Isolation and pharmacological characterization of AdTx1, a natural peptide displaying specific insurmountable antagonism of the alpha1A-adrenoceptor.

BACKGROUND AND PURPOSE: Venoms are a rich source of ligands for ion channels, but very little is known about their capacity to modulate G-protein coupled receptor (GPCR) activity. We developed a strategy to identify novel toxins targeting GPCRs. EXPERIMENTAL APPROACH: We studied the interactions of mamba venom fractions with alpha(1)-adrenoceptors in binding experiments with (3)H-prazosin. The active peptide (AdTx1) was sequenced by Edman degradation and mass spectrometry fragmentation. Its synthetic homologue was pharmacologically characterized by binding experiments using cloned receptors and by functional experiments on rabbit isolated prostatic smooth muscle. KEY RESULTS: AdTx1, a 65 amino-acid peptide stabilized by four disulphide bridges, belongs to the three-finger-fold peptide family. It has subnanomolar affinity (K(i)= 0.35 nM) and high specificity for the human alpha(1A)-adrenoceptor subtype. We showed high selectivity and affinity (K(d)= 0.6 nM) of radio-labelled AdTx1 in direct binding experiments and revealed a slow association constant (k(on)= 6 x 10(6).M(-1).min(-1)) with an unusually stable alpha(1A)-adrenoceptor/AdTx1 complex (t(1/2diss)= 3.6 h). AdTx1 displayed potent insurmountable antagonism of phenylephrine's actions in vitro (rabbit isolated prostatic muscle) at concentrations of 10 to 100 nM. CONCLUSIONS AND IMPLICATIONS: AdTx1 is the most specific and selective peptide inhibitor for the alpha(1A)-adrenoceptor identified to date. It displays insurmountable antagonism, acting as a potent relaxant of smooth muscle. Its peptidic nature can be exploited to develop new tools, as a radio-labelled-AdTx1 or a fluoro-labelled-AdTx1. Identification of AdTx1 thus offers new perspectives for developing new drugs for treating benign prostatic hyperplasia.

Structure-based secondary structure-independent approach to design protein ligands: Application to the design of Kv1.2 potassium channel blockers.

We have developed a structure-based approach to the design of protein ligands. This approach is based on the transfer of a functional binding motif of amino acids, often referred as to the "hot spot", on a host protein able to reproduce the functional topology of these residues. The scaffolds were identified by a systematic in silico search in the Protein Data Bank for proteins possessing a group of residues in a topology similar to that adopted by the functional motif in a reference ligand of known 3D structure. In contrast to previously reported studies, this search is independent of the particular secondary structure supporting the functional motif. To take into account the global properties of the host protein, two additional criteria were taken into account in the selection process: (1) Only those scaffolds sterically compatible with the positioning of the functional motif as observed in a reference complex model were retained. (2) Host proteins displaying electrostatic potentials, in the region of the transferred functional motif, similar to that of the reference ligand were selected. This approach was applied to the development of protein ligands of the Kv1.2 channel using BgK, a small protein isolated from the sea anemone Bunodosoma granulifera, as the reference ligand. Four proteins obtained by this approach were produced for experimental evaluation. The X-ray structure of one of these proteins was determined to check for similarity of the transferred functional motif with the structure it adopts in the reference ligand. Three of these protein ligands bind the Kv1.2 channel with inhibition constants of 0.5, 1.5, and 1.6 microM. Several mutants of these designed protein ligands gave binding results consistent with the presumed binding mode. These results show that protein ligands can be designed by transferring a binding motif on a protein host selected to reproduce the functional topology of this motif, irrespective to the secondary structure supporting the functional motif, if the host protein possesses steric and electrostatic properties compatible with the binding to the target. This result opens the way to the design of protein ligands by taking advantage of the considerable structural repertoire of the Protein Data Bank.

Toxicity-based selection of Escherichia coli mutants for functional recombinant protein production: application to an antibody fragment.

We propose a novel approach to the selection of Escherichia coli bacterial strains improved for the production of recombinant functional proteins. This approach is based on aggregation-induced toxicity of recombinant proteins. We show that selection of clones displaying a reduced toxicity is an efficient means of isolating bacteria producing recombinant protein with reduced aggregation in favour of correct folding. For an efficient selection, we found that time of toxicity induction must be precisely determined and recombinant protein must be expressed as a fusion with a protein whose activity is easily detectable on plates, thus allowing elimination of non-productive mutants. Choosing the expression to the periplasmic space of an scFv fragment fused to the N-terminus of alkaline phosphatase as a model, we selected chromosomal mutations that reduce aggregation-induced toxicity and showed that they concomitantly improve production of a functional recombinant hybrid. The effects of the mutations isolated could then be cumulated with those of other strategies used for recombinant scFv production. Thus, we could ensure a 6- to 16-fold increase in production of a functional scFv-PhoA hybrid. This is the first report demonstrating the possibility of directly selecting on agar plates E.coli strains improved for functional recombinant protein production from a large bacterial mutant library.

Artificial evolution of an enzyme active site: structural studies of three highly active mutants of Escherichia coli alkaline phosphatase.

The crystal structure of three mutants of Escherichia coli alkaline phosphatase with catalytic activity (k(cat)) enhancement as compare to the wild-type enzyme is described in different states. The biological aspects of this study have been reported elsewhere. The structure of the first mutant, D330N, which is threefold more active than the wild-type enzyme, was determined with phosphate in the active site, or with aluminium fluoride, which mimics the transition state. These structures reveal, in particular, that this first mutation does not alter the active site. The second mutant, D153H-D330N, is 17-fold more active than the wild-type enzyme and activated by magnesium, but its activity drops after few days. The structure of this mutant was solved under four different conditions. The phosphate-free enzyme was studied in an inactivated form with zinc at site M3, or after activation by magnesium. The comparison of these two forms free of phosphate illustrates the mechanism of the magnesium activation of the catalytic serine residue. In the presence of magnesium, the structure was determined with phosphate, or aluminium fluoride. The drop in activity of the mutant D153H-D330N could be explained by the instability of the metal ion at M3. The analysis of this mutant helped in the design of the third mutant, D153G-D330N. This mutant is up to 40-fold more active than the wild-type enzyme, with a restored robustness of the enzyme stability. The structure is presented here with covalently bound phosphate in the active site, representing the first phosphoseryl intermediate of a highly active alkaline phosphatase. This study shows how structural analysis may help to progress in the improvement of an enzyme catalytic activity (k(cat)), and explains the structural events associated with this artificial evolution.

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.

Slow folding of three-fingered toxins is associated with the accumulation of native disulfide-bonded intermediates.

Snake neurotoxins are short all-beta proteins that display a complex organization of the disulfide bonds: two bonds connect consecutive cysteine residues (C43-C54, C55-C60), and two bonds intersect when bridging (C3-C24, C17-C41) to form a particular structure classified as "disulfide beta-cross". We investigated the oxidative folding of a neurotoxin variant, named alpha62, to define the chemical nature of the three-disulfide intermediates that accumulate during the process in order to describe in detail its folding pathway. These folding intermediates were separated by reverse-phase HPLC, and their disulfide bonds were identified using a combination of tryptic hydrolysis, manual Edman degradation, and mass spectrometry. Two dominant intermediates containing three native disulfide bonds were identified, lacking the C43-C54 and C17-C41 pairing and therefore named des-[43-54] and des-[17-41], respectively. Both species were individually allowed to reoxidize under folding conditions, showing that des-[17-41] was a fast-forming nonproductive intermediate that had to interconvert into the des-[43-54] isomer before forming the native protein. Conversely, the des-[43-54] intermediate appeared to be the immediate precursor of the oxidized neurotoxin. A kinetic model for the folding of neurotoxin alpha62 which fits with the observed time-course accumulation of des-[17-41] and des-[43-54] is proposed. The effect of turn 2, located between residues 17 and 24, on the overall kinetics is discussed in view of this model.

A new omega-conotoxin that targets N-type voltage-sensitive calcium channels with unusual specificity.

A new specific voltage-sensitive calcium channel (VSCC) blocker has been isolated from the venom of the fish-hunting cone snail Conus consors. This peptide, named omega-Ctx CNVIIA, consists of 27 amino acid residues folded by 3 disulfide bridges. Interestingly, loop 4, which is supposed to be crucial for selectivity, shows an unusual sequence (SSSKGR). The synthesis of the linear peptide was performed using the Fmoc strategy, and the correct folding was achieved in the presence of guanidinium chloride, potassium buffer, and reduced/oxidized glutathione at 4 degrees C for 3 days. Both synthetic and native toxin caused an intense shaking activity, characteristic of omega-conotoxins targeting N-type VSCC when injected intracerebroventricularly to mice. Binding studies on rat brain synaptosomes revealed that the radioiodinated omega-Ctx CNVIIA specifically and reversibly binds to high-affinity sites with a K(d) of 36.3 pM. Its binding is competitive with omega-Ctx MVIIA at low concentration (K(i) = 2 pM). Moreover, omega-Ctx CNVIIA exhibits a clear selectivity for N-type VSCCs versus P/Q-type VSCCs targeted respectively by radioiodinated omega-Ctx GVIA and omega-Ctx MVIIC. Although omega-Ctx CNVIIA clearly blocked N-type Ca(2+) current in chromaffin cells, this toxin did not inhibit acetylcholine release evoked by nerve stimuli at the frog neuromuscular junction, in marked contrast to omega-Ctx GVIA. omega-Ctx CNVIIA thus represents a new selective tool for blocking N-type VSCC that displays a unique pharmacological profile and highlights the diversity of voltage-sensitive Ca(2+) channels in the animal kingdom.

Complementarity and redundancy of the binding specificity of HLA-DRB1, -DRB3, -DRB4 and -DRB5 molecules.

The second HLA-DR molecules, which are encoded by loci different from HLA-DRB1 are weakly polymorphic. Predominant alleles such as HLA-DRB3*0101, HLA-DRB4*0101 and HLA-DRB5*0101 are therefore interesting targets to define antigenic peptides with major impact for the entire population. Strikingly, they have been poorly investigated. Thus we have characterized peptides from the major bee venom allergen that bind efficiently to these molecules and compared them to peptides specific for preponderant HLA-DRB1 molecules. Interestingly, DRB5*0101 and DRB1*0701 molecules share four binding peptides and use some identical anchor residues. Similarities are also found between DRB3*0101 and its haplotype-associated molecules DRB1*0301 and DRB1*1301. In sharp contrast, DRB4*0101 exhibits a unique binding specificity, which results from particular structural features of its peptide binding site. Ybeta81 seems to alter the amino acid preferences of the P1 pocket, while Rbeta71, Ebeta74, Nbeta26 and Cbeta13 confer to the P4 pocket a unique topology. Our results show that the two HLA-DR molecules expressed in most haplotypes studied here have mostly complementary binding patterns. Only haplotype HLA-DR52 exhibits peptide binding redundancies. Finally our results document functional similarities among HLA-DR molecules and allow us to propose peptide sequences that might be useful for bee venom immunotherapy.

Structural evidence for a functional role of human tissue nonspecific alkaline phosphatase in bone mineralization.

The human tissue nonspecific alkaline phosphatase (TNAP) is found in liver, kidney, and bone. Mutations in the TNAP gene can lead to Hypophosphatasia, a rare inborn disease that is characterized by defective bone mineralization. TNAP is 74% homologous to human placental alkaline phosphatase (PLAP) whose crystal structure has been recently determined at atomic resolution (Le Du, M. H., Stigbrand, T., Taussig, M. J., Ménez, A., and Stura, E. A. (2001) J. Biol. Chem, 276, 9158-9165). The degree of homology allowed us to build a reliable TNAP model to investigate the relationship between mutations associated with hypophosphatasia and their probable consequences on the activity or the structure of the enzyme. The mutations are clustered within five crucial regions, namely the active site and its vicinity, the active site valley, the homodimer interface, the crown domain, and the metal-binding site. The crown domain and the metal-binding domain are mammalian-specific and were observed for the first time in the PLAP structure. The crown domain contains a collagen binding loop. A synchrotron radiation x-ray fluorescence study confirms that the metal in the metal-binding site is a calcium ion. Several severe mutations in TNAP occur around this calcium site, suggesting that calcium may be of critical importance for the TNAP function. The presence of this extra metal-binding site gives new insights on the controversial role observed for calcium.

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.

Cyclic dipeptide oxidase from Streptomyces noursei. Isolation, purification and partial characterization of a novel, amino acyl alpha,beta-dehydrogenase.

Cyclic dipeptide oxidase is a novel enzyme that specifically catalyzes the formation of alpha,beta-dehydro-Phe (Delta Phe) and alpha,beta-dehydro-Leu (Delta Leu) residues during the biosynthesis of albonoursin, cyclo(Delta Phe-Delta Leu), an antibiotic produced by Streptomyces noursei. It was purified 600-fold with a 30% overall recovery, and consists of the association of a single type of subunit with a relative molecular mass of 21,066 resulting in a large homopolymer of relative molecular mass over 2,000,000. The enzyme exhibits a typical flavoprotein spectrum with maxima at 343.5 and 447.5 nm, the flavin prosthetic group being covalently bound to the protein. The catalytic reaction of the natural substrate cyclo(L-Phe-L-Leu) occurs in a two-step sequential reaction leading first to cyclo(alpha,beta-dehydro-Phe-L-Leu) and finally to albonoursin. Kinetic parameters for the first step were determined (K(m) = 53 microM; k = 0.69 s(-1)). The enzyme was shown to catalyze the conversion of a variety of cyclo(dipeptides) and can be reoxidized at the expense of molecular oxygen by producing H(2)O(2). This reaction mechanism, which differs from those already described for the formation of alpha,beta-dehydro-amino acids, might consist of the transient formation of an intermediate imine followed by its rearrangement into an alpha,beta-dehydro-residue.

Crystal structure of alkaline phosphatase from human placenta at 1.8 A resolution. Implication for a substrate specificity.

Human placental alkaline phosphatase (PLAP) is one of three tissue-specific human APs extensively studied because of its ectopic expression in tumors. The crystal structure, determined at 1.8-A resolution, reveals that during evolution, only the overall features of the enzyme have been conserved with respect to Escherichia coli. The surface is deeply mutated with 8% residues in common, and in the active site, only residues strictly necessary to perform the catalysis have been preserved. Additional structural elements aid an understanding of the allosteric property that is specific for the mammalian enzyme (Hoylaerts, M. F., Manes, T., and Millán, J. L. (1997) J. Biol. Chem. 272, 22781-22787). Allostery is probably favored by the quality of the dimer interface, by a long N-terminal alpha-helix from one monomer that embraces the other one, and similarly by the exchange of a residue from one monomer in the active site of the other. In the neighborhood of the catalytic serine, the orientation of Glu-429, a residue unique to PLAP, and the presence of a hydrophobic pocket close to the phosphate product, account for the specific uncompetitive inhibition of PLAP by l-amino acids, consistent with the acquisition of substrate specificity. The location of the active site at the bottom of a large valley flanked by an interfacial crown-shaped domain and a domain containing an extra metal ion on the other side suggest that the substrate of PLAP could be a specific phosphorylated protein.

Improving Escherichia coli alkaline phosphatase efficacy by additional mutations inside and outside the catalytic pocket.

We describe a strategy that allowed us to confer on a bacterial (E. coli) alkaline phosphatase (AP) the high catalytic activity of the mammalian enzyme while maintaining its high thermostability. First, we identified mutations, at positions other than those occupied by essential catalytic residues, which inactivate the bacterial enzyme without destroying its overall conformation. We transferred concomitantly into the bacterial enzyme four residues of the mammalian enzyme, two being in the catalytic pocket and two being outside. Second, the gene encoding the inactive mutant was submitted to random mutagenesis. Enzyme activity was restored upon the single mutation D330N, at a position that is 12 A away from the center of the catalytic pocket. Third, this mutation was combined with other mutations previously reported to increase AP activity slightly in the presence of magnesium. As a result, at pH 10.0 the phosphatase activity of both mutants D330N/D153H and D330N/D153G was 17-fold higher than that of the wild-type AP. Strikingly, although the two individual mutations D153H and D153G destabilize the enzyme, the double mutant D330N/D153G remained highly stable (T(m)=87 degrees C). Moreover, when combining the phosphatase and transferase activities, the catalytic activity of the mutant D330N/D153G increased 40-fold (k(cat)=3200 s-1) relative to that of the wild-type enzyme (k(cat)=80 s-1). Due to the simultaneous increase in K(m), the resulting k(cat)/K(m) value was only increased by a factor of two. Therefore, a single mutation occurring outside a catalytic pocket can dramatically control not only the activity of an enzyme, but also its thermostability. Preliminary crystallographic data of a covalent D330N/D153G enzyme-phosphate complex show that the phosphate group has significantly moved away from the catalytic pocket, relative to its position in the structure of another mutant previously reported.

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.

Tailoring new enzyme functions by rational redesign.

Site-directed mutagenesis is still a very efficient strategy to elaborate improved enzymes. Recently, advances have been made in developing rational strategies aimed at reshaping enzyme specificities and mechanisms, and at engineering biocatalysts through molecular assembling. These knowledge-based studies greatly benefit from the most recent computational analyses of enzyme structures and functions. The combination of rational and combinatorial methods opens up new vistas in the design of stable and efficient enzymes.

Isolation of a tarantula toxin specific for a class of proton-gated Na+ channels.

Acid sensing is associated with nociception, taste transduction, and perception of extracellular pH fluctuations in the brain. Acid sensing is carried out by the simplest class of ligand-gated channels, the family of H(+)-gated Na(+) channels. These channels have recently been cloned and belong to the acid-sensitive ion channel (ASIC) family. Toxins from animal venoms have been essential for studies of voltage-sensitive and ligand-gated ion channels. This paper describes a novel 40-amino acid toxin from tarantula venom, which potently blocks (IC(50) = 0.9 nm) a particular subclass of ASIC channels that are highly expressed in both central nervous system neurons and sensory neurons from dorsal root ganglia. This channel type has properties identical to those described for the homomultimeric assembly of ASIC1a. Homomultimeric assemblies of other members of the ASIC family and heteromultimeric assemblies of ASIC1a with other ASIC subunits are insensitive to the toxin. The new toxin is the first high affinity and highly selective pharmacological agent for this novel class of ionic channels. It will be important for future studies of their physiological and physio-pathological roles.

Do structural deviations between toxins adopting the same fold reflect functional differences?

Three-finger proteins form a structurally related family of compounds that exhibit a great variety of biological properties. To address the question of the prediction of functional areas on their surfaces, we tentatively conferred the acetylcholinesterase inhibitory activity of fasciculins on a short-chain curaremimetic toxin. For this purpose, we assimilated the three-dimensional structure of fasciculin 2 with the one of toxin alpha. This comparison revealed that the tips of the first and second loops, together with the C terminus residue, deviated most. A first recombinant fasciculin/toxin alpha chimera was designed by transferring loop 1 in its entirety together with the tip of loop 2 of fasciculin 2 into the toxin alpha scaffold. A second chimera (rChII) was obtained by adding the point Asn-61 --> Tyr substitution. Comparison of functional and structural properties of both chimeras show that rChII can accommodate the imposed modifications and displays nearly all the acetylcholinesterase-blocking activities of fasciculins. The three-dimensional structure of rChII demonstrates that rChII adopts a typical three-fingered fold with structural features of both parent toxins. Taken together, these results emphasize the great structural flexibility and functional adaptability of that fold and confirm that structural deviations between fasciculins and short-chain neurotoxins do indeed reflect functional diversity.

Molecular determinants by which a long chain toxin from snake venom interacts with the neuronal alpha 7-nicotinic acetylcholine receptor.

Long chain curarimimetic toxins from snake venom bind with high affinities to both muscular type nicotinic acetylcholine receptors (AChRs) (K(d) in the pm range) and neuronal alpha 7-AChRs (K(d) in the nm range). To understand the molecular basis of this dual function, we submitted alpha-cobratoxin (alpha-Cbtx), a typical long chain curarimimetic toxin, to an extensive mutational analysis. By exploring 36 toxin mutants, we found that Trp-25, Asp-27, Phe-29, Arg-33, Arg-36, and Phe-65 are involved in binding to both neuronal and Torpedo (Antil, S., Servent, D., and Ménez, A. (1999) J. Biol. Chem. 274, 34851-34858) AChRs and that some of them (Trp-25, Asp-27, and Arg-33) have similar binding energy contributions for the two receptors. In contrast, Ala-28, Lys-35, and Cys-26-Cys-30 selectively bind to the alpha 7-AChR, whereas Lys-23 and Lys-49 bind solely to the Torpedo AChR. Therefore, alpha-Cbtx binds to two AChR subtypes using both common and specific residues. Double mutant cycle analyses suggested that Arg-33 in alpha-Cbtx is close to Tyr-187 and Pro-193 in the alpha 7 receptor. Since Arg-33 of another curarimimetic toxin is close to the homologous alpha Tyr-190 of the muscular receptor (Ackermann, E. J., Ang, E. T. H., Kanter, J. R., Tsigelny, I., and Taylor, P. (1998) J. Biol. Chem. 273, 10958-10964), toxin binding probably occurs in homologous regions of neuronal and muscular AChRs. However, no coupling was seen between alpha-Cbtx Arg-33 and alpha 7 receptor Trp-54, Leu-118, and Asp-163, in contrast to what was observed in a homologous situation involving another toxin and a muscular receptor (Osaka, H., Malany, S., Molles, B. E., Sine, S. M., and Taylor, P. (2000) J. Biol. Chem. 275, 5478-5484). Therefore, although occurring in homologous regions, the detailed modes of toxin binding to alpha 7 and muscular receptors are likely to be different. These data offer a molecular basis for the design of toxins with predetermined specificities for various members of the AChR family.

Design of a Gag pentapeptide analogue that binds human cyclophilin A more efficiently than the entire capsid protein: new insights for the development of novel anti-HIV-1 drugs.

Cyclophilin A (hCyp-18), a ubiquitous cytoplasmic peptidyl-prolyl cis/trans isomerase (PPIase), orchestrates HIV-1 core packaging. hCyp-18, incorporated into the virion, enables core uncoating and RNA release and consequently plays a critical role in the viral replication process. hCyp-18 specifically interacts with a single exposed loop of the Gag polyprotein capsid domain via a network of nine hydrogen bonds which mainly implicates a 7-mer fragment of the loop. As previously reported, the corresponding linear heptapeptide Ac-Val-His-Ala-Gly-Pro-Ile-Ala-NH(2) (2) binds to hCyp-18 with a low affinity (IC(50) = 850 +/- 220 microM) but a potentially useful selectivity for hCyp-18 relative to hFKBP-12, another abundant PPIase. On the basis of X-ray structures of Gag fragments:hCyp-18 complexes, we generated a series of modified peptides in order to probe the determinants of the interaction and hence to select a peptidic ligand displaying a higher affinity than the capsid domain of Gag. We synthesized a series of heptapeptides to test the energetic contribution of amino acids besides the Gly-Pro moiety. In particular the importance of the histidine residue for the interaction was underscored. We also investigated the influence of N- and C-terminal modifications. Hexapeptides containing either deaminovaline (Dav) in place of the N-terminal valine or substitution of the C-terminal alanine amide with a benzylamide group displayed increased affinities. Combination of both modifications gave the most potent competitor Dav-His-Ala-Gly-Pro-Ile-NHBn (28) which has a higher affinity for hCyp-18 (K(d) = 3 +/- 0.5 microM) than the entire capsid protein (K(d) = 16 +/- 4 microM) and a very low affinity for hFKBP-12. Some of our results strongly suggest that the title compound is not a substrate of hCyp-18 and interacts preferentially in the trans conformation.