Structural analysis of the evolution of steroid specificity in the mineralocorticoid and glucocorticoid receptors

Abstract

Background: The glucocorticoid receptor (GR) and mineralocorticoid receptor (MR) evolved from a common ancestor. Still not completely understood is how specificity for glucocorticoids (e.g. cortisol) and mineralocorticoids (e.g. aldosterone) evolved in these receptors.

Results: Our analysis of several vertebrate GRs and MRs in the context of 3D structures of human GR and MR indicates that with the exception of skate GR, a cartilaginous fish, there is a deletion in all GRs, at the position corresponding to Ser-949 in human MR. This deletion occurs in a loop before helix 12, which contains the activation function 2 (AF2) domain, which binds coactivator proteins and influences transcriptional activity of steroids. Unexpectedly, we find that His-950 in human MR, which is conserved in the MR in chimpanzee, orangutan and macaque, is glutamine in all teleost and land vertebrate MRs, including New World monkeys and prosimians.

Conclusion: Evolution of differences in the responses of the GR and MR to corticosteroids involved deletion in the GR of a residue corresponding to Ser-949 in human MR. A mutation corresponding to His-950 in human MR may have been important in physiological changes associated with emergence of Old World monkeys from prosimians.

Figure 1

Alignment of the steroid-binding domain of MR, GR and CR. Clustal X was used to align the MR, GR and CR sequences. AF2 site is underlined.Pro-637 and Gln-642 in human GR are important in recognition of cortisol [11,15]. Pro-637 and Gln-642 in human GR are conserved in all land vertebrate and teleost GRs. Ser-853 and Leu-848 in human MR are conserved in all land vertebrate MRs, lamprey, hagfish and the proposed ancestral CR and also skate GR.

Figure 2

Alignment of Ser-949 and His-950 in human MR with corresponding residues in other MRs, GRs and human AR and PR. Ser-949 in human MR is conserved in other MRs. The with the exception of skate GR, all other GRs contain a "signature" deletion of one amino acid, corresponding to Ser-949 in human MR. His-950 in human MR is conserved in human MR and other Old World primates. A glutamine is found in other MRs in land vertebrates and teleosts at this site. A corresponding lysine and glutamic acid and lysine are found in skate MR and hagfish CR, respectively. A corresponding glutamine is found in lamprey PR and CR.

Figure 3

Comparison of the 3D structure of the region containing Ser-949 and His-950 in human MR with other steroid receptors. Ser-949 in human MR and corresponding serine residues in human PR and AR are close to each other and have the Oγ side chain directed away from the receptor core. Human GR lacks this loop. The side chain of Lys-743 in the GR is displaced from His-950 in the MR. In the AF2 binding-domain, Oε2 on Glu-755 in the GR is displaced 3.4 A from Oε2 on Glu-962 in the MR.

Figure 4

Interaction of Ser-949 and His-950 with other residues in human MR. The Ser-949 backbone oxygen and nitrogen form hydrogen bonds with backbone oxygen and nitrogen on nearby residues. Ser-949 Cβ has a van der Waals interaction with Phe-946, Val-954 and Phe-956. The side chain on His-950 is pointed away from the MR core and could interact with proteins that bind to the MR.

Figure 5

Interaction of Asp-742 with other residues on the GR. The Asp-742 backbone oxygen and nitrogen form hydrogen bonds with backbone oxygen and nitrogen on nearby residues. Asp-742 also forms a hydrogen bond with Oγ1 on Thr-744. The side chain on Lys-743 is pointed away from the GR core and could interact with proteins that bind to the GR.

Figure 6

Pathways for synthesis of adrenal steroids. Aldosterone is at the end of the pathway, in contrast to progesterone and deoxycorticorticosterone, which are near the beginning.