The two subunits of a phospholipase A2 inhibitor from the plasma of Thailand cobra having structural similarity to urokinase-type plasminogen activator receptor and LY-6 related proteins

A comparative study of endogenous phospholipase A2 inhibitors in the serum of Brazilian pit vipers

This work compared the presence of phospholipase A₂ inhibitors (PLIs) in the serum of 19 snake species maintained at Instituto Butantan to better understand the mechanisms of venom resistance in snakes and improve the treatment of snakebite. PLI was isolated from blood of 19 snake species by one-step chromatography and identified in all samples, besides its identity was confirmed through the interaction with both phospholipase A₂ and anti-γPLI. These findings highlight the diversity of snake serum PLIs and emphasize the importance of structure-function studies.

Structural and Functional Diversity of Animal Toxins Interacting With GPCRs

Peptide toxins from venoms have undergone a long evolutionary process allowing host defense or prey capture and making them highly selective and potent for their target. This has resulted in the emergence of a large panel of toxins from a wide diversity of species, with varied structures and multiple associated biological functions. In this way, animal toxins constitute an inexhaustible reservoir of druggable molecules due to their interesting pharmacological properties. One of the most interesting classes of therapeutic targets is the G-protein coupled receptors (GPCRs). GPCRs represent the largest family of membrane receptors in mammals with approximately 800 different members. They are involved in almost all biological functions and are the target of almost 30% of drugs currently on the market. Given the interest of GPCRs in the therapeutic field, the study of toxins that can interact with and modulate their activity with the purpose of drug development is of particular importance. The present review focuses on toxins targeting GPCRs, including peptide-interacting receptors or aminergic receptors, with a particular focus on structural aspects and, when relevant, on potential medical applications. The toxins described here exhibit a great diversity in size, from 10 to 80 amino acids long, in disulfide bridges, from none to five, and belong to a large panel of structural scaffolds. Particular toxin structures developed here include inhibitory cystine knot (ICK), three-finger fold, and Kunitz-type toxins. We summarize current knowledge on the structural and functional diversity of toxins interacting with GPCRs, concerning first the agonist-mimicking toxins that act as endogenous agonists targeting the corresponding receptor, and second the toxins that differ structurally from natural agonists and which display agonist, antagonist, or allosteric properties.

Proteomic Investigations of Two Pakistani Naja Snake Venoms Species Unravel the Venom Complexity, Posttranslational Modifications, and Presence of Extracellular Vesicles

Latest advancement of omics technologies allows in-depth characterization of venom compositions. In the present work we present a proteomic study of two snake venoms of the genus Naja i.e., Naja naja (black cobra) and Naja oxiana (brown cobra) of Pakistani origin. The present study has shown that these snake venoms consist of a highly diversified proteome. Furthermore, the data also revealed variation among closely related species. High throughput mass spectrometric analysis of the venom proteome allowed to identify for the N. naja venom 34 protein families and for the N. oxiana 24 protein families. The comparative evaluation of the two venoms showed that N. naja consists of a more complex venom proteome than N. oxiana venom. Analysis also showed N-terminal acetylation (N-ace) of a few proteins in both venoms. To the best of our knowledge, this is the first study revealing this posttranslational modification in snake venom. N-ace can shed light on the mechanism of regulation of venom proteins inside the venom gland. Furthermore, our data showed the presence of other body proteins, e.g., ankyrin repeats, leucine repeats, zinc finger, cobra serum albumin, transferrin, insulin, deoxyribonuclease-2-alpha, and other regulatory proteins in these venoms. Interestingly, our data identified Ras-GTpase type of proteins, which indicate the presence of extracellular vesicles in the venom. The data can support the production of distinct and specific anti-venoms and also allow a better understanding of the envenomation and mechanism of distribution of toxins. Data are available via ProteomeXchange with identifier PXD018726.

Exploring the structural and functional aspects of the phospholipase A2 from Naja spp

Naja spp. venom is a natural source of active compounds with therapeutic application potential. Phospholipase A₂ (PLA₂) is abundant in the venom of Naja spp. and can perform neurotoxicity, cytotoxicity, cardiotoxicity, and hematological disorders. The PLA₂s from Naja spp. venoms are Asp 49 isoenzymes with the exception of PLA₂ Cys 49 from Naja sagittifera. When looking at the functional aspects, the neurotoxicity occurs by PLA₂ called β-toxins that have affinity for phosphatidylcholine in nerve endings and synaptosomes membranes, and by α-toxins that block the nicotinic acetylcholine receptors in the neuromuscular junctions. In addition, these neurotoxins may inhibit K⁺ and Ca⁺⁺ channels or even interfere with the Na⁺/K⁺/ATPase enzyme. The disturbance in the membrane fluidity also results in inhibition of the release of acetylcholine. The PLA₂ can act as anticoagulants or procoagulant. The cytotoxicity exerted by PLA₂s result from changes in the cardiomyocyte membranes, triggering cardiac failure and hemolysis. The antibacterial activity, however, is the result of alterations that decrease the stability of the lipid bilayer. Thus, the understanding of the structural and functional aspects of PLA₂s can contribute to studies on the toxic and therapeutic mechanisms involved in the envenomation by Naja spp. and in the treatment of pathologies.

Alpha-type phospholipase A2 inhibitors from snake blood

It is of popular and scientific knowledge that toxins from snake venom (among them the PLA2 and myotoxins) are neutralized by various compounds, such as antibodies and proteins purified from animal blood. Venomous and nonvenomous snakes have PLA2 inhibitory proteins, called PLIs, in their blood serum. One hypothesis that could explain the presence of these PLIs in the serum of venomous snakes would be self-protection against the enzymes of their own venom, which eventually could reach the circulatory system. However, the presence of PLIs in non-venomous snakes suggests that their physiological role might not be restricted to protection against PLA2 toxins, but could be extended to other functions, as in the innate immune system and local regulation of PLA2s. The present study aimed to review the currently available literature on PLA2 and myotoxin alpha inhibitors present in snake plasma, thus helping to improve the research on these molecules. Furthermore, this review includes current information regarding the mechanism of action of these inhibitors in an attempt to better understand their application, and proposes the use of these molecules as new models in snakebite therapy. These molecules may help in the neutralization of different types of phospholipases A2 and myotoxins, complementing the conventional serum therapy.

Endogenous phospholipase A2 inhibitors in snakes: a brief overview

The blood plasma of numerous snake species naturally comprises endogenous phospholipase A₂ inhibitors, which primarily neutralize toxic phospholipases A₂ that may eventually reach their circulation. This inhibitor type is generally known as snake blood phospholipase A₂ inhibitors (sbPLIs). Most, if not all sbPLIs are oligomeric glycosylated proteins, although the carbohydrate moiety may not be essential for PLA₂ inhibition in every case. The presently known sbPLIs belong to one of three structural classes – namely sbαPLI, sbβPLI or sbγPLI – depending on the presence of characteristic C-type lectin-like domains, leucine-rich repeats or three-finger motifs, respectively. Currently, the most numerous inhibitors described in the literature are sbαPLIs and sbγPLIs, whereas sbβPLIs are rare. When the target PLA₂ is a Lys49 homolog or an Asp49 myotoxin, the sbPLI is denominated a myotoxin inhibitor protein (MIP). In this brief overview, the most relevant data on sbPLIs will be presented. Representative examples of sbαPLIs and sbγPLIs from two Old World – Gloydius brevicaudus and Malayopython reticulatus – and two New World – Bothrops alternatus and Crotalus durissus terrificus – snake species will be emphasized.

[Relationships between venomous function and innate immune function]

Venomous function is investigated in relation to innate immune function in two cases selected from scorpion venom and serpent venom. In the first case, structural analysis of scorpion toxins and defensins reveals a close interrelation between both functions (toxic and innate immune system function). In the second case, structural and functional studies of natural inhibitors of toxic snake venom phospholipases A2 reveal homology with components of the innate immune system, leading to a similar conclusion. Although there is a clear functional distinction between neurotoxins, which act by targeting membrane ion channels, and the circulating defensins which protect the organism from pathogens, the scorpion short toxins and defensins share a common protein folding scaffold with a conserved cysteine-stabilized alpha-beta motif of three disulfide bridges linking a short alpha helix and an antiparallel beta sheet. Genomic analysis suggests that these proteins share a common ancestor (long venom toxins were separated from an early gene family which gave rise to separate short toxin and defensin families). Furthermore, a scorpion toxin has been experimentally synthetized from an insect defensin, and an antibacterial scorpion peptide, androctonin (whose structure is similar to that of a cone snail venom toxin), was shown to have a similar high affinity for the postsynaptic acetylcholine receptor of Torpedo sp. Natural inhibitors of phospholipase A2 found in the blood of snakes are associated with the resistance of venomous snakes to their own highly neurotoxic venom proteins. Three classes of phospholipases A2 inhibitors (PLI-α, PLI-β, PLI-γ) have been identified. These inhibitors display diverse structural motifs related to innate immune proteins including carbohydrate recognition domains (CRD), leucine rich repeat domains (found in Toll-like receptors) and three finger domains, which clearly differentiate them from components of the adaptive immune system. Thus, in structure, function and phylogeny, venomous function in both vertebrates and invertebrates are clearly interrelated with innate immune function.

Involvement of the prostate and testis expression (PATE)-like proteins in sperm-oocyte interaction

BACKGROUND

The prostate and testis expression (PATE)-like family of proteins are expressed mainly in the male genital tract. They are localized in the sperm head and are homologous to SP-10, the acrosomal vesicle protein also named ACRV1. Our aim was to characterize the expression and functional role of three PATE-like proteins in the testis and ejaculated sperm.

METHODS

The expression and localization of PATE-like proteins in human testis biopsies (n= 95) and sperm cells were assessed by RT-PCR, immunohistochemistry and immunofluorescence staining (at least 600 sperm cells per specimen). The function of the PATE protein was tested by the hemizona assay and hamster egg penetration test (HEPT).

RESULTS

PATE and PATE-M genes and proteins were present almost exclusively in germ cells in the testis: immunoflourescence showed that the percentage of germ cells positive for PATE, PATE-M and PATE-B was 85, 50 and 2%, respectively. PATE and PATE-M proteins were localized in the equatorial segment of the sperm head, while PATE-B protein was localized in the post-acrosomal region. A polyclonal antibody (Ab, at 1:50 and 1:200 dilutions) against the PATE protein did not inhibit sperm-zona binding in the hemizona assay (hemizona index of 89.6 ± 10 and 87 ± 36%, respectively). However, there was inhibition of sperm-oolemma fusion and penetration in the HEPT (penetration index: without Ab 7 ± 3.9; Ab dilution of 1:100, 4 ± 3.5; Ab dilution of 1:20, 0.6 ± 1.2, P < 0.001).

CONCLUSIONS

Our data suggest that PATE protein is involved in sperm-oolemma fusion and penetration but not sperm-zona binding.

Up-regulation of the expressions of phospholipase A2 inhibitors in the liver of a venomous snake by its own venom phospholipase A2

Venomous snakes such as Gloydius brevicaudus have three distinct types of phospholipase A(2) inhibitors (PLIalpha, PLIbeta, and PLIgamma) in their blood so as to protect themselves from their own venom phospholipases A(2) (PLA(2)s). Expressions of these PLIs in G. brevicaudus liver were found to be enhanced by the intramuscular injection of its own venom. The enhancement of gene expressions of PLIalpha and PLIbeta in the liver was also found to be induced by acidic PLA(2) contained in this venom. Furthermore, these effects of acidic PLA(2) on gene expression of PLIs were shown to be unrelated to its enzymatic activity. These results suggest that these venomous snakes have developed the self-protective system against their own venom, by which the venom components up-regulate the expression of anti-venom proteins in their liver.

Identification and characterization of phospholipase A2 inhibitors from the serum of the Japanese rat snake, Elaphe climacophora

Two distinct phospholipase A2 (PLA2) inhibitory proteins (PLIs) were purified from the serum of the Japanese rat snake, Elaphe climacophora. The 150-kDa inhibitor, a trimer of a 50-kDa subunit, specifically inhibited the basic PLA2 purified from the venom of Gloydius brevicaudus, whereas the 120-kDa one composed of two distinct 25-kDa subunits. A and B, inhibited both the acidic and basic PLA2s of G. brevicaudus. On the basis of their amino acid sequences, these inhibitors were assigned as PLI beta and PLI gamma, respectively. A PLI alpha homolog (PLI alpha-like protein; PLI alpha-LP) having an apparent molecular weight of 50-kDa and composed of 15-kDa subunits was also purified from the E. climacophora serum. This homolog was immunoreactive with antibody raised against the G. brevicaudus PLI alpha, but lacked in the inhibitory activity toward the acidic and basic PLA2s. The cDNAs encoding PLI alpha-LP, PLI beta, PLI gamma-A, and PLI gamma-B were cloned from liver RNA, and their nucleotide sequences were compared with those of other venomous and non-venomous snakes.

Identification of the B subtype of gamma-phospholipase A2 inhibitor from Protobothrops flavoviridis serum and molecular evolution of snake serum phospholipase A2 inhibitors

A cDNA encoding a novel phospholipase A(2) (PLA(2)) inhibitor (PLI) was isolated from a Protobothrops flavoviridis snake (Tokunoshima island, Japan) liver cDNA library. This cDNA encoded a signal peptide of 19 amino acids followed by a mature protein of 181 amino acids. Its N-terminal amino acid sequence was completely in accord with that of a PLI, named PLI-II, previously found in P. flavoviridis serum. PLI-II showed a high similarity in sequence to the B subtype of gammaPLI, denoted gammaPLI-B, isolated from Agkistrodon blomhoffii siniticus serum. Thus, PLI-II is P. flavoviridis serum gammaPLI-B. Since PLI-I, previously isolated from P. flavoviridis serum, can be assigned as gammaPLI-A, P. flavoviridis serum contains both A and B subtypes of gammaPLI. Phylogenetic analysis of gammaPLIs from the sera of various kinds of snakes, Elapinae, Colubrinae, Laticaudinae, Acanthophiinae, Crotalinae, and Pythonidae, based on the amino acid sequences revealed that A and B subtypes of gammaPLIs are clearly separated from each other. It was also found that phylogenetic topologies of gammaPLIs are in good agreement with speciation processes of snakes. The BLAST search followed by analyses with particular Internet search engines of proteins with Cys/loop frameworks similar to those of PLI-II and PLI-I revealed that gammaPLI-Bs, including PLI-II and PLI-II-like proteins from mammalian sources, form a novel PLI-II family which possesses the common Cys/loop frameworks in the anterior and posterior three-finger motifs in the molecules. Several lines of evidence suggest that PLI-II is evolutionarily ancestral to PLI-I.

Toxins in anti-nociception and anti-inflammation

The use of toxins as novel molecular probes to study the structure-function relationship of ion-channels and receptors as well as potential therapeutics in the treatment of wide variety of diseases is well documented. The high specificity and selectivity of these toxins have attracted a great deal of interest as candidates for drug development. This review highlights the involvement of the proteins and peptide toxins as well as non-proteinaceous compounds derived from both venomous and non-venomous animals, in anti-nociception and anti-inflammation. The possible mechanisms of these potential therapeutic agents and possible clinical applications in the treatment of pain and inflammation are also summarized.

The first representative of glycosylated three-fingered toxins. Cytotoxin from the Naja kaouthia cobra venom

There are different glycosylated proteins in snake venoms, but no glycosylated representatives of a large family of three-fingered toxins have previously been detected. A new glycoprotein was isolated from the venom of the Thai cobra Naja kaouthia. MALDI MS of the glycoprotein contained an array of peaks in the range from approximately 8900 to approximately 9400 Da indicating its microheterogeneity. Carbohydrate analysis showed the presence of mannose, galactose, N-acetylglucosamine, fucose and neuraminic acid. The N-terminal sequence of the glycoprotein was identical to that of cytotoxin 3 (CX3) from N. kaouthia, and CD spectra of the glycoprotein and CX3 were almost the same. Cleavage of a glycan moiety by N-glycosidase F gave a protein of molecular mass practically coinciding with that of CX3. MALDI MS of the tryptic digest of reduced glycoprotein S-pyridylethylated at cysteine residues, contained peaks corresponding to all tryptic fragments of CX3, with the exception of fragment 24-30. The peak corresponding to this peptide appeared in the mass-spectrum of similarly treated deglycosylated glycoprotein. These data show that the potential N-glycosylation site at Asn29 in CX3 is utilized for glycan attachment and that the glycoprotein is glycosylated CX3. In vivo toxicity of the glycoprotein to the cricket Gryllus assimilis was twofold lower than that of CX3. The cytotoxic activity of the glycoprotein towards HL60 cells was about two orders of magnitude lower than that of CX3, but could be made equal to the CX3 cytotoxicity by deglycosylation. Thus for the first time we have isolated a glycosylated three-fingered snake venom toxin wherein glycosylation appears to modulate its biological activity.

Natural phospholipase A(2) myotoxin inhibitor proteins from snakes, mammals and plants

A renewed interest in the phenomenon of inter- and intra-species resistance towards the toxicity of snake venoms, coupled with the search for new strategies for treatment of snake envenomations, has prompted the discovery of proteins which neutralize the major toxic components of these venoms. Among these emerging groups of proteins are inhibitors of toxic phospholipases A2 (PLA2s), many of which exhibit a wide range of toxic effects including muscle-tissue damage, neurotoxicity, and inflammation. These proteins have been isolated from both venomous and non-venomous snakes, mammals, and most recently from medicinal plant extracts. The snake blood-derived inhibitors have been grouped into three major classes, alpha, beta, and gamma, based on common structural motifs found in other proteins with diverse physiological properties. In mammals, DM64, an anti-myotoxic protein isolated from opossum serum, belongs to the immunoglobulin super gene family and is homologous to human alpha1B-glycoprotein and DM43, a metalloproteinase inhibitor from the same organism. In plants, a short note is made of WSG, a newly described anti-toxic-PLA2 glycoprotein isolated from Withania somnifera (Ashwaganda), a medicinal plant whose aqueous extracts neutralize the PLA2 activity of the Naja naja venom. The implications of these new groups of PLA2 toxin inhibitors in the context of our current understanding of snake biology as well as in the development of novel therapeutic reagents in the treatment of snake envenomations worldwide are discussed.

Phospholipase A(2) isoforms: a perspective

Several new PLA(2)s have been identified based on their nucleotide gene sequences. They were classified mainly into three groups: cytosolic PLA(2) (cPLA(2)), secretary PLA(2) (sPLA(2)), and intracellular PLA(2) (iPLA(2)). They differ from each other in terms of substrate specificity, Ca(2+) requirement and lipid modification. The questions that still remain to be addressed are the subcellular localization and differential regulation of the isoforms in various cell types and under different physiological conditions. It is required to identify the downstream events that occur upon PLA(2) activation, particularly target protein or metabolic pathway for liberated arachidonic acid or other fatty acids. Understanding the same will greatly help in the development of potent and specific pharmacological modulators that can be used for basic research and clinical applications. The information of the human and other genomes of PLA(2)s, combined with the use of proteomics and genetically manipulated mouse models of different diseases, will illuminate us about the specific and potentially overlapping roles of individual phospholipases as mediators of physiological and pathological processes. Hopefully, such understanding will enable the development of specific agents aimed at decreasing the potential contribution of individual secretary phospholipases to vascular diseases. The signaling cascades involved in the activation of cPLA(2) by mitogen activated protein kinases (MAPKs) is now evident. It has been demonstrated that p44 MAPK phosphorylates cPLA(2) and increases its activity in cells and tissues. The phosphorylation of cPLA(2) at ser505 occurs before the increase in intracellular Ca(2+) that facilitate the binding of the lipid binding domain of cPLA(2) to phospholipids, promoting its translocation to cellular membranes and AA release. Recently, a negative feed back loop for cPLA(2) activation by MAPK has been proposed. If PLA(2) activation in a given model depends on PKC, PKA, cAMP, or MAPK then inhibition of these phosphorylating enzymes may alter activities of PLA(2) isoforms during cellular injury. Understanding the signaling pathways involved in the activation/deactivation of PLA(2) during cellular injury will point to key events that can be used to prevent the cellular injury. Furthermore, to date, there is limited information available regarding the regulation of iPLA(2) or sPLA(2) by these pathways.

Identification of beta-type phospholipase A(2) inhibitor in a nonvenomous snake, Elaphe quadrivirgata

A novel serum protein inhibiting specifically the enzymatic activity of the basic phospholipase A(2) (PLA(2)) from the venom of the Chinese mamushi snake (Agkistrodon blomhoffii siniticus) was purified from a nonvenomous Colubridae snake, Elaphe quadrivirgata. The purified inhibitor was a 150-kDa glycoprotein having a trimeric structure, composed of two homologous 50-kDa subunits. Their amino acid sequences, containing leucine-rich repeats, were typical of the beta-type PLA(2) inhibitor (PLIbeta), previously identified from the serum of A. blomhoffii siniticus. The inhibitor inhibited exclusively group II basic PLA(2)s and did not inhibit other kinds of PLA(2)s. This is the first paper reporting the existence of PLIbeta in a nonvenomous snake. The existence of PLIbeta in the nonvenomous snake reflects that PLIbetas are widely distributed over the snake species and participate commonly in regulating the physiological activities of the unidentified target PLA(2)s.

Functional site of endogenous phospholipase A2 inhibitor from python serum

The functional site of 'phospholipase A2 inhibitor from python' (PIP) was predicted based on the hypothesis of proline brackets. Using different sources of secretory phospholipase A2 (sPLA2s) as enzyme, and [3H]arachidonate-labelled Escherichia coli as substrate, short synthetic peptides representing the proposed site were examined for their secretory phospholipase A2 (sPLA2) inhibitory activity. A decapeptide P-PB.III proved to be the most potent of the tested peptides in inhibiting sPLA2 enzymatic activity in vitro, and exhibited striking anti-inflammatory effects in vivo in a mouse paw oedema model. P-PB.III inhibited the enzymatic activity of class I, II and III PLA2s, including that of human synovial fluid from arthritis patients. When tested by ELISA, biotinylated P-PB.III interacted positively with various PLA2s, suggesting that the specific region of PIP corresponding to P-PB.III, is likely to be involved in the PLA2-PLI interaction. The effect of P-PB.III on the peritoneal inflammatory response after surgical trauma in rats was also examined. P-PB.III effectively reduced the extent of postsurgical peritoneal adhesions as compared to controls. sPLA2 levels at seventh postoperative day in the peritoneal tissue of P-PB.III-treated rats were also significantly reduced (P < 0.05) in comparison to those of the untreated controls. The present results shed additional insight on the essential structural elements for PLA2 binding, and may be useful as a basis for the design of novel therapeutic agents.

Inhibitory properties of the anti-bothropic complex from Didelphis albiventris serum on toxic and pharmacological actions of metalloproteases and myotoxins from Bothrops asper venom

Anti-bothropic complex (ABC) was isolated from the serum of the South American opossum (Didelphis albiventris) by single-step affinity chromatography using a Sepharose-immobilized metalloprotease (BaP1) from Bothrops asper as the binding protein. Biochemical characterization of ABC showed the presence of two glycosylated subunits of 43 and 45 kDa, respectively, with an isoelectric point < 4. The two subunits were separated by ion-exchange HPLC. The N-terminal sequences of both subunits (LKAMDPTPXLWIETESP, where X is Arg-9 and Pro-9, respectively) showed a high degree of identity with other serum inhibitors isolated from different marsupials. Functional studies pointed out that ABC inhibits the hemorrhagic and proteolytic activities on fibrin, fibrinogen, and casein induced by the metalloproteases BaP1 and BaH4 isolated from B. asper venom. In addition to the anti-hemorrhagic and anti-proteolytic activities, ABC also showed anti-myotoxic, anti-lethal, and anti-edematogenic effects against myotoxic phospholipases A(2) isolated from the same venom. Moreover, it had inhibitory effects on the phospholipase A(2) activity of the crude venom as well as the isolated venom phospholipases A(2).

Sequencing and two-dimensional structure prediction of a phospholipase A(2) inhibitor from the serum of the common tiger snake (Notechis scutatus)

A phospholipase A(2) inhibitor has been identified in the serum of the common tiger snake (Notechis scutatus). The inhibitor is composed of two chains, an alpha-chain and a beta-chain, that form a non-covalently associated complex capable of inhibiting the enzymatic activity of all phospholipase A(2) enzymes it was tested against. The alpha and beta-chains have been purified to homogeneity, digested and sequenced. From the peptide sequence generated, degenerate PCR primers were designed and used to elucidate the complete cDNA sequence of the chains using 5' and 3' RACE PCR. A total of three alpha-chain isoforms were identified, only one isoform of the beta-chain was detected. The two-dimensional structure of the three alpha-chains and one beta-chain were predicted using five prediction programs (discrimination of secondary structure class; nearest neighbour secondary structure, profile network from Heidelberg; self-optimised prediction method from multiple alignment, SSPAL). For each protein chain a consensus prediction was generated. Results are discussed in relation to the function of the protein, and how they may influence the three-dimensional structure of the inhibitor. Additionally, the sequences of several snake phospholipase A(2) inhibitors were used as the input for a motif prediction algorithm (MEME). The results are discussed in relation to the activity of these proteins.

Snake inhibitors of phospholipase A(2) enzymes

Phospholipase A(2) (PLA(2)) enzymes consist of a large family of proteins which share the same enzymatic function and display considerable sequence homology. These enzymes have been identified and characterised in mammalian tissue and snake venoms. Numerous physiological functions have been attributed to mammalian PLA(2)s and they are nontoxic. In comparison, venom PLA(2)s are toxic and induce a variety of pharmacological effects that are probably mediated via membrane receptors. Snake PLA(2) inhibitors (PLIalpha), with a similar structure to the M-type receptor, have been identified as soluble complexes in the serum of viperinae and crotalinae snakes. These inhibitors showed selective binding to crotalid group II PLA(2)s and appeared to be restricted to the serum of this snake family. Analysis of PLA(2) binding to recombinant fragments of PLIalpha indicated that the CRD region was most likely responsible for enzyme inhibition. A second type of inhibitor, PLIbeta, has been identified in serum from one viperid snake and consists of a leucine-rich structure. The third type of inhibitor, PLIgamma, was found in the serum of five snake families and contains a pattern of cysteine residues that define a three-finger structure. PLIgamma inhibitors isolated from the serum of Elapidae, Hydrophidae, Boidae and Colubridae families were able to inhibit a broad range of enzymes including the nontoxic mammalian group IB and IIA PLA(2)s, and bee venom group III PLA(2). However, differences in the binding affinities indicated specificity for particular PLA(2)s. A different representation has emerged for crotalid and viperid snakes. Their PLIgammas did not inhibit bee venom group III, mammalian group IB and IIA enzymes. Furthermore, inhibition data for the gamma-type inhibitor from Crotalus durissus terrificus (CICS) showed that this inhibitor was specific for viperid beta-neurotoxins and did not inhibit beta-neurotoxins from elapids [1]. Further studies are required to determine if this phenomenon is true for all gamma-type inhibitors from Crotalidae snakes. The relative distribution of these inhibitors, their specificities and the structural features involved in binding are discussed in this review.

The Caenorhabditis elegans odr-2 gene encodes a novel Ly-6-related protein required for olfaction

Caenorhabditis elegans odr-2 mutants are defective in the ability to chemotax to odorants that are recognized by the two AWC olfactory neurons. Like many other olfactory mutants, they retain responses to high concentrations of AWC-sensed odors; we show here that these residual responses are caused by the ability of other olfactory neurons (the AWA neurons) to be recruited at high odor concentrations. odr-2 encodes a membrane-associated protein related to the Ly-6 superfamily of GPI-linked signaling proteins and is the founding member of a C. elegans gene family with at least seven other members. Alternative splicing of odr-2 yields three predicted proteins that differ only at the extreme amino terminus. The three isoforms have different promoters, and one isoform may have a unique role in olfaction. An epitope-tagged ODR-2 protein is expressed at high levels in sensory neurons, motor neurons, and interneurons and is enriched in axons. The AWC neurons are superficially normal in their development and structure in odr-2 mutants, but their function is impaired. Our results suggest that ODR-2 may regulate AWC signaling within the neuronal network required for chemotaxis.

Increasing molecular diversity of secreted phospholipases A(2) and their receptors and binding proteins

Secreted phospholipases A(2) (sPLA(2)s) form a large family of structurally related enzymes which are widespread in nature. Snake venoms are known for decades to contain a tremendous molecular diversity of sPLA(2)s which can exert a myriad of toxic and pharmacological effects. Recent studies indicate that mammalian cells also express a variety of sPLA(2)s with ten distinct members identified so far, in addition to the various other intracellular PLA(2)s. Furthermore, scanning of nucleic acid databases fueled by the different genome projects indicates that several sPLA(2)s are also present in invertebrate animals like Drosophila melanogaster as well as in plants. All of these sPLA(2)s catalyze the hydrolysis of glycerophospholipids at the sn-2 position to release free fatty acids and lysophospholipids, and thus could be important for the biosynthesis of biologically active lipid mediators. However, the recent identification of a variety of membrane and soluble proteins that bind to sPLA(2)s suggests that the sPLA(2) enzymes could also function as high affinity ligands. So far, most of the binding data have been accumulated with venom sPLA(2)s and group IB and IIA mammalian sPLA(2)s. Collectively, venom sPLA(2)s have been shown to bind to membrane and soluble mammalian proteins of the C-type lectin superfamily (M-type sPLA(2) receptor and lung surfactant proteins), to pentraxin and reticulocalbin proteins, to factor Xa and to N-type receptors. Venom sPLA(2)s also associate with three distinct types of sPLA(2) inhibitors purified from snake serum that belong to the C-type lectin superfamily, to the three-finger protein superfamily and to proteins containing leucine-rich repeats. On the other hand, mammalian group IB and IIA sPLA(2)s can bind to the M-type receptor, and group IIA sPLA(2)s can associate with lung surfactant proteins, factor Xa and proteoglycans including glypican and decorin, a mammalian protein containing a leucine-rich repeat.

Interaction of the neurotoxic and nontoxic secretory phospholipases A2 with the crotoxin inhibitor from Crotalus serum

Crotalus durissus terrificus snakes possess a protein in their blood, named crotoxin inhibitor from Crotalus serum (CICS), which protects them against crotoxin, the main toxin of their venom. CICS neutralizes the lethal potency of crotoxin and inhibits its phospholipase A2 (PLA2) activity. The aim of the present study is to investigate the specificity of CICS towards snake venom neurotoxic PLA2s (beta-neurotoxins) and nontoxic mammalian PLA2s. This investigation shows that CICS does not affect the enzymatic activity of pancreatic and nonpancreatic PLA2s, bee venom PLA2 and Elapidae beta-neurotoxins but strongly inhibits the PLA2 activity of Viperidae beta-neurotoxins. Surface plasmon resonance and PAGE studies further demonstrated that CICS makes complexes with monomeric and multimeric Viperidae beta-neurotoxins but does not interact with nontoxic PLA2s. In the case of dimeric beta-neurotoxins from Viperidae venoms (crotoxin, Mojave toxin and CbICbII), which are made by the noncovalent association of a PLA2 with a nonenzymatic subunit, CICS does not react with the noncatalytic subunit, instead it binds tightly to the PLA2 subunit and induces the dissociation of the heterocomplex. In vitro assays performed with Torpedo synaptosomes showed a protective action of CICS against Viperidae beta-neurotoxins but not against other PLA2 neurotoxins, on primary and evoked liberation of acetylcholine. In conclusion, CICS is a specific PLA2 inhibitor of the beta-neurotoxins from the Viperidae family.

A monoclonal antibody to human SP-10 inhibits in vitro the binding of human sperm to hamster oolemma but not to human Zona pellucida

SP-10 is a sperm intra-acrosomal protein, specific to the testis, that is believed to play an important role in egg-sperm binding. While the molecular characterization of the SP-10 protein has been clarified, little is yet known of its functional role in fertilization. We therefore established a monoclonal antibody (mAb pep-SP10) against a peptide (pep-SP10) that included the most hydrophilic portion of human SP-10 between the 135th and 149th amino acids. Human SP-10 was found to be localized in the equatorial region of acrosome-reacted sperm by immunofluorescent staining using our mAb pep-SP10. Monoclonal Ab pep-SP10 inhibited sperm-oolemma binding in the zona-free hamster egg penetration test, but it did not inhibit sperm-zona binding in the hemizona assay. Furthermore, we demonstrated that the oolemmal ligands of human SP-10 did not include beta(1) integrins, the most promising candidates for oocyte ligands involved in sperm-oolemma binding, based on the findings of a human sperm-cultured cell binding assay using F9 mouse embryonal carcinoma cells and F9-transformed cells lacking beta(1) integrins. In conclusion, our present data suggest that human SP-10, expressed on the equatorial region of acrosome-reacted sperm, indeed mediates sperm-oolemma binding in a beta(1) integrin-independent manner, but not sperm-zona binding.

Functional characteristics of a phospholipase A(2) inhibitor from Notechis ater serum

A phospholipase A(2) inhibitor has been purified p6om the serum of Notechis ater using DEAE-Sephacel chromatography. The inhibitor was found to be composed of two protein subunits (alpha and beta) that form the intact complex of approximately 110 kDa. The alpha-chain is a 30-kDa glycoprotein and the beta-chain a nonglycosylated, 25-kDa protein. N-terminal sequence analysis reveals a high level of homology to other snake phospholipase A(2) inhibitors. The inhibitor was shown to be extremely pH and temperature stable. The inhibitor was tested against a wide variety of phospholipase A(2) enzymes and inhibited the enzymatic activity of all phospholipase A(2) enzymes tested, binding with micromole to nanomole affinity. Furthermore, the inhibitor was compared with the Eli-Lilly compound LY311727 and found to have a higher affinity for human secretory nonpancreatic phospholipase A(2) than this chemical inhibitor. The role of the carbohydrate moiety was investigated and found not to affect the in vitro function of the inhibitor.

Mapping part of the functional epitope for ligand binding on the receptor for urokinase-type plasminogen activator by site-directed mutagenesis

The urokinase-type plasminogen activator receptor (uPAR) is a glycolipid anchored multidomain member of the Ly-6/uPAR protein domain superfamily. Studies by site-directed photoaffinity labeling, chemical cross-linking, and ligand-induced protection against chemical modification have highlighted the possible involvement of uPAR domain I and particularly loop 3 thereof in ligand binding (Ploug, M. (1998) Biochemistry 37, 16494-16505). Guided by these results we have now performed an alanine scanning analysis of this region in uPAR by site-directed mutagenesis and subsequently measured the effects thereof on the kinetics of uPA binding in real-time by surface plasmon resonance. Only four positions in loop 3 of uPAR domain I exhibited significant changes in the contribution to the free energy of uPA binding (DeltaDeltaG >/= 1.3 kcal mol(-1)) upon single-site substitutions to alanine (i.e. Arg(53), Leu(55), Tyr(57), and Leu(66)). The energetic impact of these four alanine substitutions was not caused by gross structural perturbations, since all monoclonal antibodies tested having conformation-dependent epitopes on this domain exhibited unaltered binding kinetics. These sites together with a three-dimensional structure for uPAR may provide an appropriate target for rational drug design aimed at developing new receptor binding antagonists with potential application in cancer therapy.

cDNA cloning and bacterial expression of phospholipase A(2) inhibitor PLIalpha from the serum of the Chinese mamushi, Agkistrodon blomhoffii siniticus(1)

The cDNA encoding of a phospholipase A(2) inhibitor (PLIalpha) of the Chinese mamushi, Agkistrodon blomhoffii siniticus, was identified from a liver cDNA library by use of a probe prepared by polymerase chain reaction (PCR) on the basis of the amino acid sequence of PLIalpha. It encoded a polypeptide of 166 amino acid residues, including 19 residues of the signal sequence and 147 residues of the complete mature sequence of PLIalpha. The PLIalpha cDNA was subcloned into the expression vector pET-16b and used to transform Escherichia coli strain BL21(DE3)pLysS. The recombinant PLIalpha expressed as a fusion protein was solubilized and purified to homogeneity by use of a metal affinity resin. The purified PLIalpha fusion protein underwent folding to form a trimeric structure like the intact PLIalpha, and showed inhibitory activity against the group II acidic PLA(2) from A. blomhoffii siniticus venom; although its binding constant (1/K(i)) value was 30-fold lower than that of the natural PLIalpha. The elimination of the N-terminal additional peptide from the fusion protein resulted in a marked increase in the inhibition activity with a binding constant comparable to that of the natural PLIalpha against the acidic PLA(2). Furthermore, the carbohydrate chains of the natural PLIalpha were found to play an important role in the inhibitory activity against the basic PLA(2).

A novel phospholipase A2 inhibitor with leucine-rich repeats from the blood plasma of Agkistrodon blomhoffii siniticus. Sequence homologies with human leucine-rich alpha2-glycoprotein

The phospholipase A2 (PLA2) inhibitor PLIbeta, purified from the blood plasma of Chinese mamushi snake (Agkistrodon blomhoffii siniticus), is a 160-kDa trimer with three 50-kDa subunits; and it inhibits specifically the enzymatic activity of the basic PLA2 from its own venom (Ohkura, N., Okuhara, H., Inoue, S., Ikeda, K., and Hayashi, K. (1997) Biochem. J. 325, 527-531). In the present study, the 50-kDa subunit was found to be glycosylated with N-linked carbohydrate, and enzymatic deglycosylation decreased the molecular mass of the 50-kDa subunit to 39-kDa. One 160-kDa trimer of PLIbeta was found to form a stable complex with three basic PLA2 molecules, indicating that one basic PLA2 molecule would bind stoichiometrically to one subunit of PLIbeta. A cDNA encoding PLIbeta was isolated from a Chinese mamushi liver cDNA library by use of a probe prepared by a polymerase chain reaction on the basis of the partially determined amino acid sequence of the subunit. The cDNA contained an open reading frame encoding a 23-residue signal sequence followed by a 308-residue protein, which contained the sequences of all the peptides derived by lysyl endopeptidase digestion of the subunit. The molecular mass of the mature protein was calculated to be 34,594 Da, and the deduced amino acid sequence contained four potential N-glycosylation sites. The sequence of PLIbeta showed no significant homology with that of the known PLA2 inhibitors. But, interestingly, it exhibited 33% identity with that of human leucine-rich alpha2-glycoprotein, a serum protein of unknown function. The most striking feature of the sequence is that it contained nine leucine-rich repeats (LRRs), each of 24 amino acid residues and thus encompassing over two-thirds of the molecule. LRRs in PLIbeta might be responsible for the specific binding to basic PLA2, since LRRs are considered as the motifs involved in protein-protein interactions.

Structural elements of Trimeresurus flavoviridis serum inhibitors for recognition of its venom phospholipase A2 isozymes

Five inhibitors (PLI-I-V) against Trimeresurus flavoviridis (Tf, habu snake, Crotalinae) venom phospholipase A2 (PLA2) isozymes have been isolated from its serum. PLI-I, which is composed of two repeated three-finger motifs, and PLI-IV and PLI-V, which contain a sequence similar to the carbohydrate recognition domain (CRD) of C-type lectins, were expressed in the forms fused with glutathione S-transferase (GST). The resulting GST-PLIs showed ability to bind to three Tf venom PLA2 isozymes. The binding study with the truncated forms indicated that one of two three-finger motifs of PLI-I was able to bind to PLA2 isozymes. The N-terminal 37-amino acid fragment and the CRD-like domain of PLI-IV and PLI-V were bound to PLA2 isozymes. On the other hand, their C-terminal 12-amino acid segment also associated with PLA2 isozymes. When either of two units of a hydrophobic tripeptide in this sequence was replaced by trialanine, the binding was completely abolished, indicating that the C-terminal hydrophobic cores of PLI-IV and PLI-V were critically responsible for the binding to venom PLA2 isozymes.

Divergent and common groups of proteins in glands of venomous snakes

Protein contents of venom-producing glands from the sea-snake Laticauda colubrina (LC) and terrestrial Vipera Russelli (VR) were studied using high-resolution two-dimensional gels: isoelectric focusing followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (IEF/SDS-PAGE) and nonequilibrium pH gradient electrophoresis (NEPHGE) followed by SDS-PAGE. Tentative identities of numerous proteins were established using their amino acid compositions and in certain cases the identities were verified by microsequencing of their N-terminals and internal fragments. As expected, we found several proteins known to be present in the venom of the respective snakes. These include numerous isoforms of phospholipase A2 (PLA2) in both snake glands, various neurotoxins in LC glands and factor IX/factor X-binding protein, hemorrhagic factor and coagulation factor X activating enzyme in Russell's viper glands (VR). Not unexpectedly, we also found a number of cell housekeeping proteins, cytoskeletal proteins, proteins that are necessary for folding, such as heat-shock proteins, protein disulfide-isomerase and peptidyl-prolyl cis/trans isomerases. Unexpectedly, however, the glands of Laticauda colubrina and Russell's viper include a large quantity of antihemorrhagic factor and inhibitor of PLA2, respectively, that have been previously described in snake plasma. The possible reason associated with the presence of these components in venom glands is discussed.

RoBo-1, a novel member of the urokinase plasminogen activator receptor/CD59/Ly-6/snake toxin family selectively expressed in rat bone and growth plate cartilage

Using differential display polymerase chain reaction, we cloned a novel cDNA named RoBo-1 from rat tibia. RoBo-1 is abundantly expressed in bone, including the hypertrophic chondrocytes of the growth plate where cartilage is remodeled into bone. RoBo-1 mRNA expression increased in response to two modulators of bone metabolism, estradiol and intermittent mechanical loading, suggesting a role in bone homeostasis. The 1.6-kilobase cDNA encodes a 240-amino acid protein with a cysteine spacing pattern, suggesting that RoBo-1 is a novel member of the urokinase plasminogen activator receptor/CD59/Ly-6/snake toxin family. Furthermore, the C-terminal contains a glycosyl-phosphatidylinositol attachment site, suggesting that it is a cell surface protein similar to other mammalian members of this family. The strongest homology of RoBo-1 is to the snake serum-derived phospholipase A2 inhibitors, which uniquely contain two of the cysteine domains but are secreted proteins. Interestingly, RoBo-1 is likely the first membrane-anchored member of this family containing two cysteine domains. Thus, the tissue specificity, responsiveness to bone protective mediators, along with its relationship to the multifunctional urokinase plasminogen activator receptor/CD59/Ly-6/snake toxin family suggests that RoBo-1 may play a novel role in the growth or remodeling of bone.

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.

Characterization and evolution of a gene encoding a Trimeresurus flavoviridis serum protein that inhibits basic phospholipase A2 isozymes in the snake's venom

The proteins that bind phospholipase A2 (PLA2) isozymes of Trimeresurus flavoviridis (habu snake, crotalinae) venom were fractionated from sera on four columns, each conjugated with one of four PLA2 isozymes. Five proteins, termed PLA2 inhibitors (PLI) I-V, were obtained as the binding components. The combinations of the binding components differed depending on the PLA2 isozymes. PLI-IV and PLI-V correspond to PLI-A and PLI-B, respectively, which were known to bind to a major [Asp49]PLA2, PLA2, and contained a segment similar to the carbohydrate-recognition domain of C-type lectins. PLI-I, which is a major component of inhibitory proteins against three basic PLA2 isozymes, PLA-B (a basic [Asp49]PLA2) and basic proteins I and II (both [Lys49]PLA2s), has been isolated, and its partial amino acid sequence has been determined. A cDNA encoding PLI-I was isolated from a T. flavoviridis liver cDNA library and sequenced. PLI-I cDNA encoded 200 amino acid residues, including a signal peptide of 19 amino acid residues. One sugar chain was predicted to occur at position 157. A gene coding for PLI-I was isolated. It is 9.6-kb long and consists of five exons and four introns. Comparison of the exon-intron structure of the PLI-I gene with those of genes encoding urokinase-type-plasminogen-activator receptor (uPAR), Ly-6, CD59 and neurotoxins showed that they have characteristic unit encoding approximately 90 amino acid residues, which is divided over two exons. This strongly suggests that the PLI-I gene belongs to the uPAR, Ly-6, CD59 and neurotoxin gene family. There are two types of structurally different inhibitors against PLA2 isozymes in T. flavoviridis serum with different evolutionary origins.

Biochemical characterization and pharmacological properties of a phospholipase A2 myotoxin inhibitor from the plasma of the snake Bothrops asper

A protein that neutralizes the biological activities of basic phospholipase A2 (PLA2) myotoxin isoforms from the venom of the snake Bothrops asper was isolated from its blood by affinity chromatography with Sepharose-immobilized myotoxins. Biochemical characterization of this B. asper myotoxin inhibitor protein (BaMIP) indicated a subunit molecular mass of 23-25 kDa, an isoelectric point of 4, and glycosylation. Gel-filtration studies revealed a molecular mass of 120 kDa, suggesting that BaMIP possesses an oligomeric structure composed of five 23-25 kDa subunits. Functional studies indicated that BaMIP inhibits the PLA2 activity of B. asper basic myotoxins I and III, as well as the myotoxicity and edema-forming activity in vivo and cytolytic activity in vitro towards cultured endothelial cells, of all four myotoxin isoforms (I-IV) tested. Sequence analysis of the first 63 amino acid residues from the N-terminus of BaMIP indicated more than 65% sequence similarity to the PLA2 inhibitors isolated from the blood of the crotalid snakes Trimeresurus flavoviridis and Agkistrodon blomhoffii siniticus. These inhibitors also share sequences similar to the carbohydrate-recognition domains of human and rabbit cellular PLA2 receptors, suggesting a common domain evolution among snake plasma PLA2 inhibitors and mammalian PLA2 receptors. Despite this similarity, this is the first description of a natural anti-myotoxic factor from snake blood.

Purification and characterization of three distinct types of phospholipase A2 inhibitors from the blood plasma of the Chinese mamushi, Agkistrodon blomhoffii siniticus

Three distinct types of phospholipase A2 (PLA2) inhibitory proteins (PLIalpha, PLIbeta, and PLIgamma) were isolated from the blood plasma of the Chinese mamushi, Agkistrodon blomhoffii siniticus. PLIalpha is an inhibitor that we have already purified and whose amino acid sequence we have already determined [Ohkura, Inoue, Ikeda and Hayashi (1993) J. Biochem. (Tokyo) 113, 413-419]. It inhibited selectively the group-II acidic PLA2s from Crotalidae venom. PLIbeta was a 160-kDa glycoprotein having a trimeric structure composed of 50-kDa subunits. The amino acid sequence of the first 30 amino acids of the N-terminal part of the 50-kDa subunit was determined and found to have no significant homology to that of known proteins. PLIbeta was a selective inhibitor against the group-II basic PLA2s from Crotalidae venom. Some amino acid residues located in or close to the interfacial binding surface of the group-II basic PLA2s were suggested to be involved in selective binding to PLIbeta. PLIgamma was a 100-kDa glycoprotein containing 25-kDa and 20-kDa subunits and inhibited all of the PLA2s investigated equally, including Elapidae venom PLA2s (group I), Crotalidae and Viperidae venom PLA2s (group II) and honey-bee PLA2 (group III). From the N-terminal sequences of the two subunits, PLIgamma was found to be the same type of PLI that had been purified from Thailand cobra plasma.

The urokinase-type plasminogen activator system in cancer metastasis: a review

The urokinase-type plasminogen activator (u-PA) system consists of the serine proteinases plasmin and u-PA; the serpin inhibitors alpha2-anti-plasmin, PAI-1 and PAI-2; and the u-PA receptor (u-PAR). Two lines of evidence have strongly suggested an important and apparently causal role for the u-PA system in cancer metastasis: results from experimental model systems with animal tumor metastasis and the finding that high levels of u-PA, PAI-1 and u-PAR in many tumor types predict poor patient prognosis. We discuss here recent observations related to the molecular and cellular mechanisms underlying this role of the u-PA system. Many findings suggest that the system does not support tumor metastasis by the unrestricted enzyme activity of u-PA and plasmin. Rather, pericellular molecular and functional interactions between u-PA, u-PAR, PAI-1, extracellular matrix proteins, integrins, endocytosis receptors and growth factors appear to allow temporal and spatial re-organizations of the system during cell migration and a selective degradation of extracellular matrix proteins during invasion. Differential expression of components of the system by cancer and non-cancer cells, regulated by paracrine mechanisms, appear to determine the involvement of the system in cancer cell-directed tissue remodeling. A detailed knowledge of these processes is necessary for utilization of the therapeutic potential of interfering with the action of the system in cancers.

Structures of genes encoding phospholipase A2 inhibitors from the serum of Trimeresurus flavoviridis snake

Inhibitors (PLIs) against snake venom gland phospholipases A2 (PLA2s) have been found in their sera. A cDNA encoding a PLI from Trimeresurus flavoviridis (Tf, habu snake, Crotalinae) serum, cPLI-A, was isolated from the Tf liver cDNA library and sequenced. Northern blot analysis with cPLI-A showed that PLIs are expressed only in liver. Genes for PLIs, gPLI-A and gPLI-B, were isolated from the Tf genomic DNA library and their nucleotide (nt) sequences were determined. The genes consisted of four exons and three introns, and exon 4 encoded the carbohydrate recognition domain (CRD)-like motif. Comparison of the nt sequences between gPLI-A and gPLI-B showed that these genes are highly homologous, including introns, except that exon 3 is rich in nonsynonymous nt substitutions which are almost four times as frequent as synonymous nt substitutions. This evolutionary feature of PLI genes is different from that of venom gland PLA2 isozyme genes in which nonsynonymous nt substitutions are spread over the entire mature protein-coding region.