Somatolactin (SL) is a pituitary hormone of the GH/prolactin (PRL) family that so far has been found only in fish. Compared with GH and PRL, the primary structure of SL is highly conserved among divergent fish species, suggesting it has an important function and a discriminating receptor that constrains structural change. However, SL functions are poorly understood, and receptors for SL have not yet been identified. During cloning of GH receptor cDNA from salmon, we found a variant with relatively high (38-58%) sequence identity to vertebrate GH receptors and low (28-33%) identity to PRL receptors; however, the recombinant protein encoding the extracellular domain showed only weak binding of GH. Ligand binding of the recombinant extracellular domain for this receptor confirmed that the cDNA encoded a specific receptor for SL. The SL receptor (SLR) has common features of a GH receptor including FGEFS motif, six cysteine residues in the extracellular domain, a single transmembrane region, and Box 1 and 2 regions in the intracellular domain. These structural characteristics place the SLR in the cytokine receptor type I homodimeric group, which includes receptors for GH, PRL, erythropoietin, thrombopoietin, granulocyte-colony stimulating factor, and leptin. Transcripts for SLR were found in 11 tissues with highest levels in liver and fat, supporting the notion that a major function of SL is regulation of lipid metabolism. Cloning SLR cDNA opens the way for discovery of new SL functions and target tissues in fish, and perhaps novel members of this receptor family in other vertebrates.
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