Molecular cloning of cDNAs and structural model analysis of two gonadotropin β-subunits of snakehead fish (Channa maculata)

The cDNAs encoding β-subunits of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) have been cloned from the pituitary of snakehead fish, Channa maculata, and the three-dimensional structural models of the encoded FSH and LH were investigated. The cloned cDNAs, including 5′-untranslated region (UTR), open-reading frame, and 3′-UTR followed by a poly(A) tail, were obtained by reverse transcription-polymerase chain reaction and rapid amplification of cDNA end methods. The open-reading frames of FSH-β cDNA encodes a 120-amino acid protein with a signal peptide of 18 amino acids and a mature protein of 102 amino acids; while LH-β cDNA encodes a 140-amino acid protein with a signal peptide of 33 amino acids and a mature protein of 115 amino acids. The amino acid sequence identities of snakehead fish FSH-β and LH-β in comparison with other fish are 27.8-81.9% and 45.2-88.8%, respectively; while in comparison with tetrapods are 26.2-28.9% and 37.5-51.2%, respectively. Both FSH-β and LH-β of snakehead fish resemble most to those of Perciformes, implying their closer phylogenetic relationship. All 12 cysteine residues are conserved in snakehead fish LH-β; while 11 cysteine residues are conserved in its FSH-β. The third cysteine is absent in snakehead fish FSH-β; instead, a positionally shifted cysteine residue is present at the N-terminus, as found in some phylogenetic related fish. The structure models of snakehead fish FSH and LH, constructed by using the crystal structures of human FSH and human chorionic gonadotropin as respective template, showed that the positionally shifted N-terminal cysteine residue of snakehead fish FSH-β likely can substitute the third cysteine to form a disulfide bond with the 12th cysteine.