Determination with matrix-assisted laser desorption/ionization tandem time-of-flight mass spectrometry of the extensive disulfide bonding in tarantula venom peptide Psalmopeotoxin I.

Psalmopeotoxin I (PcFK1) is a 33-residue peptide isolated from the venom of the tarantula Psalmopoeus cambridgei. This peptide specifically inhibits the intra-erythrocyte stage of Plasmodium falciparum in vitro. It contains six cysteine residues forming three disulfide bridges and belongs to the superfamily of natural peptides containing the inhibitor cystine knot (ICK) fold. We produced the wild-type and mutated forms of the recombinant peptide to examine the mechanism of action of PcFK1. The purified toxins were consistently produced as two isobaric peptides (r-PcFK1-1 and r-PcFK1-2) with different retention properties but identical anti-plasmodial -biological activity. Comparison of (15)N-NMR heteronuclear single quantum correlation spectra revealed that although rPcFK1-1 was highly structured, rPcFK1-2 does not have a stable three-dimensional structure. We used high-energy collision-induced fragmentation of the peptides with a matrix-assisted laser desorption/ionization tandem time-of- flight mass spectrometer to further investigate the structure of the native peptides in its natural form and produced in E. coli. The fragmentation spectra of the native peptides were very complex due to the occurrence in the spectrum of ions resulting from (1) cross-linking of fragments through a disulfide bridge and (2) asymmetric fragmentations of the disulfide bridges and (3) multiple neutral losses. The tandem mass spectrometry fragmentation pattern of r-PcFK1-1 was similar to that of the natural peptide isolated from crude venom, but r-PcFK1-2 had a clearly distinct fragmentation pattern, more closely resembling the fragmentation spectra of reduced and alkylated peptides. Observed ions could be attributed to specific fragments by comparing spectra between the wild-type and selected variants with point mutations (Y11W, R20T, Y26W, K28V). The disulfide connections in r-PcFK1-2 differed from those of the native peptide and showed a rare disulfide bridge between vicinal cysteine residues. The r-PcFK1_(R20T) variant showed a very limited fragmentation pattern when analyzed in positive mode but displayed much more fragmentation in negative mode pointing out the importance of the R20 residue in the fragmentation of PcFK1. Using the reductive matrix 1,5-diaminonaphtalene promoted strongly in source decay fragmentation of the peptides in MS mode. Our findings illustrated the critical role of the electronic environment around the central Cys(18)-Cys(19) doublet in PcFK1 in internal fragmentation of the peptide.

Chemical synthesis and 1H-NMR 3D structure determination of AgTx2-MTX chimera, a new potential blocker for Kv1.2 channel, derived from MTX and AgTx2 scorpion toxins.

Agitoxin 2 (AgTx2) is a 38-residue scorpion toxin, cross-linked by three disulfide bridges, which acts on voltage-gated K(+) (Kv) channels. Maurotoxin (MTX) is a 34-residue scorpion toxin with an uncommon four-disulfide bridge reticulation, acting on both Ca(2+)-activated and Kv channels. A 39-mer chimeric peptide, named AgTx2-MTX, was designed from the sequence of the two toxins and chemically synthesized. It encompasses residues 1-5 of AgTx2, followed by the complete sequence of MTX. As established by enzyme cleavage, the new AgTx2-MTX molecule displays half-cystine pairings of the type C1-C5, C2-C6, C3-C7, and C4-C8, which is different from that of MTX. The 3D structure of AgTx2-MTX solved by (1)H-NMR, revealed both alpha-helical and beta-sheet structures, consistent with a common alpha/beta scaffold of scorpion toxins. Pharmacological assays of AgTx2-MTX revealed that this new molecule is more potent than both original toxins in blocking rat Kv1.2 channel. Docking simulations, performed with the 3D structure of AgTx2-MTX, confirmed this result and demonstrated the participation of the N-terminal domain of AgTx2 in its increased affinity for Kv1.2 through additional molecular contacts. Altogether, the data indicated that replacement of the N-terminal domain of MTX by the one of AgTx2 in the AgTx2-MTX chimera results in a reorganization of the disulfide bridge arrangement and an increase of affinity to the Kv1.2 channel.

Solution structure of PcFK1, a spider peptide active against Plasmodium falciparum.

Psalmopeotoxin I (PcFK1) is a 33-amino-acid residue peptide isolated from the venom of the tarantula Psalmopoeus cambridgei. It has been recently shown to possess strong antiplasmodial activity against the intra-erythrocyte stage of Plasmodium falciparum in vitro. Although the molecular target for PcFK1 is not yet determined, this peptide does not lyse erythrocytes, is not cytotoxic to nucleated mammalian cells, and does not inhibit neuromuscular function. We investigated the structural properties of PcFK1 to help understand the unique mechanism of action of this peptide and to enhance its utility as a lead compound for rational development of new antimalarial drugs. In this paper, we have determined the three-dimensional solution structure by (1)H two-dimensional NMR means of recombinant PcFK1, which is shown to belong to the ICK structural superfamily with structural determinants common to several neurotoxins acting as ion channels effectors.

An unusual fold for potassium channel blockers: NMR structure of three toxins from the scorpion Opisthacanthus madagascariensis.

The Om-toxins are short peptides (23-27 amino acids) purified from the venom of the scorpion Opisthacanthus madagascariensis. Their pharmacological targets are thought to be potassium channels. Like Csalpha/beta (cystine-stabilized alpha/beta) toxins, the Om-toxins alter the electrophysiological properties of these channels; however, they do not share any sequence similarity with other scorpion toxins. We herein demonstrate by electrophysiological experiments that Om-toxins decrease the amplitude of the K+ current of the rat channels Kv1.1 and Kv1.2, as well as human Kv1.3. We also determine the solution structure of three of the toxins by use of two-dimensional proton NMR techniques followed by distance geometry and molecular dynamics. The structures of these three peptides display an uncommon fold for ion-channel blockers, Csalpha/alpha (cystine-stabilized alpha-helix-loop-helix), i.e. two alpha-helices connected by a loop and stabilized by two disulphide bridges. We compare the structures obtained and the dipole moments resulting from the electrostatic anisotropy of these peptides with those of the only other toxin known to share the same fold, namely kappa-hefutoxin1.

Solution structure of APETx1 from the sea anemone Anthopleura elegantissima: a new fold for an HERG toxin.

APETx1 is a 42-amino acid toxin purified from the venom of the sea anemone Anthopleura elegantissima. This cysteine-rich peptide possesses three disulfide bridges (C4-C37, C6-C30, and C20-C38). Its pharmacological target is the Ether-a-gogo potassium channel. We herein determine the solution structure of APETx1 by use of conventional two-dimensional 1H-NMR techniques followed by torsion angle dynamics and refinement protocols. The calculated structure of APETx1 belongs to the disulfide-rich all-beta structural family, in which a three-stranded anti-parallel beta-sheet is the only secondary structure. APETx1 is the first Ether-a-gogo effector discovered to fold in this way. We therefore compare the structure of APETx1 to those of the two other known effectors of the Ether-a-gogo potassium channel, CnErg1 and BeKm-1, and analyze the topological disposition of key functional residues proposed by analysis of the electrostatic anisotropy. The interacting surface is made of a patch of aromatic residues (Y5, Y32, and F33) together with two basic residues (K8 and K18) at the periphery of the surface. We pinpoint the absence of the central lysine present in the functional surface of the two other Ether-a-gogo effectors.

MEMO associated with an ErbB2 receptor phosphopeptide reveals a new phosphotyrosine motif.

Tyrosine phosphorylations are essential in signal transduction. Recently, a new type of phosphotyrosine binding protein, MEMO (Mediator of ErbB2-driven cell motility), has been reported to bind specifically to an ErbB2-derived phosphorylated peptide encompassing Tyr-1227 (PYD). Structural and functional analyses of variants of this peptide revealed the minimum sequence required for MEMO recognition. Using a docking approach we have generated a structural model for MEMO/PYD complex and compare this new phosphotyrosine motif to SH2 and PTB phosphotyrosine motives.

Basis of recognition between the NarJ chaperone and the N-terminus of the NarG subunit from Escherichia coli nitrate reductase.

A novel class of molecular chaperones co-ordinates the assembly and targeting of complex metalloproteins by binding to an amino-terminal peptide of the cognate substrate. We have previously shown that the NarJ chaperone interacts with the N-terminus of the NarG subunit coming from the nitrate reductase complex, NarGHI. In the present study, NMR structural analysis revealed that the NarG(1-15) peptide adopts an alpha-helical conformation in solution. Moreover, NarJ recognizes and binds the helical NarG(1-15) peptide mostly via hydrophobic interactions as deduced from isothermal titration calorimetry analysis. NMR and differential scanning calorimetry analysis revealed a modification of NarJ conformation during complex formation with the NarG(1-15) peptide. Isothermal titration calorimetry and BIAcore experiments support a model whereby the protonated state of the chaperone controls the time dependence of peptide interaction.