gamma) was reduced from WT in both F1759K (by 38%) and F1759R (by 18%). The negatively charged mutant F1759E increased gamma by 14%, as expected if the charge effect were electrostatic, although F1759D was like WT. None of the charged mutations affected Na(+)/K(+) selectivity. Calculation of difference electrostatic fields in the pore model predicted that lidocaine produced the largest positive electrostatic barrier, followed by lysine and arginine, respectively. Negatively charged glutamate and aspartate both lowered the barrier, with glutamate being more effective. Experimental data were in rank order agreement with the predicted changes in the energy profile. These results demonstrate that permeation rate is sensitive to the inner pore electrostatic field, and they are consistent with creation of an electrostatic barrier to ion permeation by lidocaine's charge.

An inner pore residue (Asn406) in the Nav1.5 channel controls slow inactivation and enhances mibefradil block to T-type Ca2+ channel levels.

Mibefradil is a tetralol derivative once marketed to treat hyper-tension. Its primary target is the T-type Ca(2+) channel (IC(50), approximately 0.1-0.2 microM), but it also blocks Na(+),K(+),Cl(-), and other Ca(2+) channels at higher concentrations. We have recently reported state-dependent mibefradil block of Na(+) channels in which apparent affinity was enhanced when channels were recruited to slow-inactivated conformations. The structural determinants controlling mibefradil block have not been identified, although evidence suggests involvement of regions near or within the inner pore. We tested whether mibefradil interacts with the local anesthetic (LA) binding site, which includes residues in the S6 segments of domains (D) I, III, and IV. Mutagenesis of DIII S6 and DIVS6 did not reveal critical binding determinants. Substitution of Asn406 in DI S6 of cardiac Na(v)1.5, however, altered affinity in a manner dependent on the identity of the substituting residue. Replacing Asn406 with a phenylalanine or a cysteine increased affinity by 4- and 7-fold, respectively, thus conferring T-type Ca(2+) channel-like mibefradil sensitivity to the Na(+) channel. A series of other substitutions that varied in size, charge, and hydrophobicity had minimal effects on mibefradil block, but all mutations dramatically altered the magnitude and voltage-dependence of slow inactivation, consistent with data in other isoforms. Channels did not slow-inactivate, however, at the voltages used to assay mibefradil block, supporting the idea that Asn406 lies within or near the mibefradil binding site.

Molecular modeling of local anesthetic drug binding by voltage-gated sodium channels.

Voltage-gated sodium (Na+) channels are targets for local anesthetic (LA) drugs that bind in the inner pore of the channel with affinities related to the channel gating states. Our core model of the sodium channel (P loops and S5 and S6 segments from each of the four domains) was closed because it was developed using coordinates from the KcsA channel crystallographic structure. We developed a model of the activated, open channel based on the structure of the open MthK channel, which was characterized by bends at the S6 glycine or serine residues. This created a conformation that allowed energetically appropriate docking of the LA drugs. The alkylamino head of ionizable LA molecules was docked closer to the selectivity filter and in association with Phe-1579 of IVS6 and Leu-1280 of IIIS6 (Nav1.4), and the aromatic ring interacted with Tyr-1586 of IVS6 and Asn-434 of IS6. Comparison of multiple LA drugs showed relative binding affinities in the model consistent with experimental studies. The ionizable LA alkylamino heads interact primarily by van der Waals forces that position the charge so as to create a positive electrostatic barrier for cation permeation. Permanently uncharged benzocaine could be docked in the closed conformation as well, stabilizing the closed conformation. The structurally different anticonvulsant lamotrigine and one of its derivatives have a binding site that fully overlaps with that of the LA drugs. The open, activated channel creates the high-affinity binding site for these sodium channel blocker drugs, and block may be mainly electrostatic.

Significance of prohormone convertase 2, PC2, mediated initial cleavage at the proglucagon interdomain site, Lys70-Arg71, to generate glucagon.

To define the biological significance of the initial cleavage at the proglucagon (PG) interdomain site, K70-R71 downward arrow, we created two interdomain mutants, K70Q-R71Q and R71A. Cotransfection studies in GH4C1 cells show significant amounts of glucagon production by PC2 along with some glicentin, glicentin-related polypeptide-glucagon (GRPP-glucagon) and oxyntomodulin from wild-type PG. In contrast, a larger peptide, PG 33-158, and low amounts of GRPP-glucagon are predominantly generated from interdomain mutants. HPLC analysis shows a 5-fold increase in glucagon production by PC2 from wild-type PG and a corresponding 4-fold lower accumulation and secretion of unprocessed precursor relative to interdomain mutants. PC2 generates significant levels of glucagon from a glicentin (PG 1-69) expression plasmid, whereas PC1/3 produces only modest amounts of oxyntomodulin. Employing a major PG fragment (PG 72-158) expression plasmid, we show that PC1/3 predominantly generates glucagon-like peptide (GLP)-1, whereas PC2 produces only N-terminally extended GLP-1. Surprisingly, production of GLP-1 and GLP-2 by PC1/3 from interdomain mutants, compared with wild-type PG, is not significantly impaired. In addition to PC2 and PC1/3, PC5/6A and furin are also able to cleave the sites, K70-R71 downward arrow and R107-X-R-R110 downward arrow in PG. We show a much greater ability of furin to cleave the monobasic site, R77 downward arrow, than at the dibasic site, R124-R125 downward arrow, which is also weakly processed by PC5/6A, indicating overlapping specificities of these two convertases mainly with PC1/3. We propose here a trimer-like model of the spatial organization of the hormonal sequences within the PG molecule in which the accessibility to prohormone convertase action of most cleavage sites is restricted with the exception of the interdomain site, K70-R71, which is maximally accessible.

Accessibility of mid-segment domain IV S6 residues of the voltage-gated Na+ channel to methanethiosulfonate reagents.

The inner pore of the voltage-gated Na+ channel is predicted by the structure of bacterial potassium channels to be lined with the four S6 alpha-helical segments. Our previously published model of the closed pore based on the KcsA structure, and our new model of the open pore based on the MthK structure predict which residues in the mid-portion of S6 face the pore. We produced cysteine mutants of the mid-portion of domain IV-S6 (Ile-1575-Leu-1591) in NaV 1.4 and tested their accessibility to intracellularly and extracellularly placed positively charged methanethiosulfonate (MTS) reagents. We found that only two mutants, F1579C and V1583C, were accessible to both outside and inside 2-(aminoethyl)-methanethiosulfonate hydrobromide (MTSEA) Further study of those mutants showed that efficient closure of the fast inactivation gate prevented block by inside [2-(trimethylammonium)ethyl]methanethiosulfonate bromide (MTSET) at slow stimulation rates. When fast inactivation was inhibited by exposure to anthropleurin B (ApB), increasing channel open time, both mutants were blocked by inside MTSET at a rate that depended on the amount of time the channel was open. Consistent with the fast inactivation gate limiting access to the pore, in the absence of ApB, inside MTSET produced block when the cells were stimulated at 5 or 20 Hz. We therefore suggest that the middle of IV-S6 is an alpha-helix, and we propose a model of the open channel, based on MthK, in which Phe-1579 and Val-1583 face the pore.

Mutational analysis of predicted interactions between the catalytic and P domains of prohormone convertase 3 (PC3/PC1).

The subtilisin-like prohormone convertases (PCs) contain an essential downstream domain (P domain), which has been predicted to have a beta-barrel structure that interacts with and stabilizes the catalytic domain (CAT). To assess possible sites of hydrophobic interaction, a series of mutant PC3-enhanced GFP constructs were prepared in which selected nonpolar residues on the surface of CAT were substituted by the corresponding polar residues in subtilisin Carlsberg. To investigate the folding potential of the isolated P domain, signal peptide-P domain-enhanced GFP constructs with mutated andor truncated P domains were also made. All mutants were expressed in betaTC3 cells, and their subcellular localization and secretion were determined. The mutants fell into three main groups: (i) Golgisecreted, (ii) ERnonsecreted, and (iii) apoptosis inducing. The destabilizing CAT mutations indicate that the side chains of V292, T328, L351, Q408, H409, V412, and F441 and nonpolar fragments of the side chains of R405 and W413 form a hydrophobic patch on CAT that interacts with the P domain. We also have found that the P domain can fold independently, as indicated by its secretion. Interestingly, T594, which is near the P domain C terminus, was not essential for P domain secretion but is crucial for the stability of intact PC3. T594V produced a stable enzyme, but T594D did not, which suggests that T594 participates in important hydrophobic interactions within PC3. These findings support our conclusion that the catalytic and P domains contribute to the folding and thermodynamic stability of the convertases through reciprocal hydrophobic interactions.

Molecular modeling of interactions of dihydropyridines and phenylalkylamines with the inner pore of the L-type Ca2+ channel.

Domains IIIS5, IIIS6, and IVS6 transmembrane segments of L-type Ca(2+) channels participate in dihydropyridine (DHP) and phenylalkylamine (PAA) binding. The inner pore structure of the Ca(v)1.2 channel was reconstructed from coordinates of the transmembrane alpha-helices of the KcsA channel. S6s were aligned with M2 by comparative analysis of the pore-facing M2 side chains and those required for drug binding. Two neighboring tilted S6 helices of domains III and IV below the selectivity filter formed an interdomain crevice. Docking of DHPs inside this crevice located the DHP ring between Phe-1159 of IIIS6 and Ala-1467 of IVS6, parallel to the pore axis, whereas the 4-aryl ring participated in aromatic and polar interactions with the side chains of Tyr-1152 and Tyr-1463. Nonpolar interactions of the port side ester group with hydrophobic side chains of Ile-1156, Ile-1163, and Ile-1471 on the bottom of the binding cavity, formed by the crossover of IIIS6 and IVS6, could stabilize the channel's closed/inactivated state. Similar arrangements were found for DHP agonist drugs, except for the absence of hydrophobic interactions with the helical crossing. In this arrangement, DHPs do not physically block the pore. Locating the central amine group of desmethoxyverapamil near the selectivity filter domain III glutamic acid allows one aromatic ring through its CH(2)CH(2) linker to interact with the side chain of Tyr-1463 inside the DHP binding site, whereas the opposite aromatic ring is in contact with the side chain of Ile-1470 of IVS6, blocking the pore.