Comprehensive identification and analysis of insulin-like growth factor binding proteins family in turbot, Scophthalmus maximus.

Insulin-like growth factor binding proteins (IGFBPs) regulate IGFs and thereby influence diverse physiological processes. In this study, degenerate primers were designed for turbot (Scophthalmus maximus) based on conserved IGFBP sequences. Combined with rapid amplification of cDNA end-polymerase chain reaction (RACE-PCR), the full-length cDNA sequences of turbot IGFBP family members were successfully cloned and obtained. Results showed that the full-length cDNA of 11 IGFBPs subtypes (IGFBP1a/1b, IGFBP2a/2b, IGFBP3a/3b, IGFBP4, IGFBP5a/5b and IGFBP6a/6b) cDNA were 1060 to1802 bp long and encoded a protein composed of 189-336 amino acids, respectively. Turbot IGFBPs contain two conserved motifs (GCGCCXXC) in the IGFBP domain and a thyroglobulin type-1 domain (CWCV), and retain multiple key conserved cysteine residues. They share high homology with those of other teleosts and Atlantic halibut (Hippoglossus hippoglossus). Moreover, molecular docking predicted IGFBP3a/3b have universally high IGF affinity, IGFBP4 bind IGF3 most strongly, and other subtypes show predicted distinct ligand preferences. The isoforms of turbot igfbps were detected across all examined tissues and exhibited considerable sex differences. In female, the mRNAs levels of igfbp3a and igfbp2b in the heart and liver were significantly higher than those of other IGFBP isoforms in other tissues. In male, the expression of igfbp2b was similar to that in females, whereas igfbp3a was abound in the gill, testis, and stomach. These findings provided a foundation for the further functional characterizations of turbot IGFBPs.