Protein Name

Ancestral corticoid receptor / aldosterone complex



Biological Context

An ancestral corticoid receptor (AncCR) has been resurrected. Its maximum likelihood sequence was phylogenetically determined from a number of offspring sequences of glucocorticoid receptors (GR) and mineralocorticoid receptors (MR) in vertebrates. After that, it was biochemically synthesized, expressed in cultured cells and its structure determined by x-ray crystallography. The structure of PDB:2Q1H is a complex of the AncCR and aldosterone (Fig. 1). GR are activated by cortisol and regulate stress response, glucose homeostasis, and other functions. MR are activated by aldosterone in tetrapods and by deoxycorticosterone (DOC) in teleosts to control electrolyte homeostasis, kidney and colon function, and other processes. MR are also sensitive to cortisol, though considerably less so than to aldosterone and DOC. The resurrected AncCR has an intermediate property among GRs and MRs, can bind with aldosterone, DOC and cortisol. Thus AncCR structures in complex with aldosterone (xPSSS:2Q1H), DOC (xPSSS:2Q3Y), and cortisol (xPSSS:2Q1V) have been solved at 1.9, 2.0 and 2.4Å resolution, respectively. These three substrates are adrenal steroid hormones.

Structure Description

The AncCR resembles both offspring, human GR and human MR, as root mean square deviation (RMSD) for all backbone atoms = 0.9 and 1.2Å, respectively. Thus, from the AncCR which has sensitivity for aldosterone, DOC and cortisol, how is GR's cortisol specificity evolutionarily derived? The main reason is “conformational epistasis”, which is a coordinated effect of multiple substitutions. Structural comparison of an ancestral protein (AncGR1) which has no cortisol specificity and another (AncGR2) which gained the specificity revealed the conformational epistasis in this case (Fig. 2). The S106P substitution introduces a proline kink, which pulls helix 7 down. Subsequently, reposition of site 111 in helix 7 and the substitution L111Q generate a hydrogen bond with cortisol unique C17-hydroxyl. Thus, the stabilized receptor-hormone complex gained cortisol specificity. Neither S106P nor L111Q can generate this specificity alone. The nonadditive effect of conformational epistasis allows a few substitutions to change a protein function.

It is difficult to estimate a functional change which comes from local structural changes based on the sequence similarity. Thus, estimation of evolutionary crucial substitutions and mapping them to protein structures would be fundamental to understand structure-function relationships.

structure_2Q1H structural_change_2Q1H
(Fig. 1) The structure of ancestral corticoid receptor

The resurrected ancestral corticoid receptor (green) in complex with aldosterone (blue). Note that orange helices are helix 6 (left) and 7 (right).
(Fig. 2) structural change for a ligand specificity

Estimated structural change between AncGR1 and AncGR2. Both of them are ancestor proteins between AncCR and GR, MR.

At first, the S106P-driven proline kink pulls helix 7 down. Under this influence, site 111 is repositioned and the L111Q substitution allows the formation of a hydrogen bond with cortisol (dotted red line). This coordinated effect of multiple substitutions is called "conformational epistasis".

Protein Data Bank (PDB)



Ortlund, E.A. Bridgham, J.T. Redinbo, M.R. Thornton, J.W.; "Crystal structure of an ancient protein: evolution by conformational epistasis."; Science; (2007) 317:1544-1548 PubMed:17702911.


  • xPSSS:2Q1V ancestral corticoid receptor / cortisol complex
  • xPSSS:2Q3Y ancestral corticoid receptor / DOC complex

author: Naoya Fujita

Japanese version:PDB:2Q1H