The molecular basis of ligand specificity of the elephant progestin receptor

The progestin receptor (PR) of the African elephant is characterized by a high binding affinity to 5α-dihydroprogesterone (DHP) similar to the affinity of progesterone (P4). This unique binding specificity is of great physiological importance as DHP is the key hormone in the maintenance of pregnancy in elephants whereas P4 is not involved. In this study we investigated the molecular basis of the distinct binding specificities of elephant and human PR. Differences in the amino acid sequence of the ligand binding domains (LBD) were detected by sequence comparison and consecutively introduced into the cloned human and elephant PR LBD. Binding affinities were evaluated in a competitive binding assay using 1nM [³H]-P4 and recombinant expressed receptor proteins.

We identified a single glycine – alanine polymorphism responsible for the altered ligand binding specificity of the elephant progestin receptor. Substitution of alanine for glycine722 in the human PR (hPR) more than doubled the affinity for DHP (IC₅₀ hPR 24.8nM, hPR_A722 9.35nM). Likewise the converse substitution of glycine for alanine722 in the elephant PR (elePR) decreased the DHP affinity to half its extent (IC₅₀ elePR 4.43nM, elePR_G722 8.52nM). Furthermore a second amino acid substitution might be involved in overall change of progestin binding affinity leading to progestin hypersensitivity in the elephant PR.

Our study implies that a single polymorphism accounts for the distinct binding specificities of the elephant and human PR. Future studies include molecular modelling of the elephant PR LBD for further characterization of the conformational background affecting DHP binding.