Asymmetry in the PPARγ/RXRα Crystal Structure Reveals the Molecular Basis of Heterodimerization among Nuclear Receptors

Abstract

The nuclear receptor PPARgamma/RXRalpha heterodimer regulates glucose and lipid homeostasis and is the target for the antidiabetic drugs GI262570 and the thiazolidinediones (TZDs). We report the crystal structures of the PPARgamma and RXRalpha LBDs complexed to the RXR ligand 9-cis-retinoic acid (9cRA), the PPARgamma agonist rosiglitazone or GI262570, and coactivator peptides. The PPARgamma/RXRalpha heterodimer is asymmetric, with each LBD deviated approximately 10 degrees from the C2 symmetry, allowing the PPARgamma AF-2 helix to interact with helices 7 and 10 of RXRalpha. The heterodimer interface is composed of conserved motifs in PPARgamma and RXRalpha that form a coiled coil along helix 10 with additional charge interactions from helices 7 and 9. The structures provide a molecular understanding of the ability of RXR to heterodimerize with many nuclear receptors and of the permissive activation of the PPARgamma/RXRbeta heterodimer by 9cRA.

Effect of heterodimer partner RXRα on PPARγ activation function-2 helix in solution

“…2. The CSI consensus reveals that the PPARγ LBD has 12 α-helices, which match those in the crystal structure of the PPARγ LBD bound to GI262570 (farglitazar) and a coactivator peptide [10] and are usually shortened by up to one turn on either end of the helix except for helices H2′, H3 and H8. The analysis of helices H3 and H8 may be affected by the missing resonance assignments and helix H3′, which is not part of the canonical NR LBD structure, is not detected here either.…”

Structural basis for telmisartan-mediated partial activation of PPAR gamma

“…The overall fold was almost identical to previously determined apo and complex structures, [28][29][30] and comprised of 12 helices (H1-H12) and a small b-sheet region of four strands (Figure 1a). We superposed Ca atoms from the structure of PPARg with telmisartan onto those from the structure with rosiglitazone (PDB ID: 2PRG), 29 2-BABA (PDB ID: 1WM0) 30 and the apo structure (PDB ID: 1PRG), 29 and calculated root mean square deviations.…”

“…Although we obtained plate-like crystals that belonged to space group C2 from the solution containing PPARg and telmisartan and found lattices similar to those found when PPARg LBD formed complexes with other agonists, [28][29][30] we did not observe telmisartan in the determined structure.…”

“…Some compounds such as farglitazar induce the slide of Phe363 side chain, resulting in the additional region (branch I'), perpendicular to branch I between H3 and H7. 28 To our knowledge, three binding modes of telmisartan with PPARg have been proposed by docking simulations. In the first mode, two …”

“…Telmisartan induces only one intermolecular hydrogen bond (Tyr473), whereas full agonists rosiglitazone and farglitazar form three (Ser289, His323 and Tyr473) 29 and four (Ser289, His323, His449 and Tyr473) bonds, respectively. 28 Further, the propyl group pushes His323 out from the position it occupies in full-agonist complexes. These factors give rise to the non-canonical hydrogen-bonding network around H12, which hinders recruitment of coactivator peptides.…”