Nding specificity of the elephant PR, we aligned the amino acid sequences of human (hPR) and elephant (elePR) LBDs to find amino acid exchanges that potentially influence structure and ligand specificity of PR towards favored binding of DHP (Figure 2A). We identified 6 amino acid exchanges, none of which are involved in direct binding of the ligand according to the crystal structure of the PR-progesterone complex [16]. To examine, whether these amino acid changes are unique for the elephant PR and therefore might relate to favored binding of DHP, we aligned the elephant PR LBD with the corresponding sequences of pig, cow, dog, rabbit, rat and mouse (not shown); all mammalian species known to support pregnancy by the exclusive use of progesterone. Interestingly, the T839N exchange was present in all other species in the alignment as well, making it a human-specific exchange, while the other five substitutions appeared to be unique for the elephant PR. To investigate the role of the five unique amino acid changes on binding affinity of progesterone and DHP, we set up an in vitro assay with bacterially expressed hPR LBD, in which the amino acid exchanges were consecutively introduced by site-directed mutagenesis. Stepwise introduction of M692V, V698M, S796P and S902C did not significantly change the relative binding affinity (RBA) of DHP compared to progesterone, indicating a lack of contribution to receptor specificity (Figure 2B). Strikingly, the introduction of the remaining G722A substitution in the four-foldPartial Sequencing of PR LBD from Different MammalsExon sequences comprising the PR LBD were amplified by PCR using degenerate primer pairs deduced from sequences of related species and sequenced. Exon-intron JW 74 web boundaries were amplified and sequenced following the Site Finding PCR protocol of Tan et 18325633 al. [18]. The protocol was modified by adding a 1:Elephant Progestin ReceptorElephant Progestin ReceptorFigure 2. The G722A exchange alters receptor specificity of the PR. (A) The sequence of human PR LBD was aligned with the corresponding translated genomic DNA sequence of the African elephant (Loxodonta africana). Amino acids making van der Waals contacts with bound ligands are indicated in bold type, amino acids making hydrogen bonds to bound ligands are bold and italicized according to Williams et al. [16]. Secondarystructural elements of the PR LBD are indicated above the sequences. a-helices are pale blue, b-sheets and turns dark blue. Shaded residues indicate elephant specific amino acid exchanges. Dots resemble identical amino acids. (B) Elephant specific amino acid substitutions (+), were consecutively introduced into recombinant human PR LBD and relative binding affinity (RBA) of DHP compared to progesterone measured by competitive binding assays. (C) Competitive 1527786 binding assays for progesterone and DHP with recombinant human (hPR) and elephant (elePR) PR LBDs. 1 nM [3H]progesterone was displaced by increasing amounts of progesterone (P4) and DHP. (D) G722A and S796P exchanges were introduced into hPR, while A722G was introduced into elePR. IC50 values were measured as in (C). Data are presented as average IC50 values+SEM of at least three independent experiments. doi:10.1371/journal.pone.BI-78D3 biological activity 0050350.gmutated receptor increased the RBA of DHP 2-fold suggesting a key role in the change of elephant PR specificity (Figure 2B). To verify whether the effect was solely due to the G722A exchange or a combination of several mutations, we int.Nding specificity of the elephant PR, we aligned the amino acid sequences of human (hPR) and elephant (elePR) LBDs to find amino acid exchanges that potentially influence structure and ligand specificity of PR towards favored binding of DHP (Figure 2A). We identified 6 amino acid exchanges, none of which are involved in direct binding of the ligand according to the crystal structure of the PR-progesterone complex [16]. To examine, whether these amino acid changes are unique for the elephant PR and therefore might relate to favored binding of DHP, we aligned the elephant PR LBD with the corresponding sequences of pig, cow, dog, rabbit, rat and mouse (not shown); all mammalian species known to support pregnancy by the exclusive use of progesterone. Interestingly, the T839N exchange was present in all other species in the alignment as well, making it a human-specific exchange, while the other five substitutions appeared to be unique for the elephant PR. To investigate the role of the five unique amino acid changes on binding affinity of progesterone and DHP, we set up an in vitro assay with bacterially expressed hPR LBD, in which the amino acid exchanges were consecutively introduced by site-directed mutagenesis. Stepwise introduction of M692V, V698M, S796P and S902C did not significantly change the relative binding affinity (RBA) of DHP compared to progesterone, indicating a lack of contribution to receptor specificity (Figure 2B). Strikingly, the introduction of the remaining G722A substitution in the four-foldPartial Sequencing of PR LBD from Different MammalsExon sequences comprising the PR LBD were amplified by PCR using degenerate primer pairs deduced from sequences of related species and sequenced. Exon-intron boundaries were amplified and sequenced following the Site Finding PCR protocol of Tan et 18325633 al. [18]. The protocol was modified by adding a 1:Elephant Progestin ReceptorElephant Progestin ReceptorFigure 2. The G722A exchange alters receptor specificity of the PR. (A) The sequence of human PR LBD was aligned with the corresponding translated genomic DNA sequence of the African elephant (Loxodonta africana). Amino acids making van der Waals contacts with bound ligands are indicated in bold type, amino acids making hydrogen bonds to bound ligands are bold and italicized according to Williams et al. [16]. Secondarystructural elements of the PR LBD are indicated above the sequences. a-helices are pale blue, b-sheets and turns dark blue. Shaded residues indicate elephant specific amino acid exchanges. Dots resemble identical amino acids. (B) Elephant specific amino acid substitutions (+), were consecutively introduced into recombinant human PR LBD and relative binding affinity (RBA) of DHP compared to progesterone measured by competitive binding assays. (C) Competitive 1527786 binding assays for progesterone and DHP with recombinant human (hPR) and elephant (elePR) PR LBDs. 1 nM [3H]progesterone was displaced by increasing amounts of progesterone (P4) and DHP. (D) G722A and S796P exchanges were introduced into hPR, while A722G was introduced into elePR. IC50 values were measured as in (C). Data are presented as average IC50 values+SEM of at least three independent experiments. doi:10.1371/journal.pone.0050350.gmutated receptor increased the RBA of DHP 2-fold suggesting a key role in the change of elephant PR specificity (Figure 2B). To verify whether the effect was solely due to the G722A exchange or a combination of several mutations, we int.
