The structural features of presynaptically neurotoxic secretory phospholipases A sub(2) (sPLA sub(2)s) that are responsible for their potent and specific action are still a matter of debate. To ...identify the residues that distinguish a highly neurotoxic sPLA sub(2), ammodytoxin A (AtxA), from a structurally similar but more than two orders of magnitude less toxic Russell's viper sPLA sub(2), VIIIa, we prepared a range of mutants and compared their properties. The results show that the structural features that confer high neurotoxicity to AtxA extend from its C-terminal part, with a central role of the residues Y115, I116, R118, N119 (the YIRN cluster) and F124, across the interfacial binding surface (IBS) in the vicinity of F24, to the N-terminal helix whose residues M7 and G11 are located on the edges of the IBS. Competition binding studies indicate that the surface of interaction with the neuronal M-type sPLA sub(2) receptor R180 extends over a similar region of the molecule. In addition, the YIRN cluster of AtxA is crucial for the high-affinity interaction with two intracellular binding proteins, calmodulin and R25. The concept of a single ''presynaptic neurotoxic site'' on the surface of snake venom sPLA sub(2)s is not consistent with these results which suggest that different parts of the toxin molecule are involved in distinct steps of presynaptic neurotoxicity.
The enzymatic activity of ammodytoxins (Atxs), secreted phospholipases A sub(2) (sPLA sub(2)s) in snake venom, is essential for expression of their presynaptic neurotoxicity, but its exact role in ...the process is unknown. We have analyzed in detail the enzymatic properties of Atxs, their mutants, and homologues. The apparent rates of phospholipid hydrolysis by the sPLA sub(2)s tested vary by up to 4 orders of magnitude, and all enzymes display a strong preference for vesicles containing anionic phospholipids, phosphatidylglycerol or phosphatidylserine (PS), over those containing zwitterionic phosphatidylcholine (PC). Nevertheless, Atxs are quite efficient in hydrolyzing pure PC vesicles as well as PC-rich plasma membranes of intact HEK293 cells. The presence of anionic phospholipids in PC vesicles dramatically increases the interfacial binding affinity and catalytic activity of Atxs, but not of their nontoxic homologue ammodytin I sub(2), that displays unusually low binding affinity and enzymatic activity on PS-containing vesicles and HEK293 plasma membranes. Aromatic and hydrophobic residues on the interfacial binding surface of Atxs are important for productive binding to both zwitterionic and anionic vesicles, while basic and polar residues have a negative impact on binding to zwitterionic vesicles. When tightly bound to the membrane interface, Atxs can reach full enzymatic activity at low micromolar concentrations of Ca super(2+). Although Atxs have evolved to function as potent neurotoxins that specifically target presynaptic nerve terminals, they display a high degree of phospholipolytic efficiency on various phospholipid membranes.
Two novel acceptors for ammodytoxin C, a presynaptically neurotoxic phospholipase A(2) from snake venom, have been purified from porcine cerebral cortex by a toxin-affinity-based procedure. Using ...tandem mass spectrometry, the isolated acceptors were identified as 14-3-3 gamma and epsilon isoforms, highly conserved cytoplasmic proteins involved in the regulation of numerous physiological processes. The interaction between ammodytoxin C and 14-3-3 proteins is direct and not mediated by calmodulin, a high-affinity acceptor for both ammodytoxin C and 14-3-3 proteins, as demonstrated in pull-down experiments and by surface plasmon resonance. The latter technique gave an apparent dissociation constant of 1.0+/-0.2 microM for the interaction between chip-immobilized 14-3-3 and ammodytoxin C. 14-3-3 usually interacts with proteins through specific phospho-Ser/Thr motifs. Ammodytoxin C is not a phospho-protein, therefore the interaction must occur through a non-phosphorylated binding site, most probably the KEESEK sequence at its C-terminal end. The interaction we describe suggests an explanation for the pathophysiological effects evoked by some secreted phospholipases A(2), such as the inhibition of protein phosphorylation, of terminal ion currents, and of neurotransmission, as well as the initiation of neuronal cell death, all processes regulated by 14-3-3 proteins.
The neurotoxic activity of ammodytoxin A (AtxA), a phospholipase A(2) from Vipera ammodytes ammodytes venom, has been investigated by protein engineering. With the aim of obtaining AtxA as a ...non-fused protein in the bacterial cytoplasm and avoiding problems with incomplete cleavage in vivo of the initial Met preceding the first residue (Ser1), a double mutant (S1A/E4Q) was prepared and expressed in Escherichia coli. Immunoblotting of the bacterial lysate showed that the mutant was synthesized at a low level not exceeding 0.5% of total cell protein. Analysis of the potential secondary structure of the mutant mRNA in the translation initiation region suggested that the Ala1 (GCC) and Leu2 (CUG) codons used are likely to be involved in a hairpin structure with the Thr13 (ACG) and Gly14 (GGG) codons, hindering effective translation at the ribosome. To weaken this structure (by DeltaG of about 20 kJ/mol) the same double mutant was prepared using another mutagenic oligonucleotide with silent mutations in the Ala1 (GCU) and Leu2 (UUG) codons. The mutant was successfully produced at a level of approximately 15% of total protein, with the initial Met completely removed in the bacterial cell. Such an approach could be important in solving similar problems in bacterial production of other toxic proteins.
Ammodytoxins (Atxs) are group II phospholipases A(2) (PLA(2)s) with presynaptic toxicity from venom of the snake Vipera ammodytes ammodytes. The molecular basis of their neurotoxicity, and that of ...similar PLA(2) toxins, is still to be explained. To address this problem, a surface-exposed aromatic residue, Phe(24), in the N-terminal region of the most potent Atx, AtxA, was replaced by other aromatic (tyrosine, tryptophan), hydrophobic (alanine) and polar uncharged (serine, asparagine) residues. The mutants were produced in the bacterial expression system, refolded in vitro and purified to homogeneity. All but the Trp(24) mutant, whose activity was similar to that of the wild type, showed a considerable decrease (40-80%) in enzymic activity on a micellar phosphatidylcholine substrate. This result indicates an important role for the aromatic side chains of phenylalanine or tryptophan, but not tyrosine, in PLA(2) activity, very likely at a stage of interfacial adsorption of the enzyme to zwitterionic aggregated substrates. The substitutions of Phe(24) also significantly decreased toxicity in mice, with the most prominent decrease, of 130-fold, observed in the case of the Asn(24) mutant. The results with the mutants show that there is no correlation between enzymic activity, lethality and binding affinity for three AtxA neuronal receptors (R180, R25 and calmodulin). Our results suggest a critical involvement of Phe(24) in the neurotoxicity of AtxA, apparently at a stage which does not involve the interaction with the known Atx-binding neuronal proteins and catalytic activity.
A cDNA encoding a novel human putative member of the papain family of cysteine peptidases has been cloned. The protease, named cathepsin P, is synthesized as a preproprotein. The presumed propeptide ...of 38 amino acids is followed by a 242-residue mature protein. The mature protease region is 30% identical to human papain-like cathepsins, with all the residues important for catalysis conserved. No similarity was observed in the propeptide region. On the contrary, the proenzyme shares 51-87% residues with some precursors of cysteine proteases from other species that have not yet been characterized. They all show a nearly completely conserved "CYTRED motif" in the propeptide region, not present in other members of the family, and could therefore constitute a distinct subfamily.
R180, isolated from porcine brain cortex, is a high-affinity membrane receptor for ammodytoxin A (AtxA), a secreted phospholipase A sub(2) (sPLA sub(2)) and presynaptically active neurotoxin from ...venom of the long-nosed viper (Vipera ammodytes ammodytes). As a member of the M-type sPLA sub(2) receptors, present on the mammalian plasma membrane, R180 has been proposed to be responsible for one of the first events in the process of presynaptic neurotoxicity, the binding of the toxin to the nerve cell. To test this hypothesis, we prepared and analyzed three N-terminal fusion proteins of AtxA possessing a 12 or 5 amino acid residue peptide. The presence of such an additional "propeptide" prevented interaction of the toxin with the M-type receptor but not its lethality in mouse and neurotoxic effects on a mouse phrenic nerve-hemidiaphragm preparation. In addition, antibodies raised against the sPLA sub(2)-binding C-type lectin-like domain 5 of the M-type sPLA sub(2) receptor were unable to abolish the neurotoxic action of AtxA on the neuromuscular preparation. The specific enymatic activities of the fusion AtxAs were two to three orders of magnitude lower from that of the wild type, yet resulting in a similar but less pronounced neurotoxic profile on the neuromuscular junction. This is in accordance with other data showing that a minimal enzymatic activity suffices for presynaptic toxicity of sPLA sub(2)s to occur. Our results indicate that the interaction of AtxA with the M-type sPLA sub(2) receptor at the plasma membrane is not essential for presynaptic activity of the toxin. Interaction of AtxA with two intracellular proteins, calmodulin and the R25 receptor, was affected but not prevented by the presence of the N-terminal fusion peptides, implying that these proteins may play a role in the sPLA sub(2) neurotoxicity.
cDNA sequences of any aspartic proteinase inhibitors have not been reported so far. Two protein inhibitors of aspartic proteinases were isolated from potato tubers and sequenced. In order to find ...related inhibitors, several cDNA clones have been isolated from a potato tuber lambda gt11 cDNA library encoding aspartic proteinase inhibitor homologues. The longest full-length clone contains an open reading frame of 660 bp coding for a protein of 220 amino acid residues. The deduced protein sequence shows about 94% and 99% similarity to the novel inhibitor of cathepsin D and potato cathepsin D inhibitor, respectively.
Equinatoxin II (EqtII), a basic protein of 179 amino acids lacking cysteine residues, is the most abundant cytolysin isolated from the sea anemoneActinia equina.Its mode of action is still poorly ...understood. In order to initiate further structure–function studies by protein engineering, cDNA library was prepared from the whole animal and hybridized with a PCR-derived probe, deduced from the EqtII primary structure. The longest positive clone of 899 bp was shown to encode a 214 residue precursor of EqtII. The mature protein region was amplified by PCR, cloned into a T7 RNA polymerase-based expression vector and expressed inEscherichia coli.Recombinant toxin was isolated by a simple, two-step isolation procedure including separation on CM-cellulose and gel filtration using an FPLC system. Its biochemical properties and hemolytic activity were practically indistinguishable from those of native toxin.
The molecular mechanism of action of presynaptically toxic secreted phospholipases A
2 (sPLA
2s) isolated from snake venoms is not completely understood. It has been proposed that the positive charge ...in the β-structure region is important for their toxic activity. To test this hypothesis, we characterised several mutants of ammodytoxin A (AtxA) possessing substitution of all five basic residues in this region. The mutations had relatively little influence on the catalytic activity of AtxA, either on charge-neutral or anionic phospholipid vesicles. An exception was R72 when replaced by a hydrophobic (higher activity) or an acidic (lower activity) residue. Lethal potencies of the eight single site mutants were up to four times lower than that of the wild-type, whereas the triple mutant (K74S/H76S/R77L) was 13-fold less toxic. The substitutions also lowered the affinity of the toxin, slightly to moderately, for the neuronal receptors R25 and R180. Interaction with calmodulin was only slightly affected by substitutions of K86, more by those of the K74/H76/R77 cluster and most by those of R72 (up to 11-fold lower binding affinity). The results clearly indicate that the basic amino acid residues in the β-region of AtxA contribute to, but are not necessary for, its neurotoxic effect.
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