Ligand Binding Mechanism in Steroid Receptors: From Conserved Plasticity to Differential Evolutionary Constraints

Abstract

Steroid receptor drugs have been available for more than half a century, but details of the ligand binding mechanism have remained elusive. We solved X-ray structures of the glucocorticoid and mineralocorticoid receptors to identify a conserved plasticity at the helix 6-7 region that extends the ligand binding pocket toward the receptor surface. Since none of the endogenous ligands exploit this region, we hypothesized that it constitutes an integral part of the binding event. Extensive all-atom unbiased ligand exit and entrance simulations corroborate a ligand binding pathway that gives the observed structural plasticity a key functional role. Kinetic measurements reveal that the receptor residence time correlates with structural rearrangements observed in both structures and simulations. Ultimately, our findings reveal why nature has conserved the capacity to open up this region, and highlight how differences in the details of the ligand entry process result in differential evolutionary constraints across the steroid receptors.

30 YEARS OF THE MINERALOCORTICOID RECEPTOR: Evolution of the mineralocorticoid receptor: sequence, structure and function

“…In contrast, B, Aldo and DOC, which lack a 17α-hydroxyl, would not clash with Leu-848, explaining the stronger response of the MR to these steroids. However, a crystal structure of the MR with DEX (Edman et al 2015) did not find a steric clash between Leu-848 on the MR and the 17α-hydroxyl on DEX due to plasticity in helix6-helix7 in the MR, which allows for an open conformation that can accommodate DEX. A molecular mechanics (Monte Carlo simulations) and crystallographic analysis of the MR, GR, PR, ER and AR revealed that such plasticity is present in MR, GR, PR and ER, but not in the AR.…”

“…Figure 5 also shows the α-helices on the MR as determined by X-ray crystallography (Bledsoe et al 2005, Fagart et al 2005, Edman et al 2015 and amino acids that have been found to be important in either steroid binding or transcriptional activation of the MR or CR (Fagart et al 1998, Geller et al 2000, Bledsoe et al 2005, Hultman et al 2005, Bridgham et al 2006, Shibata et al 2013, Edman et al 2015, Jimenez-Canino et al 2016, Mani et al 2016 or the GR (Bledsoe et al 2002, He et al 2014, Edman et al 2015, Mani et al 2016. A striking feature, discussed in more detail below, is the strong conservation of amino acids among the CR, MR, GR, PR and AR, including lamprey PR and CR.…”

“…A molecular mechanics (Monte Carlo simulations) and crystallographic analysis of the MR, GR, PR, ER and AR revealed that such plasticity is present in MR, GR, PR and ER, but not in the AR. Interestingly, the MR undergoes a larger rearrangement of this region than the GR, which is in a more open configuration (Edman et al 2015).…”

“…However, studies with human MR , Mani et al 2016 find that Leu843Gln human MR mutant has a favorable response to F, unlike that of the AncCR, leaving unresolved Comparison of Ser-843/Leu-848 on human MR with Pro-637/Gln-642 on human GR. Human GR complexed with dexamethasone (PDB: 1M2Z) (Bledsoe et al 2002) and human MR complexed with dexamethasone (PDB: 4UDA) (Edman et al 2015) were superimposed. In human MR, Cδ1 on Leu-848 is 6.2 Ǻ and 3.8 Ǻ, respectively, from 17α-OH and 16α-CH3 on DEX.…”

“…In a second pathway, progesterone is hydroxylated at C17 and then hydroxylated at C21 to form 11-deoxycortisol, which is hydroxylated at C11 to form cortisol (Baker 2011, Rossier et al 2015. Fagart et al 2005, Edman et al 2015 and GR (Bledsoe et al 2002, He et al 2014 and functional studies of human MR mutants (Fagart et al 1998, Geller et al 2000, Bledsoe et al 2005, Shibata et al 2013, Jimenez-Canino et al 2016, Mani 2016 to investigate the evolution of selectivity for Aldo and other corticosteroids by the MR in terrestrial vertebrates (Baker et al 2013) and ray-finned fish (Prunet et al 2006, Bury & Sturm 2007, Arterbery et al 2011, Sugimoto et al 2016, as well as the basis for binding of some glucocorticoids by human MR and other vertebrate MRs , Mani et al 2016. Together, these data provide molecular and structural fingerprints for investigating the evolution of selectivity for 3-ketosteroids by the MR in vertebrates.…”