Cloning Of Growth Hormone Receptors (GHR2) From Southern Catfish And Nile Tilapia And Preliminary Study On The Expressions Of Two GHRs In Southern Catfish

Growth hormone receptor (GHR) is a single membrane-spanning receptor, and belongs to the class I cytokine receptor superfamily. It binds to the growth hormone (GH) to initiate biological actions. GHR is regulated by many factors, such as nutritional status, hormone presence and environmental changes, etc. In silico analysis of fish genomes revealed that there exists another GHR (GHR2) in fish.The full-length GHR2 cDNAs were cloned from the Southern catfish (Silurus meridionalis) and the Nile tilapia (Oreochromis niloticus) by reverse transcription polymerase chain reaction (RT-PCR) followed by rapid amplification of cDNA ends (RACE). The Southern catfish GHR2 (scGHR2) cDNA contains 2299bp encompassing a 5' UTR of 420bp, a coding region of 1659bp and a 3' UTR of 553 bp including the poly (A) tail. The open reading frame encodes a protein of 553 aa. The Nile tilapia GHR2 (nGHR2) cDNA contains 1896bp encompassing a 5' UTR of 39bp, a coding region of 1722bp and a 3' UTR of 135 bp including the poly (A) tail. The open reading frame encodes a protein of 574 aa. Several characteristic landmarks of conventional GHR including the ligand binding motif FG(D/E)FS in extracellular domain, conserved cytoplasmic Boxl and Box2 regions and some of the conserved extracellular cysteine residues and intracellular tyrosine residues could be found in scGHR2 and nGHR2. However, GHR2 and GHR1 of the two fish possess features that are distinctly different from each other. First, the number of extracellular cysteine residues is different. While GHR1 possesses 7 extracellular cysteine residues forming three disulfide bonds, GHR2 has only 4 or 5 which may form two disulfide bonds. Second, the number of intracellular tyrosine residues is different. GHR1 contains 8 tyrosine residues but...