Retinol binding protein in rainbow trout: Molecular properties and mRNA expression in tissues

Retinoids are important regulatory signaling molecules during embryonic dev elopment. The molecular properties of rainbow trout (Oncorhynchus mykiss) r etinol-binding protein (rtRBP), the specific retinol carrier in vertebrate plasma, were studied to elucidate its role in transporting retinols to deve loping fish oocytes. A 954-nucleotide rtRBP cDNA was cloned from the liver coding for a 176-amino-acid (aa) mature protein, with an estimated molecula r mass of 20,267 Da. The nucleotide sequence suggests a putative 16-aa sign al peptide and shows all the aa residues that were previously identified as critical for the retinol binding pocket. Five of the eight amino acid resi dues that are associated with the interaction of REP and transthyretin in m ammalian and nonmammalian species are conserved. The deduced aa sequence of rtRBP shows 60 - 66% identity with zebrafish, chicken, mouse, rat, horse, bovine, and human RBPs and 56% identity with Xenopus REP. Northern blot ana lysis revealed a similar to1.1-kb hepatic mRNA transcript. REP is highly ex pressed in the liver, but low levels were also detected in the spleen, kidn ey, ovary, and brain. In the rainbow trout, 17 beta -estradiol treatment le d to a decrease in the REP mRNA signal relative to that of the controls. Th e efficacy of the 17 beta -estradiol treatment was verified by an induction of vitellogenin (VTG) mRNA expression in the liver and occurrence of VTG i n the plasma. (C) 2001 Academic Press.