Basal repertoire of toxin-related transcripts in the zoanthid Protopalythoa variabilis(Cnidaria,Anthozoa)

OBJECTIVE To analyze the whole transcriptome of zoanthid Protopalythoa variabilis(P.variabilis),a cnidarian,and discover the potential toxic substances in P.variabilis.METHODS The P.variabilis RNA deep sequencing was performed using the HiSeq 2500 automatic sequencing platform.All the unigenes generated from the assembly process were functionally annotated based on the similarity with databases.The multiple alignments of translated toxin-related sequences were performed with Clustalw2,and amino acid identity and similarity highlighted by using BoxShade tool.Three different methods including ITASSER,PEP-FOLD and MODELLER were applied to predict tri-dimensional models of toxin-related polypeptides from translated transcript sequences of P.variabilis.The toxicity of one of the putative toxins,namely ShK/Aurelin-like peptide,was evaluated using zebrafish model.RESULTS A total of 67,549,914 pairs of quality-filtered,90-base-pair Illumina reads from an mRNA sample were obtained.The de novo assemblies yielded 276,526 contigs.The sequence comparison of 130,121 unigenes with entries in Toxin database showed that 1542 unigenes were potential peptide toxins at which 11 unigenes were related to Stichodactyla toxin(ShK)domain(Pfam ID:PF01549).ShK is a 35 residues peptide sequence that was firstly discovered from the sea anemone Stichodactyla helianthus.Here,we found out one ShK-like peptide that processed a relatively higher sequence similarity with known ShK(Uniprot ID:P29186)of Bunodosoma granuliferum(red warty sea anemone).The Protopalythoa Shk-like peptide was submitted to Probis server to detect probable binding site and found to match with a protein AURELIN(PDB id:2lg4,UniProt id:Q0MWV8)which possesses structural homology with previously identified antimicrobial peptides and K+-channel-blocking toxins.Our results showed that the ShK/Aurelin-like peptide was lethal to zebrafish embryos at concentrations above 30-μmol·L1,and could induce zebrafish locomotor deficit at 10μmol·L-1.CONCLUSION This study,for the first time,presented the whole transcriptome profile and a potential toxic peptide of P.variabilis.