cDNA sequence analysis and expression of cardiotoxins from Taiwan Cobra

Towards universal approach for bacterial production of three-finger Ly6/uPAR proteins: Case study of cytotoxin I from cobra N. oxiana

Cytotoxins or cardiotoxins is a group of polycationic toxins from cobra venom belonging to the 'three-finger' protein superfamily (Ly6/uPAR family) which includes small β-structural proteins (60-90 residues) with high disulfide bond content (4-5 disulfides). Due to a high cytotoxic activity for cancer cells, cytotoxins are considered as potential anticancer agents. Development of the high-throughput production methods is required for the prospective applications of cytotoxins. Here, efficient approach for bacterial production of recombinant analogue of cytotoxin I from N. oxiana containing additional N-terminal Met-residue (rCTX1) was developed. rCTX1 was produced in the form of E. coli inclusion bodies. Refolding in optimized conditions provided ∼6 mg of correctly folded protein from 1 L of bacterial culture. Cytotoxicity of rCTX1 for C6 rat glioma cells was found to be similar to the activity of wild type CTX1. The milligram quantities of ¹³C,¹⁵N-labeled rCTX1 were obtained. NMR study confirmed the similarity of the spatial structures of recombinant and wild-type toxins. Additional Met residue does not perturb the overall structure of the three-finger core. The analysis of available data for different Ly6/uPAR proteins of snake and human origin revealed that efficiency of their folding in vitro is correlated with the number of proline residues in the third loop and the surface area of hydrophobic residues buried within the protein interior. The obtained data indicate that hydrophobic core is important for the folding of proteins with high disulfide bond content. Developed expression method opens new possibilities for structure-function studies of CTX1 and other related three-finger proteins.

Fusogenicity of Naja naja atra cardiotoxin-like basic protein on sphingomyelin vesicles containing oxidized phosphatidylcholine and cholesterol

This study investigated the effect of oxidized phosphatidylcholine (oxPC) and cholesterol (Chol) on Naja naja atra cardiotoxin-like basic protein (CLBP)-induced fusion and leakage in sphingomyelin (SM) vesicles. Compared with those on PC/SM/Chol vesicles, CLBP showed a lower activity to induce membrane permeability but a higher fusogenicity on oxPC/SM/Chol vesicles. A reduction in inner-leaflet fusion elucidated that CLBP fusogenicity was not in parallel to its membrane-leakage activity on oxPC/SM/Chol vesicles. The lipid domain formed by Chol and SM supported CLBP fusogenicity on oxPC/SM/Chol vesicles, while oxPC altered the interacted mode of CLBP with oxPC/SM/Chol vesicles as evidenced by Fourier transform infrared spectra analyses and colorimetric phospholipid/polydiacetylene membrane assay. Although CLBP showed similar binding affinity with PC/SM/Chol and oxPC/SM/Chol vesicles, the binding capability of CLBP with PC/SM/Chol and oxPC/SM/Chol vesicles was affected differently by NaCl. This emphasized that CLBP adopted different membrane interaction modes upon binding with PC/SM/Chol and oxPC/SM/Chol vesicles. CLBP induced fusion in vesicles containing oxPC bearing the aldehyde group, and aldehyde scavenger methoxyamine abrogated the CLBP ability to induce oxPC/SM/Chol fusion. Taken together, our data indicate that Chol and oxPC bearing aldehyde group alter the CLBP membrane-binding mode, leading to fusogenicity promotion while reducing the membrane-damaging activity of CLBP.

The multiplicity of cardiotoxins from Naja naja atra (Taiwan cobra) venom

Four novel cardiotoxins were isolated from Naja naja atra (Taiwan cobra) venom by successive separation on a SP-Sephadex C-25 column and a reverse phase column. Amino acid sequences of the cardiotoxins were determined by Edman degradation and carboxypeptidase digestion. It shows that these cardiotoxins comprise 60 amino acid residues. Comparative analyses on the amino acid sequences of cardiotoxins from the venoms of N. naja atra and other Naja species indicated that amino acid substitutions of cardiotoxin isoforms frequently occurred at positions 7-11, 27-32 and 45-47. The hypervariable segments encoded by the second and third exon of cardiotoxin genes are located at or near the tips of loop structure of cardiotoxin molecules. These results, together with the suggestions that the residues at the tips of cardiotoxins' loop structure were involved in the manifestation of the biological activities of cardiotoxins, reflect that the preferential mutations may contribute to alterations in the function of cardiotoxin molecules. Analysis on the secondary structure of pre-mRNAs of N. naja atra cardiotoxin 4 gene and N. naja sputatrix cardiotoxin 3 gene has shown that the hypervariable regions of the exon 2 pertain to form intra-exon pairings and are not involved in the formation of intron-exon pairings. Since the pairings of splice sites and gene architecture were supposed to be associated with intron-exon recognition, it is likely that the preferred loci of mutations occurring with the evolution of cardiotoxin genes would not affect the processing of cardiotoxin precursors.

Structure-function studies on Taiwan cobra long neurotoxin homolog

A novel long neurotoxin homolog was purified from Naja naja atra (Taiwan cobra) venom using the combination of ion exchange chromatography and reverse phase high performance liquid chromatography. The determined protein sequence was essentially the same as that deduced from the cDNA amplified by reverse transcriptase-polymerase chain reaction. The long neurotoxin homolog exhibited an activity that inhibited acetylcholine-induced muscle contractions, as with N. naja atra cobrotoxin. The degree of inhibition caused by the addition of long neurotoxin homolog was approximately 70% of that observed with the addition of cobrotoxin. Unlike the well-known short and long neurotoxins, this neurotoxin homolog contained two additional cysteine residues forming a disulfide linkage in the N-terminal region. Circular dichroism measurement and computer models of the neurotoxin reveal that its secondary structure was not abundant in beta-sheet as noted with short and long neurotoxins. This less ordered structure may be associated with the lower activity noted with the long neurotoxin homolog. Together with the finding that the known long neurotoxin homologs exclusively appear in the venoms of the Naja and Bungarus genera, the long neurotoxin homologs should represent an evolutionary branch from the long and short neurotoxins in the Elapidae family.

Structure and organization of the cardiotoxin genes in Naja naja sputatrix

We report the genomic structure, organization and the presence of multiple isoforms of the gene encoding cardiotoxins (CTX) of Naja naja sputatrix. The cardiotoxin gene consists of six CTX isoforms, each (2.2 kb) having three exons and two introns. Two possible transcription initiation sites as well as consensus TATA boxes and transcription factor binding motifs, AP-2, NFIL-6/C/EBP, NF-kappaB and PuF have been identified in the 5'-region of the gene. The CTX gene isoforms show nucleotide variations at specific segments in exon 2 and exon 3, which correspond to the functional domains in the three-finger loop structure of the cardiotoxin molecule. The diverse functions of cardiotoxins together with our findings suggest that the cardiotoxin gene isoforms may have evolved under adaptive pressure through a positive Darwinian selection process.

Differentially expressed snoRNAs in Bungarus multicinctus (Taiwan banded krait)

Twenty novel snoRNAs forming extensive sequence complementarities to mature 5S rRNA were identified from Bungarus multicinctus by reverse transcription-polymerase chain reaction. It was found that the snoRNA species were differentially transcribed in different tissues as evidenced by single stranded conformational polymorphism analysis and direct nucleotide sequence analysis. Although the diversity in the sequences of snoRNAs is observed, comparison of these snoRNA genes reveals that the regions involved in binding to 5S rRNA are highly conserved and form two 12-nt-15-nt tracts of complementarity to phylogenetically invariant sequences in eukaryotic 5S rRNAs. Nevertheless, the lower conservation of box C/D or box H/ACA in these snoRNAs was observed. Likewise, the sequences in several fish and human genes forming perfect duplexes with 5S rRNA also did not highly retain these box elements. These results may infer that the box elements are dispensable for the function of snoRNA species identified in the present study. Moreover, the novel finding of the differentially expressed snoRNA variants in B. multicinctus suggests that the snoRNA genes are selectively processed in different tissues and are likely associated with tissue-specific regulation of their host gene transcripts.

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.

Genomic structures of cardiotoxin 4 and cobrotoxin from Naja naja atra (Taiwan cobra)

Two genomic DNAs with the size of 2.3 kb and 2.4 kb, which were isolated from the liver of Naja naja atra (Taiwan cobra), encoded the precursors of cardiotoxin 4 and cobrotoxin, respectively. Both genes shared virtually identical overall organization with three exons separated by two introns, which were inserted in the similar positions of the gene's coding regions. Moreover, their nucleotide sequences shared approximately 84.2% identity. This result reveals the evolutionary relationship between cardiotoxin and cobrotoxin. The exon/intron structures of cardiotoxin 4 and cobrotoxin genes were similar to that reported for erabutoxin c gene, a neurotoxin genomic DNA from a sea snake (Laticauda semifasciata). However, in contrast to the finding that the intron 2 of these genes had a similar size, a notable variation with the size of intron 1 was observed (1233 bp, 1269 bp and 197 bp for cardiotoxin 4, cobrotoxin and erabutoxin c genes, respectively). The different size with intron 1 is due to the middle region at the first intron of cardiotoxin 4 and cobrotoxin genes, which encoded small nucleolar RNA (snoRNA), being absent in that of erabutoxin c gene. These results, together with the finding of the potential mobility of snoRNA genes during evolution, suggest that intron insertions or deletions of snoRNA genes occur with the evolutionary divergence of snake neurotoxins and cardiotoxins.

Novel SnoRNAs from Naja naja atra (Taiwan cobra) and Bungarus multicinctus (Taiwan banded krait), form extended sequence complementarity to 5S rRNA

During the mapping and sequencing of Naja naja atra cobrotoxin and cardiotoxin 4 genes, we have found that novel small nucleolar RNAs (snoRNAs) are encoded in the first intron of the two genes. The snoRNAs in Naja naja atra were amplified from the venom glands cDNA mixtures of Naja naja atra by reverse transcription-polymerase chain reaction using the primers designed from the first intronic sequences of cobrotoxin and cardiotoxin 4 genes. Likewise, the snoRNAs in Bungarus multicinctus were also amplified by the same primers. Comparison of these snoRNA genes reveals that the regions involved in binding to 5S rRNA are highly conserved among these genes, and form 12-nt and 15-nt tracts of complementarity to phylogenetically invariant sequences in eukaryotic 5S rRNAs. The box C sequence in these snoRNAs is consensus, however, variations with the sequence of box D motif are observed. The present study is the first case of intron-encoded snoRNAs contain extended regions of perfect complementarity to mature 5S rRNA.