Molecular characterization and functional divergence of igfbp3a and igfbp3b in turbot (Scophthalmus maximus)

Insulin-like growth factor binding protein 3 (IGFBP3), an important member of the IGFBP family, is involved in growth, reproduction and metabolic regulation. In the present study, we isolated and characterized two IGFBP3 subtypes, namely IGFBP3a and IGFBP3b from turbot through rapid amplification of cDNA ends-polymerase chain reaction (RACE-PCR) cloning. The deduced amino acid sequences of turbot IGFBP3a and IGFBP3b showed high homology to those of other teleosts, particularly with Atlantic halibut (Hippoglossus hippoglossus). Turbot IGFBP3a and IGFBP3b contained two conserved motifs in the IGFBP domain (GCGCCXXC) and thyroglobulin type 1 domain (CWCV) as well as conserved cysteines. Molecular docking analysis predicted that IGFBP3a preferentially bound to IGF3, whereas IGFBP3b preferentially bound to IGF1. Both proteins localized in the cytoplasm and nucleus. The igfbp3a mRNA was most abundant in the livers of males and the pituitary of females, followed by the pituitary and heart in males, and the ovaries in females. However, igfbp3b was abundant in the hearts of males and females. igfbp3a and igfbp3b mRNAs significantly increased from the two-cell stage to the gastrula stage, peaked at gastrulation, and sharply decreased at the hatched larval stage. Moreover, the mRNA level of igfbp3a was significantly higher than that of igfbp3b throughout turbot embryonic development. These findings suggested functional divergence between IGFBP3a and IGFBP3b, IGFBP3a may play a key role during embryonic development and tissue-specific modulation in turbot.