Cloning of two Atlantic salmon growth hormone receptor isoforms and in vitro ligand-binding response

Two isoforms of the full-length cDNA of the growth hormone receptor (GHR) of the Atlantic salmon (Salmo salar; ss) were cloned by a PCR approach using RACE. Respectively, the cDNA sequences of ssGHR isoforms 1 and 2 are 2654 and 2608 nucleotides long, with 1782 and 1773 nucleotide ORFs. The resulting coded proteins are 594 and 590 aa long, with 19 and 20 aa signal peptides. The two isoforms share 86% protein and 87% cDNA sequence similarity. Isoform 1 is most similar to other salmonid GHR isoforms 1 while isoform 2 is most similar to salmonid GHR isoforms 2 (93-95%). Similarity with other teleost species was lower (37-44%). The bioactivity of the cloned ssGHR was tested by transfecting the ssGHR isoform 1 cDNA into CHO-K1 hamster cells, incubating with recombinant salmon GH (sGH) or native ovine prolactin (oPRL), and measuring cell proliferation by the MTT assay. The ssGHR-transfected cells significantly increased proliferation when stimulated by sGH at all concentrations. oPRL stimulated ssGHR-transfected cells at higher concentrations due to receptor cross reaction. ssGHR isoforms 1 and 2 contain a single transmembrane domain and the typical conserved motifs found in other teleost GHRs, including four paired cysteine residues and five potential N-glycosylation sites in the extracellular domain, Box I and Box II, as well as seven potential tyrosine phosphorylation sites in the intracellular domain. However, in salmonids, these motifs differ from those of other teleosts, and could be responsible for differentiated hormone binding, signal transduction and response.