The investigation of interactions of kappa-Hefutoxin1 with the voltage-gated potassium channels: a computational simulation

Kappa-Hefutoxin1 is a K(+) channel-blocking toxin from the scorpion Heterometrus fluvipes. It is a 22-residue protein that adapts a novel fold of two parallel helices linked by two disulfide bridges without beta-sheets. Recognition of interactions of kappa-Hefutoxin1 with the voltage-gated potassium channels, Kv1.1, Kv1.2, and Kv1.3, was studied by 3D-Dock software package. All structures of kappa-Hefutoxin1 were considered during the simulations, which indicated that even small changes in the structure of kappa-Hefutoxin1 considerably affected both the recognition and the binding between kappa-Hefutoxin1 and the Kv1 channels. kappa-Hefutoxin1 is located around the extracellular part of the Kv1 channels, making contacts with its helices. Lys 19, Tyr 5, Arg 6, Trp 9, or Arg 10 in the toxin and residues Asp 402, His 404, Thr 407,Gly 401, and Asp 386 in each subunit of the Kv potassium channel are the key residues for the toxin-channel recognition. Moreover, the simulation result demonstrates that the hydrophobic interactions are important in interaction of negatively charged toxins with potassium channels. The results of our docking/molecular dynamics simulations indicate that our 3D model structure of the kappa-Hefutoxin1-complex is both reasonable and acceptable and could be helpful for smarter drug design and the blocking agents of Kv1 channels.     

Molecular basis of inhibitory peptide maurotoxin recognizing Kv1.2 channel explored by ZDOCK and molecular dynamic simulations

Inhibitory peptide-channel interactions have been utilized to characterize both channels and peptides; however, the fundamental basis for these interactions remains elusive. Here, combined computation methods were employed to study the specific binding of maurotoxin (MTX) peptide to Kv1.2 channel. In the first stage, numerous predicted complexes were generated by docking an ensemble of all 35 NMR conformations of MTX to Kv1.2 channel with ZDOCK program. Then the resulted complexes were clustered and classified into four main binding modes, based on experimental information and interaction energy analysis after the energy minimization and molecular dynamics (MD) simulations. By examining the stability of the plausible candidates through unrestrained MD simulations and calculation of the binding free energies, a final reasonable MTX-Kv1.2 complex was identified, with an overall high degree of correlation between the calculation and experiment on mutational effects. In the obtained complex structure model, MTX mainly used its beta-sheet domains to associate the channel mouth instead of the well-recognized functionally important S5P linkers of Kv1.2 channel. Structure analysis characterized that the most essential Tyr(32) residue of MTX was surrounded by a "pocket" formed by many nonpolar and polar residues of Kv1.2 channel, and revealed a pore-blocking Lys(23) and an important Lys(7) stabilized by strong electrostatic interactions with Asp(379) of Kv1.2. Furthermore, a stepwise structural arrangement for both ligand and receptor was found to accompany the tighter interaction of MTX into the target channel. The starting conformation of MTX, the side-chain conformation of the most important residue Tyr(32), and proper introduction of flexibility for candidate complexes were demonstrated to be considerably important factors for obtaining the final reasonable complex structure model. All these findings should not only be helpful for identifying more plausible K(+) channel-inhibitory peptide complex structures, but also provide intrinsically valuable structural biology information to interpret binding affinities, specificities, and diversity of K(+) channel-nature toxin interactions.     

Brownian dynamics simulations of interaction between scorpion toxin Lq2 and potassium ion channel

The association of the scorpion toxin Lq2 and a potassium ion (K(+)) channel has been studied using the Brownian dynamics (BD) simulation method. All of the 22 available structures of Lq2 in the Brookhaven Protein Data Bank (PDB) determined by NMR were considered during the simulation, which indicated that the conformation of Lq2 affects the binding between the two proteins significantly. Among the 22 structures of Lq2, only 4 structures dock in the binding site of the K(+) channel with a high probability and favorable electrostatic interactions. From the 4 candidates of the Lq2-K(+) channel binding models, we identified a good three-dimensional model of Lq2-K(+) channel complex through triplet contact analysis, electrostatic interaction energy estimation by BD simulation and structural refinement by molecular mechanics. Lq2 locates around the extracellular mouth of the K(+) channel and contacts the K(+) channel using its beta-sheet rather than its alpha-helix. Lys27, a conserved amino acid in the scorpion toxins, plugs the pore of the K(+) channel and forms three hydrogen bonds with the conserved residues Tyr78(A-C) and two hydrophobic contacts with Gly79 of the K(+) channel. In addition, eight hydrogen-bonds are formed between residues Arg25, Cys28, Lys31, Arg34 and Tyr36 of Lq2 and residues Pro55, Tyr78, Gly79, Asp80, and Tyr82 of K(+) channel. Many of them are formed by side chains of residues of Lq2 and backbone atoms of the K(+) channel. Thirteen hydrophobic contacts exist between residues Met29, Asn30, Lys31 and Tyr36 of Lq2 and residues Pro55, Ala58, Gly79, Asp80 and Tyr82 of the K(+) channel. These favorable interactions stabilize the association between the two proteins. These observations are in good agreement with the experimental results and can explain the binding phenomena between scorpion toxins and K(+) channels at the level of molecular structure. The consistency between the BD simulation and the experimental data indicates that our three-dimensional model of Lq2-K(+) channel complex is reasonable and can be used in further biological studies such as rational design of blocking agents of K(+) channels and mutagenesis in both toxins and K(+) channels.     

Structure of the BgK-Kv1.1 complex based on distance restraints identified by double mutant cycles. Molecular basis for convergent evolution of Kv1 channel blockers

A structural model of BgK, a sea anemone toxin, complexed with the S5-S6 region of Kv1.1, a voltage-gated potassium channel, was determined by flexible docking under distance restraints identified by a double mutant cycles approach. This structure provides the molecular basis for identifying the major determinants of the BgK-Kv1.1 channel interactions involving the BgK dyad residues Lys(25) and Tyr(26). These interactions are (i) electrostatic interactions between the extremity of Lys(25) side chain and carbonyl oxygen atoms of residues from the channel selectivity filter that may be strengthened by solvent exclusion provided by (ii) hydrophobic interactions involving BgK residues Tyr(26) and Phe(6) and Kv1.1 residue Tyr(379) whose side chain protrudes in the channel vestibule. In other Kv1 channel-BgK complexes, these interactions are likely to be conserved, implicating both conserved and variable residues from the channels. The data suggest that the conservation in sea anemone and scorpion potassium channel blockers of a functional dyad composed of a lysine, and a hydrophobic residue reflects their use of convergent binding solutions based on a crucial interplay between these important conserved interactions.     

Maurotoxin and the Kv1.1 channel: voltage-dependent binding upon enantiomerization of the scorpion toxin disulfide bridge Cys31-Cys34.

Maurotoxin (MTX) is a 34-amino acid polypeptide cross-linked by four disulfide bridges that has been isolated from the venom of the scorpion Scorpio maurus palmatus and characterized. Maurotoxin competed with radiolabeled apamin and kaliotoxin for binding to rat brain synaptosomes and blocked K+ currents from Kv1 channel subtypes expressed in Xenopus oocytes. Structural characterization of the synthetic toxin identified half-cystine pairings at Cys3-Cys24, Cys9-Cys29, Cys13-Cys19 and Cys31-Cys34 This disulfide bridge pattern is unique among known scorpion toxins, particularly the existence of a C-terminal '14-membered disulfide ring' (i.e. cyclic domain 31-34), We therefore studied structure-activity relationships by investigating the structure and pharmacological properties of synthetic MTX peptides either modified at the C-terminus ?i.e. MTX(1-29), [Abu31,34]-MTX and [Cys31,34, Tyr32]D-MTX) or mimicking the cyclic C-terminal domain [i.e. MTX(31-34)]. Unexpectedly, the absence of a disulfide bridge Cys31-Cys34 in [Abu 31,34]-MTX and MTX(1-29) resulted in MTX-unrelated half-cystine pairings of the three remaining disulfide bridges for the two analogs, which is likely to be responsible for their inactivity against Kv1 channel subtypes. Cyclic MTX(31-34) was also biologically inactive. [Cys31,34, Tyr32]D-MTX, which had a 'native', MTX-related, disulfide bridge organization, but a D-residue-induced reorientation of the C-terminal disulfide bridge, was potent at blocking the Kv1.1 channel. This peptide-induced Kv1.1 blockage was voltage-dependent (a property not observed for MTX), maximal in the low depolarization range and associated with on-rate changes in ligand binding. Thus, the cyclic C-terminal domain of MTX seems to be crucial for recognition of Kv1.3, and to a lesser extent, Kv1.2 channels and it may contribute to the stabilization and strength of the interaction between the toxin and the Kv1.1 channel.     

Synthesis and characterization of Pi4, a scorpion toxin from Pandinus imperator that acts on K+ channels.

Pi4 is a 38-residue toxin cross-linked by four disulfide bridges that has been isolated from the venom of the Chactidae scorpion Pandinus imperator. Together with maurotoxin, Pi1, Pi7 and HsTx1, Pi4 belongs to the alpha KTX6 subfamily of short four-disulfide-bridged scorpion toxins acting on K+ channels. Due to its very low abundance in venom, Pi4 was chemically synthesized in order to better characterize its pharmacology and structural properties. An enzyme-based cleavage of synthetic Pi4 (sPi4) indicated half-cystine pairings between Cys6-Cys27, Cys12-32, Cys16-34 and Cys22-37, which denotes a conventional pattern of scorpion toxin reticulation (Pi1/HsTx1 type). In vivo, sPi4 was lethal after intracerebroventricular injection to mice (LD50 of 0.2 microg per mouse). In vitro, addition of sPi4 onto Xenopus laevis oocytes heterologously expressing various voltage-gated K+ channel subtypes showed potent inhibition of currents from rat Kv1.2 (IC50 of 8 pm) and Shaker B (IC50 of 3 nm) channels, whereas no effect was observed on rat Kv1.1 and Kv1.3 channels. The sPi4 was also found to compete with 125I-labeled apamin for binding to small-conductance Ca(2+)-activated K+ (SK) channels from rat brain synaptosomes (IC50 value of 0.5 microm). sPi4 is a high affinity blocker of the Kv1.2 channel. The toxin was docked (BIGGER program) on the Kv channel using the solution structure of sPi4 and a molecular model of the Kv1.2 channel pore region. The model suggests a key role for residues Arg10, Arg19, Lys26 (dyad), Ile28, Lys30, Lys33 and Tyr35 (dyad) in the interaction and the associated blockage of the Kv1.2 channel.     

Computational simulations of interactions of scorpion toxins with the voltage-gated potassium ion channel

Based on a homology model of the Kv1.3 potassium channel, the recognitions of the six scorpion toxins, viz. agitoxin2, charybdotoxin, kaliotoxin, margatoxin, noxiustoxin, and Pandinus toxin, to the human Kv1.3 potassium channel have been investigated by using an approach of the Brownian dynamics (BD) simulation integrating molecular dynamics (MD) simulation. Reasonable three-dimensional structures of the toxin-channel complexes have been obtained employing BD simulations and triplet contact analyses. All of the available structures of the six scorpion toxins in the Research Collaboratory for Structural Bioinformatics Protein Data Bank determined by NMR were considered during the simulation, which indicated that the conformations of the toxin significantly affect both the molecular recognition and binding energy between the two proteins. BD simulations predicted that all the six scorpion toxins in this study use their beta-sheets to bind to the extracellular entryway of the Kv1.3 channel, which is in line with the primary clues from the electrostatic interaction calculations and mutagenesis results. Additionally, the electrostatic interaction energies between the toxins and Kv1.3 channel correlate well with the binding affinities (-logK(d)s), R(2) = 0.603, suggesting that the electrostatic interaction is a dominant component for toxin-channel binding specificity. Most importantly, recognition residues and interaction contacts for the binding were identified. Lys-27 or Lys-28, residues Arg-24 or Arg-25 in the separate six toxins, and residues Tyr-400, Asp-402, His-404, Asp-386, and Gly-380 in each subunit of the Kv1.3 potassium channel, are the key residues for the toxin-channel recognitions. This is in agreement with the mutation results. MD simulations lasting 5 ns for the individual proteins and the toxin-channel complexes in a solvated lipid bilayer environment confirmed that the toxins are flexible and the channel is not flexible in the binding. The consistency between the results of the simulations and the experimental data indicated that our three-dimensional models of the toxin-channel complex are reasonable and can be used as a guide for future biological studies, such as the rational design of the blocking agents of the Kv1.3 channel and mutagenesis in both toxins and the Kv1.3 channel. Moreover, the simulation result demonstrates that the electrostatic interaction energies combined with the distribution frequencies from BD simulations might be used as criteria in ranking the binding configuration of a scorpion toxin to the Kv1.3 channel.     

Role of the conserved acidic residue Asp21 in the structure of phosphatidylinositol 3-kinase Src homology 3 domain: circular dichroism and nuclear magnetic resonance studies

To elucidate a role of the Src homology 3 (SH3)-conserved acidic residue Asp21 of the phosphatidylinositol 3-kinase (PI3K) SH3 domain, structural changes induced by the D21N mutation (Asp21 --> Asn) were examined by circular dichroism (CD) and nuclear magnetic resonance (NMR) spectroscopies. In the previous study, we demonstrated that environmental alterations occurred at the side chains of Trp55 and some Tyr residues from the comparison of the near-UV CD spectra of the PI3K SH3 domain with or without a D21N mutation [Okishio, N., et al. (2000) Biopolymers 57, 208-217]. In this work, the affected Tyr residues were identified as Tyr14 and Tyr73 by the CD analysis of a series of mutants, in which every single Tyr residue was replaced by a Phe residue with or without a D21N mutation. The (1)H and (15)N resonance assignments of the PI3K SH3 domain and its D21N mutant revealed that significant chemical shift changes occurred to the aromatic side-chain protons of Trp55 and Tyr14 upon the D21N mutation. All these aromatic residues are implicated in ligand recognition. In addition, the NMR analysis showed that the backbone conformations of Lys15-Asp23, Gly54-Trp55, Asn57-Gly58, and Gly67-Pro70 were affected by the D21N mutation. Furthermore, the (15)N[(1)H] nuclear Overhauser effect values of Tyr14, Glu19, and Glu20 were remarkably changed by the mutation. These results show that the D21N mutation causes structural deformation of more than half of the ligand binding cleft of the domain and provide evidence that Asp21 plays an important role in forming a well-ordered ligand binding cleft in cooperation with the RT loop (Lys15-Glu20).     

The role of the GX9GX3G motif in the gating of high voltage-activated Ca2+ channels

The putative hinge point revealed by the crystal structure of the MthK potassium channel is a glycine residue that is conserved in many ion channels. In high voltage-activated (HVA) Ca(V) channels, the mid-S6 glycine residue is only present in IS6 and IIS6, corresponding to G422 and G770 in Ca(V)1.2. Two additional glycine residues are found in the distal portion of IS6 (Gly(432) and Gly(436) in Ca(V)1.2) to form a triglycine motif unique to HVA Ca(V) channels. Lethal arrhythmias are associated with mutations of glycine residues in the human L-type Ca(2+) channel. Hence, we undertook a mutational analysis to investigate the role of S6 glycine residues in channel gating. In Ca(V)1.2, alpha-helix-breaking proline mutants (G422P and G432P) as well as the double G422A/G432A channel did not produce functional channels. The macroscopic inactivation kinetics were significantly decreased with Ca(V)1.2 wild type > G770A > G422A congruent with G436A > G432A (from the fastest to the slowest). Mutations at position Gly(432) produced mostly nonfunctional mutants. Macroscopic inactivation kinetics were markedly reduced by mutations of Gly(436) to Ala, Pro, Tyr, Glu, Arg, His, Lys, or Asp residues with stronger effects obtained with charged and polar residues. Mutations within the distal GX(3)G residues blunted Ca(2+)-dependent inactivation kinetics and prevented the increased voltage-dependent inactivation kinetics brought by positively charged residues in the I-II linker. In Ca(V)2.3, mutation of the distal glycine Gly(352) impacted significantly on the inactivation gating. Altogether, these data highlight the role of the GX(3)G motif in the voltage-dependent activation and inactivation gating of HVA Ca(V) channels with the distal glycine residue being mostly involved in the inactivation gating.     

Differences in saxitoxin and tetrodotoxin binding revealed by mutagenesis of the Na+ channel outer vestibule.

The marine guanidinium toxins, saxitoxin (STX) and tetrodotoxin (TTX), have played crucial roles in the study of voltage-gated Na+ channels. Because they have similar actions, sizes, and functional groups, they have been thought to associate with the channel in the same manner, and early mutational studies supported this idea. Recent experiments by. Biophys. J. 67:2305-2315) have suggested that the toxins bind differently to the isoform-specific domain I Phe/Tyr/Cys location. In the adult skeletal muscle Na+ channel isoform (microliter), we compared the effects on both TTX and STX affinities of mutations in eight positions known to influence toxin binding. The results permitted the assignment of energies contributed by each amino acid to the binding reaction. For neutralizing mutations of Asp400, Glu755, and Lys1237, all thought to be part of the selectivity filter of the channel, the loss of binding energy was identical for the two toxins. However, the loss of binding energy was quite different for vestibule residues considered to be more superficial. Specifically, STX affinity was reduced much more by neutralizations of Glu758 and Asp1532. On the other hand, mutation of Tyr401 to Cys reduced TTX binding energy twice as much as it reduced STX binding energy. Kinetic analysis suggested that all outer vestibule residues tested interacted with both toxins early in the binding reaction (consistent with larger changes in the binding than unbinding rates) before the transition state and formation of the final bound complex. We propose a revised model of TTX and STX binding in the Na+ channel outer vestibule in which the toxins have similar interactions at the selectivity filter, TTX has a stronger interaction with Tyr401, and STX interacts more strongly with the more extracellular residues.     

Interaction of agitoxin2, charybdotoxin,and iberiotoxin with potassium channels: selectivity between voltage-gated and Maxi-K channels

To gain insight into the molecular determinants that define the specificity of interaction of pore-blocking peptides, such as agitoxin 2 (AgTX2), charybdotoxin (ChTX), and iberiotoxin (IbTX) with the Shaker-type voltage-gated potassium channel Kv1.3, or the large-conductance Ca(2+)-activated K(+) (Maxi-K) channel, homology models of these channels were generated based on the crystal structure of the bacterial, KcsA, potassium channel. Peptide-channel complexes were analyzed to evaluate the predicted interaction interfaces between the peptides and the channels' outer vestibules. The docking model, for either AgTX2 or ChTX with the Kv1.3 channel, predicts a novel hydrogen bonding interaction between the Asn30 side-chain of the peptide and the Asp381 side-chain of the channel. This interaction is consistent with the >500-fold decreased potency of both AgTX2 and ChTX mutants at position 30 for the Shaker channel [(Ranganathan et al., Neuron 1996;16:131-139); (Goldstein et al., Neuron 1994;12:1377-1388)]. This hydrogen bonding interaction also suggests that Gly30 in IbTX may be the critical determinant for its lack of activity against Shaker Kv channels. The model of the Maxi-K channel reveals a narrower and more structurally restrained outer vestibule in which the aromatic residues Phe266 and Tyr294 may stabilize binding of IbTX and ChTX by pi-pi stacking with the aromatic residues Trp14 and Tyr36 of the peptides. This study also suggests that the extra net negative charge of IbTX is not related to the selectivity of this peptide for the Maxi-K channel.     

Brownian dynamics simulations of the recognition of the scorpion toxin P05 with the small-conductance calcium-activated potassium channels

The recognition of the scorpion toxin P05 and the small-conductance, calcium-activated potassium (SK) channels, rsk1, rsk2, and rsk3, has been studied by means of the Brownian dynamics (BD) method. All of the 25 available structures of P05 in the RCSB Protein Data Bank determined by NMR were considered during the simulation, which indicated that the conformation of P05 affects both the recognition and binding between the two proteins significantly. Comparing the top four high-frequency structures of P05 binding to the SK channels, we found that the rsk2 channel, with high frequencies and lowest electrostatic interaction energies (E (int)(ES)), is the most favorable for P05 binding, while rsk3 is intermediate, and rsk1 is the least favorable. Among the 25 structures of P05, the 13th structure docks into the binding site of the rsk2 channel with the highest probability and most favorable electrostatic interactions. From the P05-rsk2 channel binding model, we identified the residues critical for the recognition of these two proteins through triplet contact analyses. P05 locates around the extracellular mouth of the SK channels and contacts the SK channels using its alpha-helix rather than beta-sheets. The critical triplet contacts for recognition between P05 and the rsk2 channel are Arg6 (P05)-Asp364 (SK), Arg7 (P05)-Asn368 (SK), and Arg13 (P05)-Asp341 (SK). The structure of the P05-rsk2 complex with the most favorable electrostatic interaction energy was further refined by molecular mechanics, showing that six hydrogen bonding interactions exist between P05 and the rsk2 channel: one hydrogen bond is formed between Arg6 (P05) and Asp364(D) (rsk2); Arg7 (P05) forms three hydrogen bonds with Asp341(B) (rsk2)) and Asp364(C) (rsk2); two hydrogen bonds are formed by Arg13 (P05) with Asp341(A) (rsk2) and Asp364(B) (rsk2). The simulation results are in good agreement with the previous molecular biological experiments and can explain the binding phenomena between P05 and SK channels at the level of molecular structure. The consistency between the results of the BD simulations and the experimental data indicated that our 3D model of the P05-rsk2 channel complex is reasonable and can be employed in further biological studies, such as rational design of the novel therapeutic agents blocking the small-conductance, calcium-activated and apamin-sensitive potassium channels, and for mutagenesis studies in both toxins and SK channels. In particular, both the BD simulations and the molecular mechanics refinements indicate that residue Asp364 of the rsk2 channel is critical for its recognition and binding functionality towards P05. This phenomenon has not been appreciated in the previous mutagenesis experiments, indicating that this might be a new clue for further functional study of SK channels.     

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.     

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.     

Mapping the functional anatomy of BgK on Kv1.1, Kv1.2, and Kv1.3. Clues to design analogs with enhanced selectivity

BgK is a peptide from the sea anemone Bunodosoma granulifera, which blocks Kv1.1, Kv1.2, and Kv1.3 potassium channels. Using 25 analogs substituted at a single position by an alanine residue, we performed the complete mapping of the BgK binding sites for the three Kv1 channels. These binding sites included three common residues (Ser-23, Lys-25, and Tyr-26) and a variable set of additional residues depending on the particular channel. Shortening the side chain of Lys-25 by taking out the four methylene groups dramatically decreased the BgK affinity to all Kv1 channels tested. However, the analog K25Orn displayed increased potency on Kv1.2, which makes this peptide a selective blocker for Kv1.2 (K(D) 50- and 300-fold lower than for Kv1.1 and Kv1.3, respectively). BgK analogs with enhanced selectivity could also be made by substituting residues that are differentially involved in the binding to some of the three Kv1 channels. For example, the analog F6A was found to be >500-fold more potent for Kv1.1 than for Kv1.2 and Kv1.3. These results provide new information about the mechanisms by which a channel blocker distinguishes individual channels among closely related isoforms and give clues for designing analogs with enhanced selectivity.     

Participation of the Lys313-Ile333 sequence of the purinergic P2X4 receptor in agonist binding and transduction of signals to the channel gate

To study the roles of the Lys(313)-Ile(333) ectodomain sequence of the rat P2X(4) receptor in ATP binding and transduction of signals to the channel gate, the conserved Lys(313), Tyr(315), Gly(316), Ike(317), Arg(318), Asp(320), Val(323), Lys(329), Phe(330), and Ile(333) residues were mutated. Current recordings were done on lifted cells and ATP was applied using an ultrafast solution-switching system. The rates of wild type channel opening and closing in the presence of ATP, but not the rate of washout-induced closing, were dependent on agonist concentration. All mutants other than I317A were expressed in the plasma membrane at comparable levels. The majority of mutants showed significant changes in the peak amplitude of responses and the EC(50) values for ATP. When stimulated with the supramaximal (1.4 mm) ATP concentration, mutants also differed in the kinetics of their activation, deactivation, and/or desensitization. The results suggest a critical role of the Lys(313) residue in receptor function other than coordination of the phosphate group of ATP and possible contribution of the Tyr(315) residue to the agonist binding module. The pattern of changes of receptor function by mutation of other residues was consistent with the operation of the Gly(316)-Ile(333) sequence as a signal transduction module between the ligand binding domain and the channel gate in the second transmembrane domain.     

In silico detection of binding mode of J-superfamily conotoxin pl14a with Kv1.6 channel

A novel conotoxin pl14a containing 25 amino acid residues with an amidated C-terminus from vermivorous cone snail, Conus planorbis belongs to J-conotoxin superfamily and this is the first conotoxin, which inhibits both nicotinic acetylcholine receptor subtypes and Kv1.6 channel. We have attempted through bioinformatics approaches to elucidate the extent of specificity of pl14a towards Kv1 channel subtypes (Kv1.1-Kv1.6). Our work provides rationale for the relatively high specificity and binding mode of pl14a to Kv1.6 channel. The pl14a peptide contains two types of structural elements, namely the putative dyad (Lys18 and Tyr19) and basic residue ring constituted of arginine residues. We have carried out in silico docking studies so as to assess the contribution of one or combination of both structural elements of pl14a in blocking of Kv1.6 channel. For this purpose, we have built by homology modelling, the theoretical 3D structure of Kv1.6 channel based on the available crystal structure of mammalian shaker Kv1.2 channel. Docking studies suggest that positively charged residues ring may be involved in the blocking mechanism of Kv1.6 channel. The models suggest that the peptide interacts with negatively charged extracellular loops and pore-mouth of the potassium channel and blocks the channel by covering the pore as a lid, akin to previously proposed blocking mechanism of kappaM-conotoxin RIIIK from Conus radiatus to Tsha1 potassium channel. The newly detected pharmacophore for pl14a interacting with Kv1.6 channel provides a pointer to experimental work to validate the observations made here. Based on differences in the number and distribution of the positively-charged residues in other conopeptides from the J-superfamily, we hypothesize different selectivity profiles against subtypes of the potassium channels for these conopeptides.     

Evidence for domain-specific recognition of SK and Kv channels by MTX and HsTx1 scorpion toxins

Maurotoxin (MTX) and HsTx1 are two scorpion toxins belonging to the alpha-KTx6 structural family. These 34-residue toxins, cross-linked by four disulfide bridges, share 59% sequence identity and fold along the classical alpha/beta scaffold. Despite these structural similarities, they fully differ in their pharmacological profiles. MTX is highly active on small (SK) and intermediate (IK) conductance Ca(2+)-activated (K(+)) channels and on voltage-gated Kv1.2 channel, whereas HsTx1 potently blocks voltage-gated Kv1.1 and Kv1.3 channels only. Here, we designed and chemically produced MTX-HsTx1, a chimera of both toxins that contains the N-terminal helical region of MTX (sequence 1-16) and the C-terminal beta-sheet region of HsTx1 (sequence 17-34). The three-dimensional structure of the peptide in solution was solved by (1)H NMR. MTX-HsTx1 displays the activity of MTX on SK channel, whereas it exhibits the pharmacological profile of HsTx1 on Kv1.1, Kv1.2, Kv1.3, and IK channels. These data demonstrate that the helical region of MTX exerts a key role in SK channel recognition, whereas the beta-sheet region of HsTx1 is crucial for activity on all other channel types tested.     

Sequences of tryptic peptides containing the five cysteinyl residues of ribulosebisphosphate carboxylase/oxygenase from Rhodospirillum rubrum

Tryptic peptides which account for all five cysteinyl residues in ribulosebisphosphate carboxylase/oxygenase from Rhodospirillum rubrum have been purified and sequenced. Collectively, these peptides contain 94 of the approximately 500 amino acid residues per molecule of subunit. Due to one incomplete cleavage at a site for trypsin and two incomplete chymotryptic-like cleavages, eight major radioactive peptides (rather than five as predicted) were recovered from tryptic digests of the enzyme that had been carboxymethylated with [³H]iodoacetate. The established sequences are: GlyTyrThrAlaPheValHisCys¹Lys TyrValAspLeuAlaLeuLysGluGluAspLeuIleAla GlyGlyGluHisValLeuCys¹AlaTyr AlaGlyTyrGlyTyrValAlaThrAlaAlaHisPheAla AlaGluSerSerThrGlyThrAspValGluValCys¹ ThrThrAsxAsxPheThrArg AlaCys¹ThrProIleIleSerGlyGlyMetAsnAla LeuArg ProPheAlaGluAlaCys¹HisAlaPheTrpLeuGly GlyAsnPheIleLys In these peptides, radioactive carboxymethylcysteinyl residues are denoted with asterisks and the sites of incomplete cleavage with vertical wavy lines. None of the peptides appear homologous with either of two cysteinyl-containing, active-site peptides previously isolated from spinach ribulosebisphosphate carboxylase/oxygenase.