Highly specific anti-estradiol antibodies: structural characterisation and binding diversity.

Subtle modulation of antibody-binding properties by protein engineering often lies with an accurate structural and energetic description of how an antigen is recognised. Thus, with the intent to increase the affinity and add a bias in favour of natural estradiol compared with its chemically modified immunogen, we have determined the crystal structure of two anti-estradiol monoclonal antibodies, 10G6D6 and 17E12E5. Although generated against the same estradiol derivative, these antibodies share little sequence identity, which is reflected in dissimilar binding pockets and in different positioning of the steroid. In both antibodies the characteristic 17-hydroxyl group is buried deeply at the bottom of hydrophobic pockets and stabilised by hydrogen bonds. Apart from this similarity, the steroid is oriented differently in the respective binding pockets. The high specificity of both antibodies has been mapped out, and even closely related steroids show low cross-reactivity. The structural studies of the complex formed between 10G6D6 and 6-CMO-estradiol have identified contacts between the 6-CMO coupling linker and an arginine residue from the heavy chain CDR2 segment. This segment is now being targeted by random mutagenesis to select mutants with a preference for natural estradiol compared to the branched hapten.

Structural characterization of the LEM motif common to three human inner nuclear membrane proteins.

BACKGROUND: Integral membrane proteins of the inner nuclear membrane are involved in chromatin organization and postmitotic reassembly of the nucleus. The discovery that mutations in the gene encoding emerin causes X-linked Emery-Dreifuss muscular dystrophy has enhanced interest in such proteins. A common structural domain of 50 residues, called the LEM domain, has been identified in emerin MAN1, and lamina-associated polypeptide (LAP) 2. In particular, all LAP2 isoforms share an N-terminal segment composed of such a LEM domain that is connected to a highly divergent LEM-like domain by a linker that is probably unstructured. RESULTS: We have determined the three-dimensional structures of the LEM and LEM-like domains of LAP2 using nuclear magnetic resonance and molecular modeling. Both domains adopt the same fold, mainly composed of two large parallel alpha helices. CONCLUSIONS: The structural LEM motif is found in human inner nuclear membrane proteins and in protein-protein interaction domains from bacterial multienzyme complexes. This suggests that LEM and LEM-like domains are protein-protein interaction domains. A region conserved in all LEM domains, at the surface of helix 2, could mediate interaction between LEM domains and a common protein partner.

Structural analysis of emerin, an inner nuclear membrane protein mutated in X-linked Emery-Dreifuss muscular dystrophy.

Like Duchenne and Becker muscular dystrophies, Emery-Dreifuss muscular dystrophy (EDMD) is characterized by myopathic and cardiomyopathic abnormalities. EDMD has the particularity of being linked to mutations in nuclear proteins. The X-linked form of EDMD is caused by mutations in the emerin gene, whereas autosomal dominant EDMD is caused by mutations in the lamin A/C gene. Emerin colocalizes with lamin A/C in interphase cells, and binds in vitro to lamin A/C. Recent work suggests that lamin A/C might serve as a receptor for emerin. We have undertaken a structural analysis of emerin, and in particular of its N-terminal domain, which is comprised in the emerin segment critical for binding to lamin A/C. We show that region 2-54 of emerin adopts the LEM fold. This fold was originally described in the two N-terminal domains of another inner nuclear membrane protein called lamina-associated protein 2 (LAP2). The existence of a conserved solvent-exposed surface on the LEM domains of LAP2 and emerin is discussed, as well as the nature of a possible common target.

Functional characterization of two anti-estradiol antibodies as deduced from modelling and site-directed mutagenesis experiments.

Monoclonal antibodies are now widely used to measure the concentration of steroid hormones in human serum samples. The great development of molecular engineering techniques over the past 10 years has made possible the improvement of specificity and/or sensitivity of selected antibodies. We have obtained two monoclonal antibodies, 17E12E5 and 10G6D6, using estradiol-6-ethyl methoxy carbonyl (EMC)-bovine serum albumin (BSA) as immunogen. To tentatively improve their affinities for natural estradiol, we have initiated their structural and functional studies. For this purpose, we have cloned and sequenced the genes encoding the variable fragments of each antibody. Single chain variable fragments (scFv) were produced into the periplasmic space of E. coli using the pLIP6 expression vector. Mapping of the functional structures of both antibodies was obtained by combination of modelling and mutational analyses together with cross-reaction studies. The two binding pockets are described and models of estradiol complexed to 17E12E5 and 10G6D6 are proposed.

Variability in automated assignment of NOESY spectra and three-dimensional structure determination: a test case on three small disulfide-bonded proteins.

Three independent runs of automatic assignment and structure calculations were performed on three small proteins, calcicludine from the venom of the green mamba Dendroaspis angusticeps, kappa-conotoxin PVIIA from the purple cone Conus purpurascens and HsTX1, a short scorpion toxin from the venom of Heterometrus spinnifer. At the end of all the runs, the number of cross peaks which remained unassigned (0.6%, 1.4% and 2% for calcicludine, kappa-conotoxin and HsTX1, respectively), as well as the number of constraints which were rejected as producing systematic violations (2.7%, 1.0%, and 1.4% for calcicludine, kappa-conotoxin and HsTX1, respectively) were low. The conformation of the initial model used in the procedure (linear model or constructed by homology) has no influence on the final structures. Mainly two parameters control the procedure: the chemical shift tolerance and the cut-off distance. Independent runs of structure calculations, using the same parameters, yield structures for which the rmsd between averaged structures and the rmsd around each averaged structure were of the same order of magnitude. A different cut-off distance and a different chemical shift tolerance yield rmsd values on final average structures which did not differ more than 0.5 A compared to the rmsd obtained around the averaged structure for each calculation. These results show that the procedure is robust when applied to such a small disulfide-bonded protein.

Characterization of the internal motions of a chimeric protein by 13C NMR highlights the important dynamic consequences of the engineering on a millisecond time scale.

By transferring the central curaremimetic beta hairpin of the snake toxin alpha into the scaffold of the scorpion charybdotoxin, a chimeric protein was constructed that reproduced the three-dimensional structure and partially reproduced the function of the parent beta hairpin, without perturbing the three-dimensional structure of the scaffold [1]. Picosecond to hour time scale motions of charybdotoxin and the engineered protein were observed, in order to evaluate the dynamic consequences of the six deletions and eight mutations differentiating the two molecules. The chimeric protein dynamics were also compared to that of toxin alpha, in order to examine the beta hairpin motions in both structural contexts. Thus, 13C R1, R1rho and 1H-->13C nOe were measured for all the CalphaHalpha and threonine CbetaHbeta vectors. As the proteins were not labeled, accordion techniques combined to coherence selection by pulsed field gradients and preservation of magnetization following equivalent pathways were used to considerably reduce the spectrometer time needed. On one hand, we observed that the chimeric protein and charybdotoxin are subjected to similar picosecond to nanosecond time scale motions except around the modified beta sheet region. The chimeric protein also exhibits an additional millisecond time scale motion on its whole sequence, and its beta structure is less stable on a minute to hour time scale. On the other hand, when the beta hairpin dynamics is compared in two different structural contexts, i.e. in the chimeric protein and the curaremimetic toxin alpha, the picosecond to nanosecond time scale motions are fairly conserved. However, the microsecond to millisecond time scale motions are different on most of the beta hairpin sequence, and the beta sheet seems more stable in toxin alpha than in the chimera. The slower microsecond to hour time scale motions seem to be extremely sensitive to the structural context, and thus poorly transferred from one protein to another.

Unfolding of hen egg lysozyme by molecular dynamics simulations at 300K: insight into the role of the interdomain interface.

We present the results of two 1.2 ns molecular dynamics (MD) unfolding simulations on hen egg lysozyme in water at 300K, performed using a new procedure called PEDC (Path Exploration With Distance Constraints). This procedure allows exploration of low energy structures as a function of increasing RMSD from the native structure, and offers especially the possibility of extensive exploration of the conformational space during the initial unfolding stages. The two independent MD simulations gave similar chronology of unfolding events: disruption of the active site, kinking of helix C, partial unfolding of the three-stranded beta-sheet to a two-stranded sheet (during which the helices A, B, and D remain to a great extent native), and finally unfolding of the beta-domain and partial unfolding of the alpha-domain in which hydrophobic clusters persist. We show particularly that the loss of hydrophobic contacts between the beta-sheet turn residues Leu55 and Ile56 and the hydrobic patch of the alpha-domain destabilizes the beta-domain and leads to its unfolding, suggesting that the correct embedding of these residues in the alpha-beta interface may constitute the rate limiting step in folding. These results are in accord with experimental observations on the folding/unfolding behavior of hen egg lysozyme at room temperature. They would also explain the loss of stability and the tendency to aggregation observed for the mutant Leu55Thr, and the slow refolding kinetics observed in the analogous amyloidogenic variant of human lysozyme.

Relative spatial position of a snake neurotoxin and the reduced disulfide bond alpha (Cys192-Cys193) at the alpha gamma interface of the nicotinic acetylcholine receptor.

We determined the distances separating five functionally important residues (Gln(10), Lys(27), Trp(29), Arg(33), and Lys(47)) of a three-fingered snake neurotoxin from the reduced disulfide bond alpha(Cys(192)-Cys(193)) located at the alphagamma interface of the Torpedo nicotinic acetylcholine receptor. Each toxin position was substituted individually for a cysteine, which was then linked to a maleimido moiety through three different spacers, varying in length from 10 to 22 A. We estimated the coupling efficiency between the 15 toxin derivatives and the reduced cystine alpha(192-193) by gel densitometry of Coomassie Blue-stained gels. A nearly quantitative coupling was observed between alphaCys(192) and/or alphaCys(193) and all probes introduced at the tip of the first (position 10) and second (position 33) loops of Naja nigricollis alpha-neurotoxin. These data sufficed to locate the reactive thiolate in a "croissant-shaped" volume comprised between the first two loops of the toxin. The volume was further restrained by taking into account the absence or partial coupling of the other derivatives. Altogether, the data suggest that alphaCys(192) and/or alphaCys(193), at the alphagamma interface of a muscular-type acetylcholine receptor, is (are) located in a volume located between 11.5 and 15.5 A from the alpha-carbons at positions 10 and 33 of the toxin, under the tip of the toxin first loop and close to the second one.

Internal motion time scales of a small, highly stable and disulfide-rich protein: a 15N, 13C NMR and molecular dynamics study.

Motions of the backbone C alpha H alpha and threonine C beta H beta bonds of toxin alpha were investigated using natural abundance 13C NMR and molecular dynamics. Measurement of the 13C longitudinal and transverse relaxation rates employed ACCORDION techniques together with coherence selection by pulsed field gradients and sensitivity enhancement through the use of preservation of equivalent pathway, thus allowing a considerable reduction of the required spectrometer time. 13C R1, R2, 1H-->13C NOE were obtained, as well as the variations of R1 rho (90 degrees) as a function of the rf field strength. These data were compared to those recorded by 1H and 15N NMR on a labelled sample of the toxin [Guenneugues et al. (1997) Biochemistry, 36, 16097-16108]. Both sets of data showed that picosecond to nanosecond time scale motions are well correlated to the secondary structure of the protein. This was further reinforced by the analysis of a 1 ns molecular dynamics simulation in water. Several C alpha H alpha and threonine C beta H beta experimentally exhibit fast motions with a correlation time longer than 500 ps, that cannot be sampled along the simulation. In addition, the backbone exhibits motions on the microsecond to millisecond time scale on more than half of its length. Thus, toxin alpha, a highly stable protein (Tm = 75 degrees C at acidic pH) containing 61 amino acids and 4 disulfides, shows important internal motions on time scales ranging from 0.1-0.5 ps, to 10-100 ps, 1 ns, and about 30 microseconds to 10 ms.

Conformational and functional variability supported by the BPTI fold: solution structure of the Ca2+ channel blocker calcicludine.

Calcicludine, a 60-amino acid protein isolated from the green mamba venom, has been recently identified as blocking a large set (i.e., L-, N- and P-type) of Ca2+ channels. The three-dimensional structure of calcicludine has been determined by NMR and molecular modeling using a data set of 723 unambiguous and 265 ambiguous distance restraints, as 33 phi and 13 chi1 dihedral angle restraints. Analysis of the 15 final structures (backbone root-mean-square deviation = 0.6 A) shows that calcicludine adopts the Kunitz-type protease inhibitor fold. Its three-dimensional structure is similar to that of snake K+ channel blockers dendrotoxins. Conformational differences with protease inhibitors and dendrotoxins are localized in the 3(10) helix and loop 1 (segments 1-7 and 10-19), the extremity of the beta-hairpin (segment 27-30), and loop 2 (segment 39-44). These regions correspond to the functional sites of bovine pancreatic trypsin inhibitor (BPTI) and dendrotoxins. The positioning of the N-terminal segment 1-7 relative to the rest of the protein is characteristic of calcicludine. The involvement of this segment and the positively charged K31 at the tip of the beta-hairpin in the biological activity of calcicludine is discussed.

A gradual disruption of tight side-chain packing: 2D 1H-NMR characterization of acid-induced unfolding of CHABII.

Little is known about the mechanism of the transition between native proteins and partially folded intermediates. Complete assignments of 2D 1H-NOESY spectra of CHABII at 5 degrees C, pH 6.3, 5.5, 4.6 and 4.0, reveal that lowering of pH results in an extensive but gradual disappearance of NOEs, implying a gradual disruption of tight side-chain packing. Moreover, a tertiary packing core is identified at 5 degrees C and pH 4.0, characterized by persistent long-range NOEs. Thus, we suggest that severe disruption of tight side-chain packing of CHABII can occur at a stage where its secondary structure and tertiary topology remain highly native-like.

Structural and functional consequences of the presence of a fourth disulfide bridge in the scorpion short toxins: solution structure of the potassium channel inhibitor HsTX1.

We have determined the three-dimensional structure of the potassium channel inhibitor HsTX1, using nuclear magnetic resonance and molecular modeling. This protein belongs to the scorpion short toxin family, which essentially contains potassium channel blockers of 29 to 39 amino acids and three disulfide bridges. It is highly active on voltage-gated Kv1.3 potassium channels. Furthermore, it has the particularity to possess a fourth disulfide bridge. We show that HsTX1 has a fold similar to that of the three-disulfide-bridged toxins and conserves the hydrophobic core found in the scorpion short toxins. Thus, the fourth bridge has no influence on the global conformation of HsTX1. Most residues spatially analogous to those interacting with voltage-gated potassium channels in the three-disulfide-bridged toxins are conserved in HsTX1. Thus, we propose that Tyr21, Lys23, Met25, and Asn26 are involved in the biological activity of HsTX1. As an additional positively charged residue is always spatially close to the aromatic residue in toxins blocking the voltage-gated potassium channels, and as previous mutagenesis experiments have shown the critical role played by the C-terminus in HsTX1, we suggest that Arg33 is also important for the activity of the four disulfide-bridged toxin. Docking calculations confirm that, if Lys23 and Met25 interact with the GYGDMH motif of Kv1.3, Arg33 can contact Asp386 and, thus, play the role of the additional positively charged residue of the toxin functional site. This original configuration of the binding site of HsTX1 for Kv1.3, if confirmed experimentally, offers new structural possibilities for the construction of a molecule blocking the voltage-gated potassium channels.

Novel miniproteins engineered by the transfer of active sites to small natural scaffolds.

Small multidisulfide-containing proteins are attractive structural templates to produce a biologically active conformation that mimics the binding surface of natural large proteins. In particular, the structural motif that is evolutionary conserved in all scorpion toxins has a small size (30-40 amino acid residues), a great structural stability, and high permissiveness for sequence mutation. This motif is composed of a beta-sheet and an alpha-helix bridged in the interior core by three disulfides. We have used this motif successfully to transfer within its beta-sheet new functional sites, including the curaremimetic loop of a snake neurotoxin and the CDR2-like site of human CD4. Accumulated evidence indicated that the two miniproteins produced, the curaremimetic miniprotein and the CD4 mimetic, contain the alpha/beta fold that is characteristic of the scaffold used and bind respectively to the acetylcholine receptor and to the envelope gp120 of HIV-1. Furthermore, the latter was shown to prevent viral infection of lymphocytes. These examples illustrate that, by the transfer of active sites to small and stable natural scaffolds, it is possible to engineer miniproteins reproducing, in part, the function of much larger proteins. Such miniproteins may be of great utility as tools in structure-function studies and as leads in drug design.

Three-dimensional structure of kappa-conotoxin PVIIA, a novel potassium channel-blocking toxin from cone snails.

kappa-Conotoxin PVIIA from the venom of Conus purpurascens is the first cone snail toxin that was described to block potassium channels. We synthesized chemically this toxin and showed that its disulfide bridge pattern is similar to those of omega- and delta-conotoxins. kappa-conotoxin competes with radioactive alpha-dendrotoxin for binding to rat brain synaptosomes, confirming its capacity to bind to potassium channels; however, it behaves as a weak competitor. The three-dimensional structure of kappa-conotoxin PVIIA, as elucidated by NMR spectroscopy and molecular modeling, comprises two large parallel loops stabilized by a triple-stranded antiparallel beta-sheet and three disulfide bridges. The overall fold of kappa-conotoxin is similar to that of calcium channel-blocking omega-conotoxins but differs from those of potassium channel-blocking toxins from sea anemones, scorpions, and snakes. Local topographies of kappa-conotoxin PVIIA that might account for its capacity to recognize Kv1-type potassium channels are discussed.

NMR solution structure of a two-disulfide derivative of charybdotoxin: structural evidence for conservation of scorpion toxin alpha/beta motif and its hydrophobic side chain packing.

The alpha/beta scorpion fold consisting of a short alpha-helix and beta-sheet is a structural motif common to scorpion toxins, insect defensins, and plant gamma-thionins that invariably contains three disulfides. CHABII is a two-disulfide derivative of the scorpion toxin charybdotoxin (ChTX), chemically synthesized by inserting two L-alpha-aminobutyric acids in place of the two half-cystine residues involved in the disulfide 13-33. This disulfide is one of the two disulfides which connect the alpha-helix to the beta-sheet. The solution structure of CHABII was determined at pH 6.3 and 5 degrees C using 2D NMR and simulated annealing from 513 distance and 46 dihedral angle constraints. The NMR structure of CHABII is well-defined as judged from the low value of the averaged backbone rms deviation between the 30 lowest energy structures and the energy-minimized mean structure ((rmsd) = 0.65 A for the entire sequence and 0.48 A for the segment 3-36). Analysis and comparison of the solution structures of CHABII and ChTX lead to the following conclusions: (i) the fold of CHABII is similar to that of ChTX as indicated by the low value of the averaged backbone atomic rms deviation between the 10 lowest energy solution structures of the two proteins (1.44 A); (ii) the packing of the hydrophobic core is well-preserved, underlying the critical structural role of the hydrophobic interactions even for such a small and cysteine-rich protein as ChTX.

On the convergent evolution of animal toxins. Conservation of a diad of functional residues in potassium channel-blocking toxins with unrelated structures.

BgK is a K+ channel-blocking toxin from the sea anemone Bunodosoma granulifera. It is a 37-residue protein that adopts a novel fold, as determined by NMR and modeling. An alanine-scanning-based analysis revealed the functional importance of five residues, which include a critical lysine and an aromatic residue separated by 6.6 +/- 1.0 A. The same diad is found in the three known homologous toxins from sea anemones. More strikingly, a similar functional diad is present in all K+ channel-blocking toxins from scorpions, although these toxins adopt a distinct scaffold. Moreover, the functional diads of potassium channel-blocking toxins from sea anemone and scorpions superimpose in the three-dimensional structures. Therefore, toxins that have unrelated structures but similar functions possess conserved key functional residues, organized in an identical topology, suggesting a convergent functional evolution for these small proteins.

[Body composition during HIV infect: physiopathology, methods of measurement and review of results]. / Composition corporelle au cours de l'infection à VIH: physiopathologie, méthodes de mesure et revue des résultats.

The immunological linkage between infection and nutritional status was well established even before the AIDS epidemic. AIDS-associated malnutrition alters cellular immunocompetence by decreasing phagocytic function, antibodies secretion, complement system, and number of lymphocytes. Vitamin and oligofactors deficiency increases the dysimmunity. Relations between nutritional status, infectious disease, and immunity system suggest that nutrition could act as a co-factor in the HIV progression. Correlation between survival and weight loss has been shown. Death occurs when loss of 50% of the lean body mass or of 34% of body weight is observed. Furthermore body composition studies have displayed a decrease of body cell mass out of proportion with the discharge in fat or weight. But what is not yet correctly identified is the impact of malnutrition on immunocompetence of a given subject. A significantly longer survival seems likely in patients whose body cell mass is over 30% of total weight, or whose albumin level goes beyond 30 g/L, this criterian being non-specific; parameters of nutritional status are independent of CD4 lymphocytosis, and would have a high predictive value of the survival time. So, early screening of body composition alterations must be used to identify the sligh test erosion of lean mass, as soon as seroconversion is known, even during treatments, using non-invasive, non-expensive, reproducible methods (and no longer weight loss only). This preventive attitude will increase metabolism knowledge about seropositivity status, in time for carrying out adjusted realimentation measures, for providing planned physical activity advice to develop muscular mass.

Picosecond to hour time scale dynamics of a "three finger" toxin: correlation with its toxic and antigenic properties.

Toxin alpha from Naja nigricollis (61 amino acids, four disulfide bridges) belongs to the "three finger" fold family, which contains snake toxins with various biological activities and nontoxic proteins from different origins. In this paper, we report an extensive 1H and 15N NMR study of the dynamics of toxin alpha in solution. 15N relaxation, 1H off-resonance ROESY, and H-D exchange experiments allowed us to probe picosecond to hour motions in the protein. Analysis of these NMR measurements demonstrates that toxin alpha exhibits various time scale motions, i.e., particularly large amplitude picosecond to nanosecond motions at the tips of the loops, observable microsecond to millisecond motions around two disulfide bridges, second time scale motions around the C-N bonds of asparagine and glutamine side chains which are more or less rapid depending on their amino acid solvent accessibility, and minute to hour motions in the beta-sheet structure. The less well-defined regions of toxin alpha solution structures are subject to important picosecond to nanosecond motions. The toxic site is organized around residues belonging to the rigid core of the molecule but also comprises residues exhibiting dynamics on various time scales. The Malpha1 epitope is subject to large picosecond to millisecond motions, which are probably modified by the interaction with the antibody. This phenomenon could be linked to the neutralizing properties of the antibody.

Transfer of a beta-hairpin from the functional site of snake curaremimetic toxins to the alpha/beta scaffold of scorpion toxins: three-dimensional solution structure of the chimeric protein.

The alpha/beta scorpion fold is shared by scorpion toxins, insect defensins, and plant thionins. This small and functionally versatile template contains an alpha-helix and a triple beta-sheet linked by three disulfide bridges. With the view to introduce novel functional centers within this fold, we replaced the sequence (the cysteines and glycines excepted) of the original beta-hairpin of a scorpion toxin by the sequence of a beta-hairpin that forms part of the site by which snake neurotoxins bind to nicotinic acetylcholine receptors (AcChOR). The resulting chimeric protein, synthesized by chemical means, binds to AcChOR, though with a lower affinity than the snake toxins [Drakopoulou; E., Zinn-Justin, S., Guenneugues, M., Gilquin, B., Ménez, A., & Vita, C. (1996) J. Biol. Chem. 271, 11979-11987]. The work described in this paper is an attempt to clarify the structural consequences associated with the transfer of the beta-hairpin. We report the determination of the three-dimensional solution structure of the chimeric protein by proton NMR spectroscopy and molecular dynamics calculations. Comparison of the structure of the chimera with those of the scorpion alpha/beta toxin and of the snake neurotoxin shows that (i) the new protein folds as an alpha/beta motif and (ii) the beta-hairpins of the chimera and of the curaremimetic toxin adopt a similar conformation. A closer inspection of the differences between the structures of the original and transferred beta-hairpins allows rationalization of the biological properties of the chimera.

Determination of the three-dimensional solution structure of noxiustoxin: analysis of structural differences with related short-chain scorpion toxins.

The 3D structure of noxiustoxin, the first identified scorpion toxin acting on K+ channels, has been elucidated by NMR and molecular modeling. Thirty-nine solution structures were calculated using 572 distance and 42 dihedral restraints. The average atomic rms deviation between the refined structures and the mean structure is 0.75 A for the backbone atoms. Noxiustoxin adopts a alpha/beta scaffold constituted of a three-stranded beta-sheet (residues 2-3, 25-30, 33-38) linked to a helix (residues 10-20) through two disulfide bridges. A comparison between the 3D structure of noxiustoxin and those of other structurally and functionally related scorpion toxins (charybdotoxin, PO5-NH2, kaliotoxin) revealed a bending capacity of the helix and a variability in the relative orientations between the helix and the beta-sheet. These two features highlight the plasticity of the alpha/beta scaffold and offer a structural explanation for the capacity of the fold to accommodate an additional alanine residue in the Gly-x-Cys pattern of a previously proposed consensus sequence [Bontems et al. (1991) Science 254, 1521-1523]. Our structural data also emphasize the possibility that the beta-sheet of NTX is implicated in the capacity of NTX to recognize voltage-dependent K+ channels.