An inhibitor IB homologue from bay scallop Argopecten irradians

IIB is an important member of NF-IB pathway in the innate immune system. In the present study, the full-length cDNA sequence encoding IIB protein (designated AiIIB) was isolated from bay scallop Argopecten irradians. The complete sequence of AiIIB cDNA containing a 5' untranslated region (UTR) of 237bp, a 3'UTR of 1023bp with a poly (A) tail, and an open reading frame (ORF) of 1086bp encoding a polypeptide of 361 amino acids with the predicted molecular weight of 39.9kDa and theoretical isoelectric point of 4.7. Six ankyrin repeats which were necessary for specific binding to NF-IB and two potential phosphorylation sites responsible for IIB degradation were identified in the N-terminus of AiIIB. No PEST domain but phosphorylation site motif (S357DSD360) was present at the C-terminus of AiIIB. Predicted three-dimensional structure of AiIIB shared high similarity with mammalian IIBa. Similarity and phylogenetic analysis revealed that AiIIB was clustered into IIBs from invertebrate. All these typical characteristics indicated that the AiIIB should be classified into IIB family proteins. Quantitative real-time RT-PCR was employed to assess the mRNA expression of AiIIB in various tissues and its temporal expression in haemocytes of scallops challenged with Listonella anguillarum. The mRNA transcript of AiIIB could be detected in all the examined tissues with highest expression level in hepatopancreas. Bacteria infection inhibited the transcription level of AiIIB. The results suggested the involvement of AiIIB in responses against bacterial infection and further highlighted its functional importance in the immune system of A. irradians.