Localization of the toxic site of Naja mossambica cardiotoxins: small synthetic peptides express an in vivo lethality

The role of hydrophobic /hydrophilic balance in the activity of structurally flexible vs. rigid cytolytic polypeptides and analogs developed on their basis

INTRODUCTION

Being important representatives of various proteomes, membrane-active cationic peptides (CPs) are attractive objects as lead compounds in the design of new antibacterial, anticancer, antifungal, and antiviral molecules. Numerous CPs are found in insect and snake venoms, where many of them reveal cytolytic properties. Due to advances in omics technologies, the number of such peptides is growing dramatically. Areas covered: To understand structure-function relationships for CPs in a living cell, detailed analysis of their hydrophobic/hydrophilic properties is indispensable. We consider two structural classes of membrane-active CPs: latarcins (Ltc) from spider and cardiotoxins (CTXs) from snake venoms. While the former are void off disulfide bonds and conformationally flexible, the latter are structurally rigid and cross-linked with disulfide bonds. In order to elucidate structure-activity relationships behind their antibacterial, anticancer, and hemolytic effects, the properties of these polypeptides are considered on a side-by-side basis. Expert commentary: An ever-increasing number of venom-derived membrane-active polypeptides require new methods for identification of their functional propensities and sequence-based design of novel pharmacological substances. We address these issues considering a number of the designed peptides, based either on Ltc or CTX sequences. Experimental and computer modeling techniques required for these purposes are delineated.

The mechanism of cytotoxicity by Naja naja atra cardiotoxin 3 is physically distant from its membrane-damaging effect

In order to dissect out whether multiple activities of cardiotoxins (CTXs) are connected, to some extent, with each other, studies on reduced and S-carboxyamidomethylated (Rcam) Naja naja atra CTX3 were carried out in the present study. Although both CTX3 and Rcam-CTX3 induced apoptotic death of PC-3 cells as evidenced by propodium iodide/annexin V double staining, degradation of procaspases and DNA fragmentation, the cytotoxicity of Rcam-CTX3 was mostly 100-fold lower than that noted with native toxin. However, Rcam-CTX3 retained approximately 38% of the membrane-damaging activity of native toxin as revealed by the decrease in calcein self-quenching from phospholipid vesicles. These results are likely to reflect that the mechanism of cytotoxicity by CTX3 is not heavily dependent on its membrane-perturbing effect, and suggest that the structural elements within CTX3 responsible for the two activities are probably separated.

Molecular evolution of snake toxins: is the functional diversity of snake toxins associated with a mechanism of accelerated evolution?

Recent studies revealed that animal toxins with unrelated biological functions often possess a similar architecture. To tentatively understand the evolutionary mechanisms that may govern this principle of functional prodigality associated with a structural economy, two complementary approaches were considered. One of them consisted of investigating the rates of mutations that occur in cDNAs and/or genes that encode a variety of toxins with the same fold. This approach was largely adopted with phospholipases A2 from Viperidae and to a lesser extent with three-fingered toxins from Elapidae and Hydrophiidae. Another approach consisted of investigating how a given fold can accommodate distinct functional topographies. Thus, a number of topologies by which three-fingered toxins exert distinct functions were investigated either by making chemical modifications and/or mutational analyses or by studying the three-dimensional structure of toxin-target complexes. This review shows that, although the two approaches are different, they commonly indicate that most if not all the surface of a snake toxin fold undergoes natural engineering, which may be associated with an accelerated rate of evolution. The biochemical process by which this phenomenon occurs remains unknown.

Snake venom cardiotoxins-structure, dynamics, function and folding

Snake cardiotoxins are highly basic (pI > 10) small molecular weight (approximately 6.5 kDa), all beta-sheet proteins. They exhibit a broad spectrum of interesting biological activities. The secondary structural elements in these toxins include antiparallel double and triple stranded beta-sheets. The three dimensional structures of these toxins reveal an unique asymmetric distribution of the hydrophobic and hydrophilic amino acids. The 3D structures of closely related snake venom toxins such as neurotoxins and cardiotoxin-like basic proteins (CLBP) fail to show similar pattern(s) in the distribution of polar and nonpolar residues. Recently, many novel biological activities have been reported for cardiotoxins. However, to-date, there is no clear structure-function correlation(s) available for snake venom cardiotoxins. The aim of this comprehensive review is to summarize and critically evaluate the progress in research on the structure, dynamics, function and folding aspects of snake venom cardiotoxins.

Determination of the nuclear-magnetic-resonance solution structure of cardiotoxin CTX IIb from Naja mossambica mossambica

The NMR structure of cardiotoxin CTX IIb from Naja mossambica mossambica in aqueous solution was determined from a total of 593 nuclear Overhauser enhancement distance constraints and 135 dihedral angle constraints, which were collected using two-dimensional homonuclear 1H-NMR experiments. Structure calculations were performed with the program DIANA, using the redundant dihedral angle constraints strategy for improved convergence, followed by restrained energy minimization with the program FANTOM and a modified version of the program AMBER. The CTX IIb structure is represented by a group of 20 conformers with an average root-mean-square deviation relative to the mean solution structure of 0.072 nm for the backbone atoms, and 0.116 nm for all heavy atoms. The molecular structure of CTX IIb is characterized by a three-stranded beta-sheet made up of residues 20-26, 32-39 and 48-54, and a two-stranded beta-sheet composed of residues 1-5 and 10-14. A cluster of four disulfide bonds, 3-21, 14-38, 42-53 and 54-59, form the core of the molecule and crosslink the individual polypeptide strands. The NMR structure is similar to the previously reported X-ray crystal structure of the cardiotoxin CTX VII4 from the same species. Differences between the two structures were noted in the tips of the two loops formed by residues 6-9 and 27-31, which connect the beta-strand 1-5 with 10-14, and 20-26 with 32-39, respectively. For these loops the NMR data also indicate significantly increased dynamic disorder in the solution structure. These observations are discussed with respect to earlier suggestions by others that these two loops are essential structural elements for function and specificity of a wide variety of homologous toxins.

Do cardiotoxins possess a functional site? Structural and chemical modification studies reveal the functional site of the cardiotoxin from Naja nigricollis

Examination of the literature has revealed that regarding the amino acid sequences, cardiotoxins constitute a family of homogeneous compounds. In contrast, cardiotoxins appear heterogeneous as far as their biological and spectroscopic properties are concerned. As a result, comparison between these molecules with a view to establishing structure-activity correlations is complicated. We have therefore reviewed recent works aiming at identifying the functional site of a defined cardiotoxin, ie toxin gamma from the venom of the spitting cobra Naja nigricollis. The biological and structural properties of toxin gamma are first described. In particular, a model depicting the 3-dimensional structure of the toxin studied by NMR spectroscopy is proposed. The toxin polypeptide chain is folded into 3 adjacent loops rich in beta-sheet structure connected to a small globular core containing the 4 disulfide bonds. A number of derivatives chemically modified at a single aromatic or amino group have been prepared. The structure of each derivative was probed by emission fluorescence, circular dichroism and NMR spectroscopy. Also tested was the ability of the derivatives to kill mice, depolarize excitable cell membranes and lyse epithelial cells. Modification of some residues in the first loop, in particular Lys-12 and at the base of the second loop substantially affected biological properties, with no sign of concomitant structural modifications other than local changes. Modifications in other regions much less affected the biological properties of the toxin. A plausible functional site for toxin gamma involving loop I and the base of loop II is presented. It is stressed that the functional site of other cardiotoxins may be different.

Delineation of the functional site of a snake venom cardiotoxin: preparation, structure, and function of monoacetylated derivatives

Toxin gamma, a cardiotoxin from the venom of the cobra Naja nigricollis, was modified with acetic anhydride, and the derivatives were separated by cation-exchange and reverse-phase chromatography. Nine monoacetylated derivatives were obtained, and those modified at positions 1, 2, 12, 23, and 35 were readily identified by automated sequencing. The overall structure of toxin gamma, composed of three adjacent loops (I, II, and III) rich in beta-sheet, was not affected by monoacetylation as revealed by circular dichroic analysis. Trp-11, Tyr-22, and Tyr-51 fluorescence intensities were not affected by modifications at Lys-12 and Lys-35, whereas Trp-11 fluorescence intensity slightly increased when Lys-1 and Lys-23 were modified. The cytotoxic activity of toxin gamma to FL cells in culture was unchanged after modification at positions 1 and 2, whereas it was 3-fold lower after modification at Lys-23 and Lys-35. The derivative modified at Lys-12 was 10-fold less active than native toxin. Using two isotoxins, we found that substitutions at positions 28, 30, 31, and 57 did not change the cytotoxic potency of toxin gamma. A good correlation between cytotoxicity, lethality, and, to some extent, depolarizing activity on cultured skeletal muscle cells was found. In particular, the derivative modified at Lys-12 always had the lowest potency. Our data show that the site responsible for cytotoxicity, lethality, and depolarizing activity is not diffuse but is well localized on loop I and perhaps at the base of loop II. This site is topographically different from the AcChoR binding site of the structurally similar snake neurotoxins.

Selective cytolysis by a protein toxin as a consequence of direct interaction with the lymphocyte plasma membrane

Two lines of evidence support the concept that cardiotoxin from Naja naja siamensis interacts directly with the plasma membrane to produce selective cytolysis of lymphocytes. Toxin adsorbed to the wells of microtiter plates retained the ability to lyse murine T-lymphocytes, but lost the ability to destroy natural killer cells, whereas soluble toxin obliterated both cell types. Second, toxin covalently coupled to 100-microns-diameter agarose beads, such that endocytosis would be precluded, effectively lysed L1210 tumor T-lymphocytes. Although differences were observed among susceptibilities of a variety of mouse and human tumor lymphocyte cell lines to toxin-mediated lysis, these differences were not so great as the differences between tumor and normal lymphocytes. The intrinsic selectivity of the toxin for T-lymphocytes, plus its retention of cytolytic potential when affixed to a solid support, suggests that such a protein could be applied therapeutically. In addition, based upon activity which is temperature-independent and not influenced by the absence or presence of external calcium, it appears that the toxin's mode of action may be different from that involved with erythrocyte hemolysis or with skeletal or cardiac muscle depolarization.