Functional divergence of glycoprotein hormone receptors

Two lamprey glycoprotein hormone receptors (lGpH-R I and II) highly similar with gnathostome GpH-Rs were cloned from sea lamprey testes and thyroid, respectively. Vertebrate glycoprotein protein receptors have a large extracellular domain (ED) containing a leu rich domain (LRD) linked to a rhodopsin-like transmembrane domain (TMD) through a highly divergent linker region (signal specificity domain, SSD or 'hinge' region) and a third major segment, the intracellular domain. To determine the potential roles of the different domains in the activation of the receptor following ligand-receptor binding, functional assays were performed on lGpH-R I/rat luteinizing hormone (LH)-R domain swapped chimeric receptors. These results show that the functional roles of the lamprey glycoprotein-receptor I (lGpH-R I) domains are conserved compared with its Gnathostome homologs. The ability of different glycoprotein hormones to activate chimeric lamprey/rat receptors suggests that the selectivity of the GpH-Rs in respect to their ligands is not controlled exclusively by a single domain but is the result of specific interactions between domains. We hypothesize that these interactions were refined during millions of years of co-evolution of the receptors with their cognate ligands under particular intramolecular, intermolecular and physiological constraints.