Mapping the structural requirements for nicotinic acetylcholine receptor activation by using tethered alkyltrimethylammonium agonists and antagonists

A molecule as simple in structure as tetramethylammonium gates the nicotinic acetylcholine receptor (nAChR) with high efficacy. To compare the structure of the nAChR transmitter binding site in the open channel state with that of the ACh binding protein, we determined the efficacy of nAChR gating by -S(CH(2))(n)N(CH(3))(3)(+) (n = 1-4) tethered to substituted cysteines at positions in the alpha subunits or gamma and delta subunits predicted to contribute to the ACh binding sites in mutant Torpedo nAChRs expressed in Xenopus oocytes. For tethered thiocholine [-S(CH(2))(2)N(CH(3))(3)(+)], we previously reported that within alpha195-201 gating was observed only at alphaY198C while at alphaY93C it acted as an antagonist. We now show that within alpha191-194, thiocholine activates when tethered at alphaCys192 or alphaCys193. Thiocholine also activates when tethered at alphaY190C or alphaW149C in nAChRs containing a beta subunit mutation (betaL257S) that destabilizes the closed channel, but not from gammaW55C/deltaW57C, where longer adducts can activate. When tethered at positions in binding site segment E, thiocholine activates only from gammaL119C/deltaL121C, where the shorter -S(CH(2))(1)N(CH3)(3)(+) acts as an antagonist. Longer adducts tethered at gammaL109C/deltaL111C or gammaL119C/deltaL121C also activate, but less efficiently. The length requirements for efficient gating by tethered agonists agree closely with predictions based upon the structure of the agonist site in a nAChR homology model derived from the ACh binding protein structure, which suggests that this structure is an excellent model of the nAChR agonist binding site in the open channel conformation. The inability of thiocholine to activate from alphaY93C, which is not predicted by the model, is discussed in terms of the structure of the nAChR in the closed state.     

Additional acetylcholine (ACh) binding site at alpha4/alpha4 interface of (alpha4beta2)2alpha4 nicotinic receptor influences agonist sensitivity

Nicotinic acetylcholine receptor (nAChR) α4 and β2 subunits assemble in two alternate stoichiometries to produce (α4β2)(2)α4 and (α4β2)(2)β2, which display different agonist sensitivities. Functionally relevant agonist binding sites are thought to be located at α4(+)/β2(-) subunit interfaces, but because these interfaces are present in both receptor isoforms, it is unlikely that they account for differences in agonist sensitivities. In contrast, incorporation of either α4 or β2 as auxiliary subunits produces isoform-specific α4(+)/α4(-) or β2(+)/β2(-) interfaces. Using fully concatenated (α4β2)(2)α4 nAChRs in conjunction with structural modeling, chimeric receptors, and functional mutagenesis, we have identified an additional site at the α4(+)/α4(-) interface that accounts for isoform-specific agonist sensitivity of the (α4β2)(2)α4 nAChR. The additional site resides in a region that also contains a potentiating Zn(2+) site but is engaged by agonists to contribute to receptor activation. By engineering α4 subunits to provide a free cysteine in loop C at the α4(+)α4(-) interface, we demonstrated that the acetylcholine responses of the mutated receptors are attenuated or enhanced, respectively, following treatment with the sulfhydryl reagent [2-(trimethylammonium)ethyl]methanethiosulfonate or aminoethyl methanethiosulfonate. The findings suggest that agonist occupation of the site at the α4(+)/(α4(-) interface leads to channel gating through a coupling mechanism involving loop C. Overall, we propose that the additional agonist site at the α4(+)/α4(-) interface, when occupied by agonist, contributes to receptor activation and that this additional contribution underlies the agonist sensitivity signature of (α4β2)(2)α4 nAChRs.     

Alternating access and a pore-loop structure in the Na+-citrate transporter CitS of Klebsiella pneumoniae

CitS of Klebsiella pneumoniae is a secondary transporter that transports citrate in symport with 2 Na(+) ions. Reaction of Cys-398 and Cys-414, which are located in a cytoplasmic loop of the protein that is believed to be involved in catalysis, with thiol reagents resulted in significant inhibition of uptake activity. The reactivity of the two residues was determined in single Cys mutants in different catalytic states of the transporter and from both sides of the membrane. The single Cys mutants were shown to have the same transport stoichiometry as wild type CitS, but the C398S mutation was responsible for a 10-fold loss of affinity for Na(+). Both cysteine residues were accessible from the periplasmic as well as from the cytoplasmic side of the membrane by the membrane-impermeable thiol reagent [2-(trimethylammonium)ethyl] methanethiosulfonate bromide (MTSET) suggesting that the residues are part of the translocation site. Binding of citrate to the outward facing binding site of the transporter resulted in partial protection against inactivation by N-ethylmaleimide, whereas binding to the inward facing binding site resulted in essentially complete protection. A 10-fold higher concentration of citrate was required at the cytoplasmic rather than at the periplasmic side of the membrane to promote protection. Only marginal effects of citrate binding were seen on reactivity with MTSET. Binding of Na(+) at the periplasmic side of the transporter protected both Cys-398 and Cys-414 against reaction with the thiol reagents, whereas binding at the cytoplasmic side was less effective and discriminated between Cys-398 and Cys-414. A model is presented in which part of the cytoplasmic loop containing Cys-398 and Cys-414 folds back into the translocation pore as a pore-loop structure. The loop protrudes into the pore beyond the citrate-binding site that is situated at the membrane-cytoplasm interface.     

Photolabeling the Torpedo nicotinic acetylcholine receptor with 4-azido-2,3,5,6-tetrafluorobenzoylcholine, a partial agonist

The interactions of a photoreactive analogue of benzoylcholine, 4-azido-2,3,5,6-tetrafluorobenzoylcholine (APFBzcholine), with nicotinic acetylcholine receptors (nAChRs) were studied using electrophysiology and photolabeling. APFBzcholine acted as a low-efficacy partial agonist, eliciting maximal responses that were 0.3 and 0.1% of that of acetylcholine for embryonic mouse and Torpedo nAChRs expressed in Xenopus oocytes, respectively. Equilibrium binding studies of [3H]APFBzcholine with nAChR-rich membranes from Torpedo electric organ revealed equal affinities (K(eq) = 12 microM) for the two agonist binding sites. Upon UV irradiation at 254 nm, [3H]APFBzcholine was photoincorporated into the nAChR alpha, gamma, and delta subunits in an agonist-inhibitable manner. Photolabeled amino acids in the agonist binding sites were identified by Edman degradation of isolated, labeled subunit fragments. [3H]APFBzcholine photolabeled gammaLeu-109/deltaLeu-111, gammaTyr-111, and gammaTyr-117 in binding site segment E as well as alphaTyr-198 in alpha subunit binding site segment C. The observed pattern of photolabeling is examined in relation to the predicted orientation of the azide when APFBzcholine is docked in the agonist binding site of a homology model of the nAChR extracellular domain based upon the structure of the snail acetylcholine binding protein.     

Probing the agonist domain of the nicotinic acetylcholine receptor by cysteine scanning mutagenesis reveals residues in proximity to the alpha-bungarotoxin binding site

We have constructed a series of cysteine-substitution mutants in order to identify residues in the mouse muscle nicotinic acetylcholine receptor (AChR) that are involved in alpha-bungarotoxin (alpha-Bgtx) binding. Following transient expression in HEK 293-derived TSA-201 cells, covalent modification of the introduced cysteines with thiol-specific reagents reveals that alpha subunit residues W187, V188, F189, Y190, and P194 are solvent accessible and are in a position to contribute to the alpha-Bgtx binding site in native receptors. These results with the intact receptor are consistent with NMR studies of an alpha-Bgtx/receptor-dodecapeptide complex [Basus, V., Song., G., and Hawrot, E. (1993) Biochemistry 32, 12290-12298]. We pursued a more detailed analysis of the F189C mutant as this site varies substantially between AChRs that bind Bgtx and certain neuronal AChRs that do not. Treatment of intact cells expressing F189C with either bromoacetylcholine (BrACh) or [2-(trimethylammonium)ethyl] methane-thiosulfonate (MTSET), both methylammonium-containing thiol-modifying reagents with agonist properties, results in a marked decrease ( approximately 55-70%) in the number of alpha-Bgtx binding sites, as measured under saturating conditions. The decrease in sites appears to affect both alpha/gamma and alpha/delta sites to the same extent, as shown for alphaW187C and alphaF189C which were the two mutants examined on this issue. In contrast to the results obtained with MTSET and BrACh, modification with reagents that lack the alkylammonium entity, such as methylmethanethiosulfonate (MMTS), the negatively charged 2-sulfonatoethyl methane-thiosulfonate (MTSES), or the positively charged aminoethyl methylthiosulfonate (MTSEA), has little or no effect on the maximal binding of alpha-Bgtx to the alphaW187C, alphaV188C, or alphaF189C mutant receptors. The striking alkylammonium dependency suggests that an interaction of the tethered modifying group with the negative subsite within the agonist binding domain is primarily responsible for the observed blockade of toxin binding.     

Crystal structures of a cysteine-modified mutant in loop D of acetylcholine-binding protein

Covalent modification of α7 W55C nicotinic acetylcholine receptors (nAChR) with the cysteine-modifying reagent [2-(trimethylammonium)ethyl] methanethiosulfonate (MTSET(+)) produces receptors that are unresponsive to acetylcholine, whereas methyl methanethiolsulfonate (MMTS) produces enhanced acetylcholine-gated currents. Here, we investigate structural changes that underlie the opposite effects of MTSET(+) and MMTS using acetylcholine-binding protein (AChBP), a homolog of the extracellular domain of the nAChR. Crystal structures of Y53C AChBP show that MTSET(+)-modification stabilizes loop C in an extended conformation that resembles the antagonist-bound state, which parallels our observation that MTSET(+) produces unresponsive W55C nAChRs. The MMTS-modified mutant in complex with acetylcholine is characterized by a contracted C-loop, similar to other agonist-bound complexes. Surprisingly, we find two acetylcholine molecules bound in the ligand-binding site, which might explain the potentiating effect of MMTS modification in W55C nAChRs. Unexpectedly, we observed in the MMTS-Y53C structure that ten phosphate ions arranged in two rings at adjacent sites are bound in the vestibule of AChBP. We mutated homologous residues in the vestibule of α1 GlyR and observed a reduction in the single channel conductance, suggesting a role of this site in ion permeation. Taken together, our results demonstrate that targeted modification of a conserved aromatic residue in loop D is sufficient for a conformational switch of AChBP and that a defined region in the vestibule of the extracellular domain contributes to ion conduction in anion-selective Cys-loop receptors.     

Serotonin and cocaine-sensitive inactivation of human serotonin transporters by methanethiosulfonates targeted to transmembrane domain I

To explore aqueous accessibility and functional contributions of transmembrane domain (TM) 1 in human serotonin transporter (hSERT) proteins, we utilized the largely methanethiosulfonate (MTS) insensitive hSERT C109A mutant and mutated individual residues of hSERT TM1 to Cys followed by tests of MTS inactivation of 5-hydroxytryptamine (5-HT) transport. Residues in TM1 cytoplasmic to Gly-94 were largely unaffected by Cys substitution, whereas the mutation of residues extracellular to Ile-93 variably diminished transport activity. TM1 Cys substitutions displayed differential sensitivity to MTS reagents, with residues more cytoplasmic to Asp-98 being largely insensitive to MTS inactivation. Aminoethylmethanethiosulfonate (MTSEA), [2-(trimethylammonium) ethyl]methanethiosulfonate bromide (MTSET), and sodium (2-sulfonatoethyl)-methanethiosulfonate (MTSES) similarly and profoundly inactivated 5-HT transport by SERT mutants D98C, G100C, W103C, and Y107C. MTSEA uniquely inactivated transport activity of S91C, G94C, Y95C but increased activity at I108C. MTSEA and MTSET, but not MTSES, inactivated transport function at N101C. Notably, 5-HT provided partial to complete protection from MTSET inactivation for D98C, G100C, N101C, and Y107C. Equivalent blockade of MTSET inactivation at N101C was observed with 5-HT at both room temperature and at 4 degrees C, inconsistent with major conformational changes leading to protection. Notably, cocaine also protected MTSET inactivation of G100C and N101C, although MTS incubations with N101C that eliminate 5-HT transport do not preclude cocaine analog binding nor its inhibition by 5-HT. 5-HT modestly enhanced the inactivation by MTSET at I93C and Y95C, whereas cocaine significantly enhanced MTSET sensitivity at Y107C and I108C. In summary, our studies reveal physical differences in TM1 accessibility to externally applied MTS reagents and reveal sites supporting substrate and antagonist modulation of MTS inactivation. Moreover, we identify a limit to accessibility for membrane-impermeant MTS reagents that may reflect aspects of an occluded permeation pathway.     

Epithelial sodium channel pore region. structure and role in gating

Epithelial sodium channels (ENaC) have a crucial role in the regulation of extracellular fluid volume and blood pressure. To study the structure of the pore region of ENaC, the susceptibility of introduced cysteine residues to sulfhydryl-reactive methanethiosulfonate derivatives ((2-aminoethyl)methanethiosulfonate hydrobromide (MTSEA) and [(2-(trimethylammonium)ethyl]methanethiosulfonate bromide (MTSET)) and to Cd(2+) was determined. Selected mutants within the amino-terminal portion (alphaVal(569)-alphaTrp(582)) of the pore region responded to MTSEA, MTSET, or Cd(2+) with stimulation or inhibition of whole cell Na(+) current. The reactive residues were not contiguous but were separated by 2-3 residues where substituted cysteine residues did not respond to the reagents and line one face of an alpha-helix. The activation of alphaS580Cbetagamma mENaC by MTSET was associated with a large increase in channel open probability. Within the carboxyl-terminal portion (alphaSer(583)-alphaSer(592)) of the pore region, only one mutation (alphaS583C) conferred a rapid, nearly complete block by MTSEA, MTSET, and Cd(2+), whereas several other mutant channels were partially blocked by MTSEA or Cd(2+) but not by MTSET. Our data suggest that the outer pore of ENaC is formed by an alpha-helix, followed by an extended region that forms a selectivity filter. Furthermore, our data suggest that the pore region participates in ENaC gating.     

Catalytic phosphorylation of Na,K-ATPase drives the outward movement of its cation-binding H5-H6 hairpin

The Na,K-ATPase undergoes conformational transitions during its catalytic cycle that mediate energy transduction between the phosphorylation and cation-binding sites. Structure-function studies have shown that transmembrane segments H5 and H6 in the alpha subunit of the enzyme participate in cation binding and transport. The Ca-ATPase crystal structure indicates that the H5 helix extends into the cytoplasmic ATP binding domain, finishing 4-5 A from the phosphorylation site. Here, we test whether the phosphorylation of the Na,K-ATPase leads to conformational changes in the cation-binding H5-H6 hairpin. Using as background an enzyme where all wild-type Cys in the transmembrane region were replaced, Cys were introduced in the joining loop and extracellular ends of H5 and H6. Mutated proteins were expressed in COS cells and probed with Hg(2+), [2-(trimethylammonium)ethyl]methanethiosulfonate (MTSET), and biotin-maleimide, applied to the extracellular media while placing the cells in two different media (K-medium and Na-medium). We assumed that under these treatment conditions most of the enzyme would be in one of two predominant conformations: E1 (K-medium) and E2P (Na-medium). The extent of enzyme inactivation by Hg(2+) or MTSET treatment was dependent on the targeted position; i.e., proteins carrying Cys in the outermost positions were more affected by treatment. Moreover, in the case of proteins carrying Cys at positions 785, 787, and 797, driving the enzyme to phosphorylated conformations (Na-media) led to a larger inactivation. Similarly, biotinylation of introduced Cys was also influenced by the enzyme conformation, with a larger extent of modification after treatment of cells in the Na-medium (E2P form). These results can be explained by the enzyme phosphorylation driving the outward movement of the H5 helix. Thus, they provide experimental evidence for a structure-function mechanism where, via H5, enzyme phosphorylation leads to a conformational change at the cation-binding site and the consequent cation translocation.     

We were there..

Polyamine block of inwardly rectifying potassium (Kir) channels underlies their steep voltage dependence observed in vivo. We have examined the potency, voltage dependence, and kinetics of spermine block in dimeric Kir2.1 constructs containing one nonreactive subunit and one cysteine-substituted subunit before and after modification by methanethiosulfonate (MTS) reagents. At position 169C (between the D172 “rectification controller” and the selectivity filter), modification by either 2-aminoethyl MTS (MTSEA) or 2-(trimethylammonium)ethyl MTS (MTSET) reduced the potency and voltage dependence of spermine block, consistent with this position overlapping the spermine binding site. At position 176C (between D172 and the M2 helix bundle crossing), modification by MTSEA also weakened spermine block. In contrast, MTSET modification of 176C dramatically slowed the kinetics of spermine unblock, with almost no effect on potency or voltage dependence. The data are consistent with MTSET modification of 176C introducing a localized barrier in the inner cavity, resulting in slower spermine entry into and exit from a “deep” binding site (likely between the D172 rectification controller and the selectivity filter), but leaving the spermine binding site mostly unaffected. These findings constrain the location of deep spermine binding that underlies steeply voltage-dependent block, and further suggest important chemical details of high affinity binding of spermine in Kir2.1 channels—the archetypal model of strong inward rectification.     

Analysis of the glucagon receptor first extracellular loop by the substituted cysteine accessibility method

Glucagon is an important hormone for the prevention of hypoglycemia, and contributes to the hyperglycemia observed in diabetic patients, yet very little is known about its receptor structure and the receptor-glucagon interaction. In related receptors, the first extracellular loop, ECL1, is highly variable in length and sequence, suggesting that it might participate in ligand recognition. We applied a variant of the SCAM (Substituted Cysteine Accessibility Method) to the glucagon receptor ECL1 and sequentially mutated positions 197 to 223 to cysteine. Most of the mutations (15/27) affected the glucagon potency, due either to a modification of the glucagon binding site, or to the destabilization of the active receptor conformation. We reasoned that side chains accessible to glucagon must also be accessible to large, hydrophilic cysteine reagents. We therefore evaluated the accessibility of the introduced cysteines to maleimide-PEO₂-biotin ((+)-biotinyl-3-maleimido-propionamidyl-3,6-dioxa-octanediamine), and tested the effect of pretreatment of intact cells with a large cationic cysteine reagent, MTSET ([2-(trimethylammonium)ethyl]methanethiosulfonate bromide), on glucagon potency. Our results suggest that the second and third transmembrane helices (TM2 and TM3) are extended to position 202 and from position 215, respectively, and separated by a short β stretch (positions 203-209). Glucagon binding induced a conformational change close to TM2: L198C was accessible to the biotin reagent only in the presence of glucagon. Most other mutations affected the receptor activation rather than glucagon recognition, but S217 and D218 (at the top of TM3) were good candidates for glucagon recognition and V221 was very close to the binding site.     

Evidence for distinct sodium-, dopamine-, and cocaine-dependent conformational changes in transmembrane segments 7 and 8 of the dopamine transporter

Previously we obtained evidence based on engineering of Zn2+ binding sites that the extracellular parts of transmembrane segment 7 (TM7) and TM8 in the human dopamine transporter are important for transporter function. To further evaluate the role of this domain, we have employed the substituted cysteine accessibility method and performed 10 single cysteine substitutions at the extracellular ends of TM7 and TM8. The mutants were made in background mutants of the human dopamine transporter with either two (E2C) or five endogenous cysteines substituted (X5C) that render the transporter largely insensitive to cysteine modification. In two mutants (M371C and A399C), treatment with the sulfhydryl-reactive reagent [2-(trimethylammonium)-ethyl]methanethiosulfonate (MTSET) led to a substantial inhibition of [3H]dopamine uptake. In M371C this inactivation was enhanced by Na+ and blocked by dopamine. Inhibitors such as cocaine did not alter the effect of MTSET in M371C. The protection of M371C inactivation by dopamine required Na+. Because dopamine binding is believed to be Na+-independent, this suggests that dopamine induces a transport-associated conformational change that decreases the reactivity of M371C with MTSET. In contrast to M371C, cocaine decreased the reaction rate of A399C with MTSET, whereas dopamine had no effect. The protection by cocaine can either reflect that Ala-399 lines the cocaine binding crevice or that cocaine induces a conformational change that decreases the reactivity of A399C. The present findings add new functionality to the TM7/8 region by providing evidence for the occurrence of distinct Na+-, substrate-, and perhaps inhibitor-induced conformational changes critical for the proper function of the transporter.     

Three surface subdomains form the vestibule of the Na+/glucose cotransporter SGLT1

A combination of biophysical and biochemical approaches was employed to probe the topology, arrangement, and function of the large surface subdomains of SGLT1 in living cells. Using atomic force microscopy on the single molecule level, Chinese hamster ovary cells overexpressing SGLT1 were probed with atomic force microscopy tips carrying antibodies against epitopes of different subdomains. Specific single molecule recognition events were observed with antibodies against loop 6-7, loop 8-9, and loop 13-14, demonstrating the extracellular orientation of these subdomains. The addition of D-glucose in Na+-containing medium decreased the binding probability of the loop 8-9 antibody, suggesting a transport-related conformational change in the region between amino acids 339 and 356. Transport studies with mutants C345A, C351A, C355A, or C361S supported a role for these amino acids in determining the affinity of SGLT1 for D-glucose. MTSET, [2-(trimethylammonium)ethyl] methanethiosulfonate and dithiothreitol inhibition patterns on alpha-methyl-glucoside uptake by COS-7 cells expressing C255A, C560A, or C608A suggested the presence of a disulfide bridge between Cys255 and Cys608. This assumption was corroborated by matrix-assisted laser desorption ionization time-of-flight mass spectrometry showing mass differences in peptides derived from transporters biotinylated in the absence and presence of dithiothreitol. These results indicate that loop 6-7 and loop 13-14 are connected by a disulfide bridge. This bridge brings also loop 8-9 into close vicinity with the former subdomains to create a vestibule for sugar binding.     

The drug-binding pocket of the human multidrug resistance P-glycoprotein is accessible to the aqueous medium

P-Glycoprotein (P-gp) is an ATP-dependent drug pump that transports a broad range of compounds out of the cell. Cross-linking studies have shown that the drug-binding pocket is at the interface between the transmembrane (TM) domains and can simultaneously bind two different drug substrates. Here, we determined whether cysteine residues within the drug-binding pocket were accessible to the aqueous medium. Cysteine mutants were tested for their reactivity with the charged thiol-reactive compounds sodium (2-sulfonatoethyl)methanethiosulfonate (MTSES) and [2-(trimethylammonium)ethyl)]methanethiosulfonate (MTSET). Residue Ile-306(TM5) is close to the verapamil-binding site. It was changed to cysteine, reacted with MTSES or MTSET, and assayed for verapamil-stimulated ATPase activity. Reaction of mutant I306C(TM5) with either compound reduced its affinity for verapamil. We confirmed that the reduced affinity for verapamil was indeed due to introduction of a charge at position 306 by demonstrating that similar effects were observed when Ile-306 was replaced with arginine or glutamic acid. Mutant I306R showed a 50-fold reduction in affinity for verapamil and very little change in the affinity for rhodamine B or colchicine. MTSES or MTSET modification also affected the cross-linking pattern between pairs of cysteines in the drug-binding pocket. For example, both MTSES and MTSET inhibited cross-linking between I306C(TM5) and I868C(TM10). Inhibition was enhanced by ATP hydrolysis. By contrast, cross-linking of cysteine residues located outside the drug-binding pocket (such as G300C(TM5)/F770C(TM8)) was not affected by MTSES or MTSET. These results indicate that the drug-binding pocket is accessible to water.     

Conformationally sensitive residues in transmembrane domain 9 of the Na+/dicarboxylate co-transporter

The Na(+)/dicarboxylate co-transporter, NaDC-1, couples the transport of sodium and Krebs cycle intermediates, such as succinate and citrate. Previous studies identified two functionally important amino acids, Glu-475 and Cys-476, located in transmembrane domain (TMD) 9 of NaDC-1. In the present study, each amino acid in TMD-9 was mutated to cysteine, one at a time, and the accessibility of the membrane-impermeant reagent [2-(trimethylammonium)ethyl]methanethiosulfonate (MTSET) to the replacement cysteines was determined. Cysteine substitution was tolerated at all but five of the sites: the A461C mutant was not present at the plasma membrane, whereas the F473C, T474C, E475C, and N479C mutants were inactive proteins located on the plasma membrane. Cysteine substitution of four residues found near the extracellular surface of TMD-9 (Ser-478, Ala-480, Ala-481, and Thr-482) resulted in proteins that were sensitive to inhibition by MTSET. The accessibility of MTSET to the four substituted cysteines was highest in the presence of the transported cations, sodium or lithium, and low in choline. The four mutants also exhibited substrate protection of MTSET accessibility. The MTSET accessibility to S478C, A481C, and A480C was independent of voltage. In contrast, T482C was more accessible to MTSET in choline buffer at negative holding potentials, but there was no effect of voltage in sodium buffer. In conclusion, TMD-9 may be involved in transducing conformational changes between the cation-binding sites and the substrate-binding site in NaDC-1, and it may also form part of the translocation pathway through the transporter.     

Cysteine mutagenesis and computer modeling of the S6 region of an intermediate conductance IKCa channel

Cysteine-scanning mutagenesis (SCAM) and computer-based modeling were used to investigate key structural features of the S6 transmembrane segment of the calcium-activated K(+) channel of intermediate conductance IKCa. Our SCAM results show that the interaction of [2-(trimethylammonium)ethyl] methanethiosulfonate bromide (MTSET) with cysteines engineered at positions 275, 278, and 282 leads to current inhibition. This effect was state dependent as MTSET appeared less effective at inhibiting IKCa in the closed (zero Ca(2+) conditions) than open state configuration. Our results also indicate that the last four residues in S6, from A283 to A286, are entirely exposed to water in open IKCa channels, whereas MTSET can still reach the 283C and 286C residues with IKCa maintained in a closed state configuration. Notably, the internal application of MTSET or sodium (2-sulfonatoethyl) methanethiosulfonate (MTSES) caused a strong Ca(2+)-dependent stimulation of the A283C, V285C, and A286C currents. However, in contrast to the wild-type IKCa, the MTSET-stimulated A283C and A286C currents appeared to be TEA insensitive, indicating that the MTSET binding at positions 283 and 286 impaired the access of TEA to the channel pore. Three-dimensional structural data were next generated through homology modeling using the KcsA structure as template. In accordance with the SCAM results, the three-dimensional models predict that the V275, T278, and V282 residues should be lining the channel pore. However, the pore dimensions derived for the A283-A286 region cannot account for the MTSET effect on the closed A283C and A286 mutants. Our results suggest that the S6 domain extending from V275 to V282 possesses features corresponding to the inner cavity region of KcsA, and that the COOH terminus end of S6, from A283 to A286, is more flexible than predicted on the basis of the closed KcsA crystallographic structure alone. According to this model, closure by the gate should occur at a point located between the T278 and V282 residues.     

Defining nicotinic agonist binding surfaces through photoaffinity labeling

Nicotinic acetylcholine (ACh) receptor (nAChR) agonists are potential therapeutic agents for neurological dysfunction. In the present study, the homopentameric mollusk ACh binding protein (AChBP), used as a surrogate for the extracellular ligand-binding domain of the nAChR, was specifically derivatized by the highly potent agonist azidoepibatidine (AzEPI) prepared as a photoaffinity probe and radioligand. One EPI-nitrene photoactivated molecule was incorporated in each subunit interface binding site based on analysis of the intact derivatized protein. Tryptic fragments of the modified AChBP were analyzed by collision-induced dissociation and Edman sequencing of radiolabeled peptides. Each specific EPI-nitrene-modified site involved either Tyr195 of loop C on the principal or (+)-face or Met116 of loop E on the complementary or (-)-face. The two derivatization sites were observed in similar frequency, providing evidence of the reactivity of the azido/nitrene probe substituent and close proximity to both residues. [3H]AzEPI binds to the alpha4beta2 nAChR at a single high-affinity site and photoaffinity-labels only the alpha4 subunit, presumably modifying Tyr225 spatially corresponding to Tyr195 of AChBP. Phe137 of the beta2 nAChR subunit, equivalent to Met116 of AChBP, conceivably lacks sufficient reactivity with the nitrene generated from the probe. The present photoaffinity labeling in a physiologically relevant condition combined with the crystal structure of AChBP allows development of precise structural models for the AzEPI interactions with AChBP and alpha4beta2 nAChR. These findings enabled us to use AChBP as a structural surrogate to define the nAChR agonist site.     

External nickel inhibits epithelial sodium channel by binding to histidine residues within the extracellular domains of alpha and gamma subunits and reducing channel open probability

Epithelial sodium channels (ENaC) are regulated by various intracellular and extracellular factors including divalent cations. We studied the inhibitory effect and mechanism of external Ni(2+) on cloned mouse alpha-beta-gamma ENaC expressed in Xenopus oocytes. Ni(2+) reduced amiloride-sensitive Na(+) currents of the wild type mouse ENaC in a dose-dependent manner. The Ni(2+) block was fast and partially reversible at low concentrations and irreversible at high concentrations. ENaC inhibition by Ni(2+) was accompanied by moderate inward rectification at concentrations higher than 0.1 mm. ENaC currents were also blocked by the histidine-reactive reagent diethyl pyrocarbonate. Pretreatment of the oocytes with the reagent reduced Ni(2+) inhibition of the remaining current. Mutations at alphaHis(282) and gammaHis(239) located within the extracellular loops significantly decreased Ni(2+) inhibition of ENaC currents. The mutation alphaH282D or double mutations alphaH282R/gammaH239R eliminated Ni(2+) block. All mutations at gammaHis(239) eliminated Ni(2+)-induced inward current rectification. Ni(2+) block was significantly enhanced by introduction of a histidine at alphaArg(280). Lowering extracellular pH to 5.5 and 4.4 decreased or eliminated Ni(2+) block. Although alphaH282C-beta-gamma channels were partially inhibited by the sulfhydryl-reactive reagent [2-(trimethylammonium)ethyl] methanethiosulfonate bromide (MTSET), alpha-beta-gamma H239C channels were insensitive to MTSET. From patch clamp studies, Ni(2+) did not affect unitary current but decreased open probability when perfused into the recording pipette. Our results suggest that external Ni(2+) reduces ENaC open probability by binding to a site consisting of alphaHis(282) and gammaHis(239) and that these histidine residues may participate in ENaC gating.     

Identification of a functionally important conformation-sensitive region of the secretory Na+-K+-2Cl- cotransporter (NKCC1)

The secretory Na(+)-K(+)-2Cl(-) cotransporter (NKCC1) is a member of a small gene family of electroneutral salt transporters that play essential roles in salt and water homeostasis in many mammalian tissues. We have identified a highly conserved residue (Ala-483) in the sixth membrane-spanning segment of rat NKCC1 that when mutated to cysteine renders the transporter sensitive to inhibition by the sulfhydryl reagents 2-aminoethyl methanethiosulfonate (MTSEA) and 2-(trimethylammonium)ethyl methanethiosulfonate (MTSET). The mutation of Ala-483 to cysteine (A483C) results in little or no change in the affinities of NKCC1 for substrate ions but produces a 6-fold increase in sensitivity to the inhibitor bumetanide, suggesting a specific modification of the bumetanide binding site. When residues surrounding Ala-483 were mutated to cysteine, only I484C was sensitive to inhibition by MTSEA and MTSET. Surprisingly I484C showed increased transport activity in the presence of low concentrations of mercury (1-10 microm), whereas A483C showed inhibition. The inhibition of A483C by MTSEA was unaffected by the presence or absence of sodium and potassium but required the presence of extracellular chloride. Taken together, our results indicate that Ala-483 lies at or near an important functional site of NKCC1 and that the exposure of this site to the extracellular medium is dependent on the conformation of the transporter. Specifically, our results indicate that the cysteine introduced at residue 483 is only available for interaction with MTSEA when chloride is bound to NKCC1 at the extracellular surface.     

Analysis of transmembrane domain 2 of rat serotonin transporter by cysteine scanning mutagenesis

The second transmembrane domain (TM2) of neurotransmitter transporters has been invoked to control oligomerization and surface expression. This transmembrane domain lies between TM1 and TM3, which have both been proposed to contain residues that contribute to the substrate binding site. Rat serotonin transporter (SERT) TM2 was investigated by cysteine scanning mutagenesis. Six mutants in which cysteine replaced an endogenous TM2 residue had low transport activity, and two were inactive. Most of the reduction in transport activity was due to decreased surface expression. In contrast, M124C and G128C showed increased activity and surface expression. Random mutagenesis at positions 124 and 128 revealed that hydrophobic residues at these positions also increased activity. When modeled as an alpha-helix, positions where mutation to cysteine strongly affects expression levels clustered on the face of TM2 surrounding the leucine heptad repeat conserved within this transporter family. 2-(Aminoethyl)-methanethiosulfonate hydrobromide (MTSEA)-biotin labeled A116C and Y136C but not F117C, M135C, or Y134C, suggesting that these residues may delimit the transmembrane domain. None of the cysteine substitution mutants from 117 through 135 were sensitive to [2-(trimethylammonium)ethyl]methanethiosulfonate bromide (MTSET) or MTSEA. However, treatment with MTSEA increased 5-hydroxytryptamine transport by A116C. Activation of A116C by MTSEA was observed only in mutants containing Cys to Ile mutation at position 357, suggesting that modification of Cys-116 activated transport by compensating for a disruption in transport in response to Cys-357 replacement. The reactivity of A116C toward MTSEA was substantially increased in the presence of substrates but not inhibitors. This increase required Na+ and Cl-, and was likely to result from conformational changes during the transport process.