Positive charges at the intracellular mouth of the pore regulate anion conduction in the CFTR chloride channel

Novel residues lining the CFTR chloride channel pore identified by functional modification of introduced cysteines

Substituted cysteine accessibility mutagenesis (SCAM) has been used widely to identify pore-lining amino acid side chains in ion channel proteins. However, functional effects on permeation and gating can be difficult to separate, leading to uncertainty concerning the location of reactive cysteine side chains. We have combined SCAM with investigation of the charge-dependent effects of methanethiosulfonate (MTS) reagents on the functional permeation properties of cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channels. We find that cysteines substituted for seven out of 21 continuous amino acids in the eleventh and twelfth transmembrane (TM) regions can be modified by external application of positively charged [2-(trimethylammonium)ethyl] MTS bromide (MTSET) and negatively charged sodium [2-sulfonatoethyl] MTS (MTSES). Modification of these cysteines leads to changes in the open channel current-voltage relationship at both the macroscopic and single-channel current levels that reflect specific, charge-dependent effects on the rate of Cl(-) permeation through the channel from the external solution. This approach therefore identifies amino acid side chains that lie within the permeation pathway. Cysteine mutagenesis of pore-lining residues also affects intrapore anion binding and anion selectivity, giving more information regarding the roles of these residues. Our results demonstrate a straightforward method of screening for pore-lining amino acids in ion channels. We suggest that TM11 contributes to the CFTR pore and that the extracellular loop between TMs 11 and 12 lies close to the outer mouth of the pore.

Changes in negative charge at the luminal mouth of the pore alter ion handling and gating in the cardiac ryanodine-receptor

We have tested the hypothesis that a high density of negative charge at the luminal mouth of the RyR2 pore plays a pivotal role in the high cation conductance and limited selectivity observed in this channel by introducing into each monomer a double point mutation to neutralize acidic residues in this region of the mouse RyR2 channel. The resultant channel, ED4832AA, is capable of functioning as a calcium-release channel in situ. Consistent with our hypothesis, the ED4832AA mutation altered the ion handling characteristics of single RyR2 channels. The mutant channel retains the ability to discriminate between cations and anions but cation conductance is altered significantly. Unitary K+ conductance is reduced at low levels of activity but increases dramatically as activity is raised and shows little sign of saturation. ED4832AA no longer discriminates between divalent and monovalent cations. In addition, the gating characteristics of single RyR2 channels are altered markedly by residue neutralization. Open probability in the ED4832AA channel is substantially higher than that of the wild-type channel. Moreover, at holding potentials in excess of +/-50 mV several subconductance states become apparent in ED4832AA and are more prevalent at very high holding potentials. These observations are discussed within the structural framework provided by a previously developed model of the RyR2 pore. Our data indicates that neutralization of acidic residues in the luminal mouth of the pore produces wide-ranging changes in the electric field in the pore, the interaction energies of permeant ions in the pore and the stability of the selectivity filter region of the pore, which together contribute to the observed changes ion handling and gating.

Mechanism of direct bicarbonate transport by the CFTR anion channel

BACKGROUND

CFTR contributes to HCO(3)(-) transport in epithelial cells both directly (by HCO(3)(-) permeation through the channel) and indirectly (by regulating Cl(-)/HCO(3)(-) exchange proteins). While loss of HCO(3)(-) transport is highly relevant to cystic fibrosis, the relative importance of direct and indirect HCO(3)(-) transport it is currently unknown.

METHODS

Patch clamp recordings from membrane patches excised from cells heterologously expressing wild type and mutant forms of human CFTR were used to isolate directly CFTR-mediated HCO(3)(-) transport and characterize its functional properties.

RESULTS

The permeability of HCO(3)(-) was approximately 25% that of Cl(-) and was invariable under all ionic conditions studied. CFTR-mediated HCO(3)(-) currents were inhibited by open channel blockers DNDS, glibenclamide and suramin, and these inhibitions were affected by mutations within the channel pore. Cystic fibrosis mutations previously associated with disrupted cellular HCO(3)(-) transport did not affect direct HCO(3)(-) permeability.

CONCLUSIONS

Cl(-) and HCO(3)(-) share a common transport pathway in CFTR, and selectivity between Cl(-) and HCO(3)(-) is independent of ionic conditions. The mechanism of transport is therefore effectively identical for both ions. We suggest that mutations in CFTR that cause cystic fibrosis by selectively disrupting HCO(3)(-) transport do not impair direct CFTR-mediated HCO(3)(-) transport, but may predominantly alter CFTR regulation of other HCO(3)(-) transport pathways.

Evidence for direct CFTR inhibition by CFTR(inh)-172 based on Arg347 mutagenesis

CFTR (cystic fibrosis transmembrane conductance regulator) is an epithelial Cl- channel inhibited with high affinity and selectivity by the thiazolidinone compound CFTR(inh)-172. In the present study, we provide evidence that CFTR(inh)-172 acts directly on the CFTR. We introduced mutations in amino acid residues of the sixth transmembrane helix of the CFTR protein, a domain that has an important role in the formation of the channel pore. Basic and hydrophilic amino acids at positions 334-352 were replaced with alanine residues and the sensitivity to CFTR(inh)-172 was assessed using functional assays. We found that an arginine-to-alanine change at position 347 reduced the inhibitory potency of CFTR(inh)-172 by 20-30-fold. Mutagenesis of Arg347 to other amino acids also decreased the inhibitory potency, with aspartate producing near total loss of CFTR(inh)-172 activity. The results of the present study provide evidence that CFTR(inh)-172 interacts directly with CFTR, and that Arg347 is important for the interaction.

Identification of positive charges situated at the outer mouth of the CFTR chloride channel pore

We have used site-directed mutagenesis and functional analysis to identify positively charged amino acid residues in the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel that interact with extracellular anions. Mutation of two positively charged arginine residues in the first extracellular loop (ECL) of CFTR, R104, and R117, as well as lysine residue K335 in the sixth transmembrane region, leads to inward rectification of the current-voltage relationship and decreased single channel conductance. These effects are dependent on the charge of the substituted side chain and on the Cl(-) concentration, suggesting that these positive charges normally act to concentrate extracellular Cl(-) ions near the outer mouth of the pore. Side chain charge-dependent effects are mimicked by manipulating charge in situ by mutating these amino acids to cysteine followed by covalent modification with charged cysteine-reactive reagents, confirming the location of these side chains within the pore outer vestibule. State-independent modification of R104C and R117C suggests that these residues are located at the outermost part of the pore. We suggest that ECL1 contributes to the CFTR pore external vestibule and that positively charged amino acid side chains in this region act to attract Cl(-) ions into the pore. In contrast, we find no evidence that fixed positive charges in other ECLs contribute to the permeation properties of the pore.

Mutations at arginine 352 alter the pore architecture of CFTR

Arginine 352 (R352) in the sixth transmembrane domain of the cystic fibrosis transmembrane conductance regulator (CFTR) previously was reported to form an anion/cation selectivity filter and to provide positive charge in the intracellular vestibule. However, mutations at this site have nonspecific effects, such as inducing susceptibility of endogenous cysteines to chemical modification. We hypothesized that R352 stabilizes channel structure and that charge-destroying mutations at this site disrupt pore architecture, with multiple consequences. We tested the effects of mutations at R352 on conductance, anion selectivity and block by the sulfonylurea drug glipizide, using recordings of wild-type and mutant channels. Charge-altering mutations at R352 destabilized the open state and altered both selectivity and block. In contrast, R352K-CFTR was similar to wild-type. Full conductance state amplitude was similar to that of wild-type CFTR in all mutants except R352E, suggesting that R352 does not itself form an anion coordination site. In an attempt to identify an acidic residue that may interact with R352, we found that permeation properties were similarly affected by charge-reversing mutations at D993. Wild-type-like properties were rescued in R352E/D993R-CFTR, suggesting that R352 and D993 in the wild-type channel may interact to stabilize pore architecture. Finally, R352A-CFTR was sensitive to modification by externally applied MTSEA+, while wild-type and R352E/D993R-CFTR were not. These data suggest that R352 plays an important structural role in CFTR, perhaps reflecting its involvement in forming a salt bridge with residue D993.

Molecular basis for the ATPase activity of CFTR

CFTR is a member of the ABC (ATP binding cassette) superfamily of transporters. It is a multidomain membrane protein, which utilizes ATP to regulate the flux of its substrate through the membrane. CFTR is distinct in that it functions as a channel and it possesses a unique regulatory R domain. There has been significant progress in understanding the molecular basis for CFTR activity as an ATPase. The dimeric complex of NBD structures seen in prokaryotic ABC transporters, together with the structure of an isolated CF-NBD1, provide a unifying molecular template to model the structural basis for the ATPase activity of CFTR. The dynamic nature of the interaction between the NBDs and the R domain has been revealed in NMR studies. On the other hand, understanding the mechanisms mediating the transmission of information from the cytosolic domains to the membrane and the channel gate of CFTR remains a central challenge.

On the nature of antimicrobial activity: a model for protegrin-1 pores

We conducted over 150 ns of simulation of a protegrin-1 octamer pore in a lipid bilayer composed of palmitoyloleoyl-phosphatidylethanolamine (POPE) and palmitoyloleoyl-phosphatidylglycerol (POPG) lipids mimicking the inner membrane of a bacterial cell. The simulations improve on a model of a pore proposed from recent NMR experiments and provide a coherent understanding of the molecular mechanism of antimicrobial activity. Although lipids tilt somewhat toward the peptides, the simulated protegrin-1 pore more closely follows the barrel-stave model than the toroidal-pore model. The movement of ions is investigated through the pore. The pore selectively allows negatively charged chloride ions to pass through at an average rate of one ion every two nanoseconds. Only two events are observed of sodium ions crossing through the pore. The potential of mean force is calculated for the water and both ion types. It is determined that the chloride ions move through the pore with ease, similarly to the water molecules with the exception of a zone of restricted movement midway through the pore. In bacteria, ions moving through the pore will compromise the integrity of the transmembrane potential. Without the transmembrane potential as a countermeasure, water will readily flow inside the higher osmolality cytoplasm. We determine that the diffusivity of water through a single PG-1 pore is sufficient to cause fast cell death by osmotic lysis.

Direct and indirect effects of mutations at the outer mouth of the cystic fibrosis transmembrane conductance regulator chloride channel pore

The cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel pore is thought to contain multiple binding sites for permeant and impermeant anions. Here, we investigate the effects of mutation of different positively charged residues in the pore on current inhibition by impermeant Pt(NO(2)) (4) (2-) and suramin anions. We show that mutations that remove positive charges (K95, R303) influence interactions with intracellular, but not extracellular, Pt(NO(2))(4)(2-) ions, consistent with these residues being situated within the pore inner vestibule. In contrast, mutation of R334, supposedly located in the outer vestibule of the pore, affects block by both extracellular and intracellular Pt(NO(2))(4)(2-). Inhibition by extracellular Pt(NO(2))(4)(2-) requires a positive charge at position 334, consistent with a direct electrostatic interaction resulting in either open channel block or surface charge screening. In contrast, inhibition by intracellular Pt(NO(2))(4)(2-) is weakened in all R334-mutant forms of the channel studied, inconsistent with a direct interaction. Furthermore, mutation of R334 had similar effects on block by intracellular suramin, a large organic molecule that is apparently unable to enter deeply into the channel pore. Mutation of R334 altered interactions between intracellular Pt(NO(2))(4)(2-) and extracellular Cl(-) but not those between intracellular Pt(NO(2))(4)(2-) and extracellular Pt(NO(2))(4)(2-). We propose that while the positive charge of R334 interacts directly with extracellular anions, mutation of this residue also alters interactions with intracellular anions by an indirect mechanism, due to mutation-induced conformational changes in the protein that are propagated some distance from the site of the mutation in the outer mouth of the pore.

CFTR inhibition by glibenclamide requires a positive charge in cytoplasmic loop three

The sulfonylurea glibenclamide is widely used as an open-channel blocker of the CFTR chloride channel. Here, we used site-directed mutagenesis to identify glibenclamide site of interaction: a positively charged residue K978, located in the cytoplasmic loop 3. Charge-neutralizing mutations K978A, K978Q, K978S abolished the inhibition of forskolin-activated CFTR chloride current by glibenclamide but not by CFTR(inh)-172. The charge-conservative mutation K978R did not alter glibenclamide sensitivity of CFTR current. Mutations of the neighbouring R975 (R975A, R975S, R975Q) did not affect electrophysiological and pharmacological properties of CFTR. No alteration of halide selectivity was observed with any of these CFTR mutant channels. This study identifies a novel potential inhibitor site within the CFTR molecule, and suggests a novel role of cytoplasmic loop three, within the second transmembrane domain of CFTR protein. This work is the first to report on the role of a residue in a cytoplasmic loop in the mechanism of action of the channel blocker glibenclamide.

Structural basis of Na(+)/K(+)-ATPase adaptation to marine environments

Throughout evolution, enzymes have adapted to perform in different environments. The Na(+)/K(+) pump, an enzyme crucial for maintaining ionic gradients across cell membranes, is strongly influenced by the ionic environment. In vertebrates, the pump sees much less external Na(+) (100-160 mM) than it does in osmoconformers such as squid (450 mM), which live in seawater. If the extracellular architecture of the squid pump were identical to that of vertebrates, then at the resting potential, the pump's function would be severely compromised because the negative voltage would drive Na(+) ions back to their binding sites, practically abolishing forward transport. Here we show that four amino acids that ring the external mouth of the ion translocation pathway are more positive in squid, thereby reducing the pump's sensitivity to external Na(+) and explaining how it can perform optimally in the marine environment.

Molecular mechanism of arachidonic acid inhibition of the CFTR chloride channel

Arachidonic acid inhibits the activity of a number of different Cl- channels, however its molecular mechanism of action is not known. Here we show that inhibition of cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channels by arachidonic acid is weakened following mutagenesis of two positively charged pore-lining amino acids. Charge-neutralizing mutants K95Q and R303Q both increased the Kd for inhibition from approximately 3.5 microM in wild type to approximately 17 microM. At both sites, the effects of mutagenesis were dependent of the charge of the substituted side chain. We suggest that arachidonic acid interacts electrostatically with positively charged amino acid side chains in the cytoplasmic vestibule of the CFTR channel pore to block Cl- permeation.

Role of nonconserved charged residues of the AE2 transmembrane domain in regulation of anion exchange by pH

The ubiquitous AE2/SLC4A2 anion exchanger is acutely and independently regulated by intracellular (pH(i)) and extracellular pH (pH(o)), whereas the closely related AE1/SLC4A1 of the red cell and renal intercalated cell is relatively pH-insensitive. We have investigated the contribution of nonconserved charged residues within the C-terminal transmembrane domain (TMD) of AE2 to regulation by pH through mutation to the corresponding AE1 residues. AE2-mediated Cl(-)/Cl(-) exchange was measured as 4,4'-di-isothiocyanatostilbene-2,2'-disulfonic acid-sensitive (36)Cl(-) efflux from Xenopus oocytes by varying pH(i) at constant pH(o), and by varying pH(o) at near-constant pH(i). All mutations of nonconserved charged residues of the AE2 TMD yielded functional protein, but mutations of some conserved charged residues (R789E, R1056A, R1134C) reduced or abolished function. Individual mutation of AE2 TMD residues R921, F922, P1077, and R1107 exhibited reduced pH(i) sensitivity compared to wt AE2, whereas TMD mutants K1153R, R1155K, R1202L displayed enhanced sensitivity to acidic pH(i). In addition, pH(o) sensitivity was significantly acid- shifted when nonconserved AE2 TMD residues E981, K982, and D1075 were individually converted to the corresponding AE1 residues. These results demonstrate that multiple conserved charged residues are important for basal transport function of AE2 and that certain nonconserved charged residues of the AE2 TMD are essential for wild-type regulation of anion exchange by pH(i) and pH(o).

Identification of a second blocker binding site at the cytoplasmic mouth of the cystic fibrosis transmembrane conductance regulator chloride channel pore

Chloride transport by the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel is inhibited by a broad range of substances that bind within a wide inner vestibule in the pore and physically occlude Cl(-) permeation. Binding of many of these so-called open-channel blockers involves electrostatic interactions with a positively charged lysine residue (Lys95) located in the pore. Here, we use site-directed mutagenesis to identify a second blocker binding site located at the cytoplasmic mouth of the pore. Mutagenesis of a positively charged arginine at the cytoplasmic mouth of the pore, Arg303, leads to significant weakening of the blocking effects of suramin, a large negatively charged organic molecule. Apparent suramin affinity is correlated with the side chain charge at this position, consistent with an electrostatic interaction. In contrast, block by suramin is unaffected by mutagenesis of Lys95, suggesting that it does not approach close to this important pore-forming lysine residue. We propose that the CFTR pore inner vestibule contains two distinct blocker binding sites. Relatively small organic anions enter deeply into the pore to interact with Lys95, causing an open-channel block that is sensitive to both the membrane potential and the extracellular Cl(-) concentration. Larger anionic molecules can become lodged in the cytoplasmic mouth of the pore where they interact with Arg303, causing a distinct type of open-channel block that is insensitive to membrane potential or extracellular Cl(-) ions. The pore may narrow significantly between the locations of these two blocker binding sites.