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Original Article |
sdudley{at}emory.edu
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ABSTRACT |
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 Top ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Key Words: electrophysiology • site-directed mutagenesis • molecular models • kinetics • binding sites
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INTRODUCTION |
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TopABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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), the Na+ channel has been resistant so far to such analysis. The Na+ channel consists of four homologous domains of a single polypeptide arranged circumferentially around a central ion permeation pathway. These similar, but nonidentical, domains could be organized either in a clockwise or in a counterclockwise pattern around the central pore. No experimental data are available to distinguish between these alternative patterns. Molecular models of the channel have been divided on this point: four proposing a clockwise arrangement (Lipkind and Fozzard 1994; Pérez-García et al. 1996; Schlief et al. 1996; Bénitah et al. 1997), and four proposing a counterclockwise arrangement (Guy 1988; Schild and Moczydlowski 1994; Chiamvimonvat et al. 1996; Pérez-García et al. 1997).
µ-Conotoxin GIIIA (µ-CTX) is a 22–amino acid peptide toxin, originally isolated from piscivorous cone snails, that binds Na+ channels (Olivera et al. 1990). Three disulfide bonds confer structural rigidity on the toxin, and its solution structure has been solved by nuclear magnetic resonance (Lancelin et al. 1991; Ott et al. 1991; Wakamatsu et al. 1992). The µ-CTX binding site overlaps that of tetrodotoxin (TTX) and saxitoxin (STX; Moczydlowski et al. 1986; Stephan et al. 1994; Chahine et al. 1995; Dudley et al. 1995; Li et al. 1997; Chang et al. 1998), but it is not identical to it. The blocking mechanism of these site 1 toxins appears to be binding to the outer vestibule and occlusion of the pore by a guanidinium group (Kao 1986; Sato et al. 1991; Becker et al. 1992; Chahine et al. 1995; French et al. 1996; Chang et al. 1998; Todt et al. 1999).
Previously, we have shown interactions of the critical guanidinium group on Arg-13 of µ-CTX with predominantly two acidic residues of the adult rat skeletal muscle Na+ channel (µI) outer vestibule, Glu-403 and Glu-758 (Chang et al. 1998). These two residues are thought to be within the outer vestibule, but located extracellular to the selectivity filter (Terlau et al. 1991; Chiamvimonvat et al. 1996; Yamagishi et al. 1997). Directly above Arg-13 of µ-CTX is a group of four residues (Asp-12, Gln-14, Hyp-17, and Lys-16) arranged in a plane and at approximately right angles to each other, forming a collar around Arg-13 (DQHypK collar; Fig. 1). Three of these residues are known to have significant effects on µ-CTX binding (Sato et al. 1991; Becker et al. 1992; Chahine et al. 1995). We suggested that the DQHypK collar interacted with the outer vestibule to prevent Arg-13 of µ-CTX from more closely approaching the selectivity filter (Chang et al. 1998). In a molecular model explaining the observed channel interactions with µ-CTX Arg-13, the Na+ channel domains were arranged in a clockwise manner around the central axis of the pore as viewed from the extracellular surface; the DQHypK collar tetrad was oriented such that Asp-12, Gln-14, Hyp-17, and Lys-16 were approximating domains I, II, III, and IV, respectively.
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; Serrano et al. 1990; Fersht et al. 1992). With this approach, the interdependence of the effects on the toxin blocking efficacy of toxin and channel mutations is taken as evidence of an interaction. The degree of interdependence can be used to calculate a coupling energy between the reactive groups (Schreiber and Fersht 1995). In this study, we tested our previous hypotheses about the interactions of the collar tetrad with outer vestibule amino acids from each of the four channel domains using mutant cycle analysis. |
MATERIALS AND METHODS |
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TopABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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). Native µ-CTX was synthesized as detailed below or obtained from either RBI or Sigma-Aldrich. µ-CTX from all three sources was equivalent in blocking activity.
µ-CTX Mutations
µ-CTX mutations were made by solid phase synthesis using 9-fluorenylmethoxycarbonyl (Fmoc) chemistry (Becker et al. 1989, Becker et al. 1990; Chang et al. 1998). The syntheses were performed on a polystyrene-based Rink amide resin that delivered the required amidated toxin upon cleavage. The peptides were air oxidized and HPLC purified, showing a single major peak on the chromatogram. Peptide composition was verified by quantitative amino acid analysis, supplemented as necessary with molecular weight determinations by mass spectroscopy for some derivatives. In three cases (Q14D, K16A, and Hyp17P), one dimensional proton nuclear magnetic resonance spectra for the toxin mutants were compared with published data for native µ-CTX. No evidence of improper folding was found. A complete structure has been reported for R13A, revealing no significant change in backbone arrangement when this critical residue is substituted (Wakamatsu et al. 1992), which is consistent with our own partial analyses. The synthetic technique yielded peptides that were 
95% pure, as determined by analytical HPLC. The total peptide content of the samples was determined by quantitative amino acid analysis. Variance in weighing was estimated to lead to no more than a 10% error in calculating the toxin concentration.
Reagents were obtained from the following sources: Fmoc amino acids from Calbiochem-Novabiochem, Bachem, Genzyme, and Richelieu Biotechnologies; Rink amide resin was from Calbiochem-Novabiochem; and the coupling reagents were from Richelieu and Sigma-Aldrich.
Na+ Channel Mutagenesis
Oligonucleotide-directed point mutations were introduced into the rat adult skeletal muscle Na+ channel cDNA. DNA sequencing of the entire polymerized regions insured that only the intended mutations were present. Stage V and VI Xenopus oocytes from female frogs (NASCO or Xenopus 1) were injected with 
50–100 ng of cRNA. Oocytes were incubated at 16 for 12–72 h before examination.
Electrophysiology
Recordings were made by two-electrode voltage clamp at room temperature (20–22°C). The oocytes were placed in a bath chamber with a solution exchange time sufficiently short to resolve toxin blocking kinetics. The standard bath solution consisted of the following (in mM): 90 NaCl, 2.5 KCl, 1 CaCl2, 1 MgCl2, and 5 HEPES, titrated to pH 7.2 with 1 N NaOH. Recordings of the peak currents were made every 20 s upon step pulses from –100 to 0 mV. Only oocytes with between 1 and 10 µA of peak current were studied. The change in peak INa with time was fitted by single exponential functions and was used to estimate kinetic rate constants. The 50% inhibitory concentration (IC50) for toxin binding was calculated from the ratio of peak currents in the absence and presence of toxin (Cruz et al. 1985; Moczydlowski et al. 1986). As previously described (Chang et al. 1998), agreement between the kinetic Kd and the equilibrium IC50s was used to validate the measurements and was taken as evidence that toxin binding resulted in complete block for all mutations tested.
The free energy change in toxin binding to a wild-type/mutant channel pair (
G) was calculated as the difference of the average RTln(IC50) for the wild-type and mutant, where R is the gas constant and T is temperature. The standard errors (SEMs) for G were estimated as the square of the variance of the RTln(IC50) averages divided by the square root of the sum of the number of observations. G was taken as the difference of the Gs for µ-CTX and the toxin mutant (G = (GWT,native – Gmutant,native) – (GWT,mutant – Gmutant,mutant), where the first and second subscript positions refer to the channel and the toxin, respectively, and the standard error of this number was reported as the square root of the sum of the variances of the RTln(IC50) averages (Bevington 1969) divided by the square root of the number of degrees of freedom. Note that G may be positive or negative, both representing a coupling interaction. The negative values represent less coupling energy between the mutant pair as compared with the native residue pair. A positive G indicates that the introduced pair has more coupling energy after mutation relative to the native pair. This might occur as a result of relief of a preexisting repulsion or by the creation of a novel attraction between the new pair. Data are presented as means ± SEM. Statistical comparisons of Gs and Gs were performed using two-tailed t tests assuming unequal variances (Excel 97; Microsoft Corp.).
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RESULTS |
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TopABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The Effect of Mutations on Toxin Blocking Efficacy
Fig. 2 (top) shows toxin blocking efficacy of the µ-CTX derivatives with the wild-type channel. The native µ-CTX affinity for the wild-type µI Na+ channel was in the range of values reported by others for the rat skeletal muscle channel (Moczydlowski et al. 1986; Becker et al. 1992; Chen et al. 1992; Stephan et al. 1994; Chahine et al. 1995, Chahine et al. 1998; Dudley et al. 1995; Li et al. 1997; Chang et al. 1998). All mutants other than D12A resulted in a significant decrease in toxin IC50.
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). Inspection of the macroscopic current kinetics and current–voltage curves showed no significant alteration in kinetics or reversal potential with any of the channel mutations. Of the two domain I mutations, Glu-403 appeared to be moderately important for native µ-CTX blocking, and Asn-404 appeared not to be important in determining the toxin equilibrium IC50. N404R resulted in a modest increase in µ-CTX blocking efficacy, but this change was not statistically significant (P = 0.28). Since almost the entire loss of binding energy when mutating Arg-13 could be explained by the loss of the Arg-13/Glu-758 interaction (Chang et al. 1998), Glu-758 was not evaluated for other interactions in the present study. Isoleucine was introduced in place of Thr-759. This channel mutation resulted in modest increase in native µ-CTX blocking efficacy (P = 0.02). This is the first report of change in µ-CTX affinity with mutations at this channel site, and the modest change in toxin IC50 suggests little alteration in the outer vestibule by this channel mutant. The effects of channel mutations in domain III were highly selective. The IC50 of native µ-CTX for M1240A was not statistically different from the wild-type channel (P = 0.1). D1241A reduced µ-CTX IC50 by 10-fold. Of the channel residues tested in our experiments, domain IV Asp-1532 appeared to be the most important determinant of µ-CTX blocking efficacy. The size-conserving mutation, D1532N, resulted in a 15-fold reduction in the IC50 of the native toxin for the channel. Table summarizes the effect of all combinations of toxin and channel mutants on IC50.
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G for the Q14D–T759I interaction relative to the native complex was statistically different (P < 0.01) from the G values for the Q14D/N404R, Q14D/E403Q, and Q14D/D1532N.
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of Met acting as a hydrogen bond receptor for the 
-OH of Hyp. The energy of this type of interaction has been shown to be 1.1 kcal/mol (Wilkinson et al. 1983). The Hyp17P mutation resulted in a loss of toxin binding energy (G) of 1.5 kcal/mol. Therefore, most of this loss could be explained by elimination of the Hyp-17/Met-1240 interaction. The Hyp-17 interaction appeared specific for Met-1240 of domain III. No interaction was identified between the adjacent Asp-1241 and Hyp-17 (G = 0.2 ± 0.1 kcal/mol), even though D1241A had a large effect on native µ-CTX affinity, and no other significant interactions were identified between Hyp-17 and channel residues of the other domains. Lys-16 is opposite Gln-14 in the µ-CTX collar tetrad, and the K16A mutation resulted in the next largest reduction in blocking efficacy among the collar tetrad derivatives tested. As anticipated from the structure of µ-CTX and the interaction of Gln-14 with domain II, the toxin 16 site showed the strongest interaction with domain IV Asp-1532 (Fig. 3 B). However, the Lys-16 interaction was not confined to Asp-1532. An interaction of lesser energy was demonstrated with the domain I channel residue, Glu-403 (P < 0.01). Of less clear significance, an energetically opposite K16A–T759I interaction was observed that was statistically different from zero (P < 0.01).
No strong Asp-12 interactions were resolved by these experiments (Fig. 3 B). Because of the previously demonstrated interactions, the expectation was that Asp-12 would be adjacent to channel residues of domain I. The coupling with Asn-404 was small but statistically significant. Asp-12 showed no coupling with residues in domain II (Thr-759) or domain III (Met-1240).
The structure of the toxin (Lancelin et al. 1991; Wakamatsu et al. 1992) shows that µ-CTX Arg-1 and Gln-14 are both on the same side of the molecule. Therefore, we looked for possible interactions between Arg-1 and domain II. The toxin mutation R1A resulted in a 152-fold reduction in the toxin IC50 for the wild-type µI channel, confirming the importance of Arg-1 in µ-CTX binding. As predicted from the coupling between toxin residue Gln-14 and the domain II residue Thr-759, a significant interaction between Arg-1 and domain II was demonstrated. In summary, the coupling energy data identified most clearly the following pairs of interacting toxin–channel residues: Gln-14/Thr-759, Arg-1/Thr-759, Hyp-17/Met-1240, Lys-16/Asp-1532, and Lys-16/Glu-403.
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DISCUSSION |
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TopABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Consistency with Previous Results
For wild-type channels, the IC50s of µ-CTX mutations D12A, Q14D, K16A, and Hyp17P were 0.6-, 31-, 24-, and 14-fold change, respectively, from the native toxin IC50. These changes are consistent with previous reports. The slight increase in blocking efficacy seen with D12A is consistent with all other reports of the effect of neutralization of the negative charge at this site (Sato et al. 1991; Becker et al. 1992; Chahine et al. 1995). Similar to our 31-fold Q14D effect, Chahine et al. 1995 reported that the Q14E mutation decreased toxin blocking efficacy by 55-fold. The modest difference in these results may be caused by the larger size of the glutamic acid side chain compared with aspartic acid. The normalized increase in IC50 with the K16A was slightly larger than that of a previous report by Sato et al. 1991 using a rat diaphragm contraction bioassay. Nevertheless, the 24-fold change was similar to the 21-fold change in the dissociation constant seen for K16Q in bilayers incorporating rat skeletal muscle Na+ channels (Becker et al. 1992). The 14-fold increase in IC50 with Hyp17P mutations fell in the middle of the range noted previously for the elimination of the β-OH of Hyp (6–21-fold; Sato et al. 1991; Becker et al. 1992; Chahine et al. 1995). Finally, the effect of elimination of the charge at the Arg-1 site was larger than previous reports (Sato et al. 1991; Becker et al. 1992; Chahine et al. 1995). Differences in the assay method or differences in the amino acid substituted may explain the quantitative differences among the data.
The effects of channel mutants seen in this study were generally consistent with previous reports. The mutation E403Q increased the native µ-CTX IC50 by 8.9-fold, similar to a 4-fold increase reported previously for the identical mutation (Stephan et al. 1994). However, the reason for the discrepancy between these similar results and the lack of effect on µ-CTX binding by E403C (Li et al. 1997) is unclear. N404R resulted in a modest increase in µ-CTX blocking efficacy, and, under similar conditions, Chen et al. 1992 reported a slight increase in the IC50 (1.5-fold). Li et al. 1997 described mutations of Met-1240 and Asp-1241, substituting Cys, and came to the same conclusions about the effects of these two residues on µ-CTX blocking efficacy. Li et al. 1997 reported no effect on the IC50 with the D1532C mutation, in contrast to the large effect that we observed for the D1532N substitution. Moreover, Chiamvimonvat et al. 1996 and Tsushima et al. 1997 found that the mutation D1532C had a large effect on selectivity for NH4+ and K+. Further studies are needed to identify the reason for these differences.
Interpretation of Interaction Energies
Toxins of known structure have proven extremely useful in probing the outer vestibule of K+ channels (MacKinnon and Miller 1989; MacKinnon et al. 1990, MacKinnon et al. 1998; Park and Miller 1992; Aiyar et al. 1995, Aiyar et al. 1996; Hidalgo and MacKinnon 1995; Lu and MacKinnon 1995; Gross and MacKinnon 1996; Naranjo and Miller 1996; Ranganathan et al. 1996; Swartz and MacKinnon 1997). In principle, any individual amino acid may have multiple energetic interactions that contribute to the energy of binding. Mutant cycle analysis is a technique designed to isolate the coupling between a particular toxin–channel pair.
The major potential source of error with mutant cycle analysis arises from the possibility of structural changes in either of the interacting molecules as the direct result of mutations, or resulting secondarily from changes in the nature of the ligand–protein interaction. Allosteric effects can often be identified by generalized disruption of normal function of the channel. The mutated channels in this study were evaluated functionally, and they showed no significant alterations in the macroscopic gating behavior or reversal potential. Several toxin mutants have been screened for major structural changes by nuclear magnetic resonance with negative results. All toxin–channel pairs show blocking interactions, suggesting that the interacting surfaces were not grossly altered. Furthermore, the pattern of domain- and residue-specific Gs supported the specificity of interactions.
Of the interactions noted above, four showed negative Gs, and there was at least one positive G. In performing mutant cycle analysis, mutations are usually chosen to eliminate interactions without inducing new ones. We defined the interaction energy such that negative energies of interaction would represent a loss of binding energy in the mutant pair complex compared with the native, bound complex. This idea is consistent with the negative Gs in the R1A/T759I, K16A/D1532N, K16A/E403Q, and Hyp17P/Met1240A pairs. Alternatively, a negative G could arise from new repulsions resulting from the substituted residues as compared with the native ones. In either event, an interaction exists, implying that the residues are near enough to interact with each other in the toxin–channel complex. This dependence of G on the choice of substituted residue was predicted by Faiman and Horovitz 1996.
The Q14D–T759I interaction showed a positive G. Since the overall effect of the Q14D mutation was to decrease toxin blocking efficacy, the Gln-14/Thr-759 interaction is not sufficient to explain the entire Q14D effect. This could be the result of the double mutant complex eliminating a repulsion in the native complex or, less likely, adding a new attractive force between the toxin Asp and the channel Ile relative to the Gln/Thr pair. The overall effect of T759I on native toxin binding is small, which is consistent with the opposing effects. It is plausible that there could be electrostatic repulsion between the substituted Asp-14 of the toxin and the negative residues of the vestibule, but Glu-403 and Asp-1532 must be too far away to contribute significantly. Consistent with this possibility, Li and his co-workers (Li, R.A., I.L. Ennis, S.C. Dudley Jr., R.J. French, G.F. Tomaselli, and E. Marban, manuscript submitted for publication) have noted significant interactions between Gln-14 and channel residues Asp-762 and Glu-765. This observation supports the conclusion that Gln-14 is oriented toward domain II.
The failure to show a Gln-14/Glu-403 interaction is consistent with the implication derived from the data of Stephan et al. 1994. In that study, the effects of E403Q on IC50s for µ-CTX GIIIA and µ-CTX GIIIB IC50s were determined. Among four amino acid differences with µ-CTX GIIIA, µ-CTX GIIIB contains a Q14R substitution. The ratio of the toxin IC50s for wild-type and E403Q- mutated channels derived from these data were similar for µ-CTX GIIIA and µ-CTX GIIIB, suggesting that toxin site 14 and channel site 403 were not interacting significantly (G = –0.3 kcal/mol).
Our failure to demonstrate a strong interaction of Asp-12 with the channel does not preclude the possibility that Asp-12 is near domain I. In the Barstar–Barnase complex wherethe crystal structure could be determined, Schreiber and Fersht 1995 showed that there was a general relationship between the coupling energy and separation of the interaction partners, but that a lack of coupling could occur in closely approximated residues. Structural proximity alone was not sufficient to produce interactions. On the other hand, there was no coupling when the tested pair was separated by >8 Å. Therefore, they concluded that demonstration of coupling set a distance limit on the separation of two residues. Perhaps, the charge of Asp-12 is neutralized in the native toxin by an intramolecular interaction with an adjacent toxin residue such as Lys-8, and therefore, elimination of charge at this residue has little effect on toxin–channel interactions. In any event, the absence of strong coupling is consistent with the lack of any significant influence of Asp-12 substitutions on toxin affinity.
The Lys-16 interaction pattern was more difficult to interpret. At a level considered to identify confidently interactions (Ranganathan et al. 1996), Lys-16 showed significant Gs with Asp-1532 of domain IV and Glu-403 of domain I, suggesting that Lys-16 might be located between these two domains. Multidomain interactions suggest that caution should be used when interpreting the effects of charge-changing mutations. In this case, the largest energy associated with the Lys-16/Asp-1532 interaction hints that Lys-16 might be closest to domain IV.
Because the M1240C mutation did not change the IC50 of native µ-CTX binding significantly, Backx and his coworkers (Li et al. 1997) concluded that Met-1240 played no role in toxin binding. Consistent with this assertion, our results demonstrated that the effect of M1240A on native µ-CTX blocking efficacy was small. On the other hand, a significant negative G was calculated for the Hyp17Pro/M1240A interacting pair. This suggests that the net effect of Met-1240 mutations is the sum of at least two interactions with opposing energetic effects. Our study supports previous demonstrations that Asp-1241 is important for µ-CTX binding (Li et al. 1997), but the toxin residue interacting with Asp-1241 is not identified in either study.
Structural Implications of Coupling Data
Our experimentally derived coupling data are most consistent with a circumferentially sequential, clockwise arrangement of the domains around the ion permeation pathway. Gln-14, Hyp-17, and Lys-16 are arranged at approximately right angles to each other in a plane perpendicular to the axis of the pore. These toxin residues interact most strongly with residues of domains II, III, and IV, respectively. These interactions are best explained if the domains are arranged in the clockwise pattern, as shown in Fig. 4. In the structure of µ-CTX, Arg-1 is on the same side of the toxin as Gln-14, and its coupling with a domain II residue also supports the clockwise domain arrangement.
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Gs for each of the eight possible sequential clockwise and counterclockwise configurations of the domain–collar interactions were made and the variances were calculated. The most favorable clockwise configuration was as shown in Fig. 4 A. This was tested against the two most favorable counterclockwise conformations. In both comparisons, the clockwise configuration was favored, with P < 0.001. There are a limited number of toxin–channel interactions that can be elucidated with a single toxin, so it is important to test multiple toxins of differing shapes to constrain models adequately. A multiple toxin approach minimizes the possibility of being misled by allosteric changes produced by mutagenesis. The conclusions about channel architecture inferred here from µ-CTX–channel interactions are similar to those derived from STX–channel interactions (Penzotti, J.L., G.M. Lipkind, H.A. Fozzard, and S.C. Dudley Jr., manuscript submitted for publication). Interactions derived using neoSTX and µ-CTX, two toxins with significantly different structures and chemical interactions, support the validity of the general features of the model in Fig. 4 A, and set the stage for further tests of outer vestibule structural predictions.
G, the change in free energy of binding; G, interaction energy; Hyp, hydroxyproline; IC50, 50% inhibitory concentration; koff, off rate; kon, on rate; µ-CTX, µ-conotoxin GIIIA; µI, rat adult skeletal muscle Na+ channel; neoSTX, neosaxitoxin; STX, saxitoxin; 
off, exponential time constant for relief of toxin block; on, exponential time constant for toxin block; TTX, tetrodotoxin.
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ACKNOWLEDGMENTS |
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This research was supported by an American Heart Association, Southeast Affiliate Beginning Grant-in-Aid (to S.C. Dudley), by a Program Project Award from the National Institutes of Health (to H.A. Fozzard, No. P01-HL20592), and by an operating grant from the Medical Research Council of Canada (to R.J. French). S.C. Dudley is supported by a Scientist Development Award from the American Heart Association, a Procter and Gamble University Research Exploratory Award, and National Institutes of Health Grant (HL64828). N.S. Chang was supported by the training grant TO1-GM07019. R.J. French received salary support as an Alberta Heritage Foundation for Medical Research Medical Scientist and a Medical Research Council Distinguished Scientist.
Submitted: 4 April 2000
Revised: 13 September 2000
Accepted: 15 September 2000
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