cDNA cloning of the beta subunit of teleost thyrotropin

cDNA clones encoding the beta subunit of thyrotropin (thyroid-stimulating hormone; TSH) were isolated from a cDNA library made from the pituitaries of immature rainbow trout and sequenced. The precursor of rainbow trout TSH beta consists of 147 aa, which can be cleaved into a signal peptide (20 aa) and a mature protein (127 aa) containing one potential N-glycosylation site and 12 cysteine residues. The protein showed highest homology with human TSH beta (51%) and lesser homology with human follitropin (42%), human lutropin (32%), and salmon gonadotropin (31-33%) beta subunits. The identification of TSH in addition to two gonadotropins (gonadotropins I and II) in the teleost fish suggests that the divergence of three kinds of glycoprotein hormones from an ancestral molecule took place earlier than the time of divergence of teleosts from the main line of evolution leading to tetrapods. Northern blot analysis showed that the expression of the rainbow trout TSH beta gene is specific to the pituitary gland and is significantly higher in immature fish than in mature fish, suggesting that TSH plays some role in the biological processes of immature fish.