A putative endocrine factor SIBD (single insulin binding domain protein) involved in immune response of Chinese mitten crab Eriocheir sinensis

Insulin growth factor binding proteins (IGFBPs), characterized by the conserved insulin binding (IB) domains, are important components of endocrine system and play key roles in metabolism and growth. In the present study, the full-length cDNA of a single IB domain protein (designated EsSIBD) was identified from Chinese mitten crab Eriocheir sinensis based on expressed sequence tag (EST) sequence. The 1187 bp EsSIBD cDNA contained a 321 bp open reading frame (ORF) encoding a putative protein of 106 amino acids, a 5′-untranslated region (UTR) of 189 bp, and a 3′-UTR of 677 bp. Multiple sequence alignment presented ten conserved cysteine residues critical for the fundamental structure and function of IB domain. BLAST analysis revealed that EsSIBD shared high similarity with previously known IB domains of IGFBPs with the identities ranging from 40% to 46%. The sequence similarity and domain conservation indicated that EsSIBD was a potential member of the IGFBP family. Phylogenic analysis presented that EsSIBD was closer to IGFBP7 than to the other IB domain containing proteins, suggesting its functional similarity with the endocrine factor IFGBP7. The mRNA expression of EsSIBD in different tissues including hepatopancreas, gill, gonad, muscle, heart and haemocytes, and the temporal expression in haemocytes challenged by Listonella anguillarum were measured by real-time RT-PCR. EsSIBD mRNA transcripts could be detected in all examined tissues with the highest expression level in gill. The EsSIBD mRNA expression in haemocytes was sensitive to L. anguillarum stimulation and it was up-regulated from 3 to 24 h after challenge. The structure conservation and functional similarity to IFGBPs, and its sensitivity to L. anguillarum stimulation collectively implied that EsSIBD was probably involved in endocrine and immune systems of Chinese mitten crab, and provided insight into the cross-talk between the invertebrate endocrine and immune system.