Molecular cloning and characterization of FSH and LH receptors in Atlantic salmon (Salmo salar L.)

Two cDNAs encoding the FSH receptor (FSHR) and the LH receptor (LHR) from Atlantic salmon (Salmo salar) were cloned and characterized. The predicted protein sequence for FSHR comprises a mature protein of 635 amino acids (aa) and a signal peptide of 23 aa, and for LHR a mature protein of 701 aa and a signal peptide of 27 aa. Multiple sequence alignment of Atlantic salmon FSHR and LHR with gonadotropin receptor sequences of available teleosts and representative vertebrates revealed high sequence homology with other salmonids (97–98% for both receptors); amino acid identities ranged from 59 to 67% for FSHR and 47–79% for LHR compared with other teleosts, and between 50 and 52% compared with other vertebrates. The salmon FSHR and LHR showed the typical characteristics of glycoprotein receptors, including a long N-terminal extracellular domain (ECD), seven transmembrane domains and a short C-terminal intracellular domain. The ECD of the Atlantic salmon FSHR and LHR were composed of nine imperfect leucine-rich repeats forming the potential recognition sites for the corresponding hormone. The comparative analysis of the recognition sites in the Atlantic salmon gonadotropin receptors with the corresponding sites in the human receptors showed that the nature of the residues involved in the key contacts with the glycoprotein α-subunit were highly conserved. In contrast the recognition sites for the specific β-subunits showed clear differences between the two salmon gonadotropin receptors and the human receptors. In the salmon LHR the recognition sites for the LH β-subunit were relatively conserved, while the recognition sites for the FSH β-subunit in the salmon FSHR showed a higher divergence, suggesting different evolution rates for the two teleost gonadotropin receptors. Both FSHR and LHR were mainly expressed in the ovary and testis, but were also detected at low abundance in extra-gonadal tissues such as gills, brain, liver and heart.