The high degree of amino acid sequence homology and the divergent ligand binding affinities of the rat (r) and human (h) lutropin receptors (LHR) allowed us to identify amino acid residues of its extracellular domain that are responsible for the different binding affinities of bovine (b) and human lutropin (LH), and human choriogonadotropin (hCG) to the hLHR and rLHR.

Because of the proposed importance of the -sheets of the leucine rich repeats (LRRs) of the extracellular domain of the LHR on hormone binding, we examined ten divergent residues present in these regions by analyzing two complementary sets of mutants in which hLHR residues were substituted with the corresponding rLHR residues and vice versa. These experiments resulted in the identification of a single residue (a Ile or Ser in the C-terminal end of LRR2 of the hLHR or rLHR, respectively) that is important for hLH binding affinity. Surprisingly, however, this residue does not affect hCG or for bLH binding affinity. In fact, the results obtained with bLH and hCG show that several of the divergent residues in the -sheets of LRR1-9 affect bLH binding affinity but none of them affect hCG binding affinity. Importantly, our results also emphasize the involvement of residues outside of the -sheets of the LRRs of the LHR in ligand binding affinity. This finding has to be considered in future models of the interaction of LH/CG with the LHR.