The charge densities rho(r) of the six amino acids L-Asn.H(2)O, DL-Glu.H(2)O, DL-Lys.HCl, DL-Pro.H(2)O, DL-Ser, and DL-Val were determined from high-resolution X-ray diffraction experiments at 100 K using synchrotron radiation and area detection (CCD) techniques. Bond topological parameters derived from these densities and from those of six additional amino acids published earlier are compared to each other and to the results of ab initio calculations. Experimental and theoretical properties for each chemically equivalent bond are in a fair agreement, and their variances are of similar magnitude. A noticeable outlier is the positive curvature of the density at the bond critical point, for which no correlation between the experimental and theoretical values can be established. The location of nonbonded valence shell charge concentrations derived from the crystalline densities scatter in a wider range than those obtained for the isolated molecules.

The estrogen-related receptor (ERR) ␥ behaves as a constitutive activator of transcription. Although no natural ligand is known, ERR␥ is deactivated by the estrogen receptor (ER) agonist diethylstilbestrol and the selective ER modulator 4-hydroxytamoxifen but does not significantly respond to estradiol or raloxifene. Here we report the crystal structures of the ERR␥ ligand binding domain (LBD) complexed with diethylstilbestrol or 4-hydroxytamoxifen. Antagonist binding to ERR␥ results in a rotation of the side chain of Phe-435 that partially fills the cavity of the apoLBD. The new rotamer of Phe-435 displaces the "activation helix" (helix 12) from the agonist position observed in the absence of ligand. In contrast to the complexes of the ER␣ LBD with 4-hydroxytamoxifen or raloxifene, helix 12 of antagonist-bound ERR␥ does not occupy the coactivator groove but appears to be completely dissociated from the LBD body. Comparison of the ligand-bound LBDs of ERR␥ and ER␣ reveals small but significant differences in the architecture of the ligand binding pockets that result in a slightly shifted binding position of diethylstilbestrol and a small rotation of 4-hydroxytamoxifen in the cavity of ERR␥ relative to ER␣. Our results provide detailed molecular insight into the conformational changes occurring upon binding of synthetic antagonists to the constitutive orphan receptor ERR␥ and reveal structural differences with ERs that explain why ERR␥ does not bind estradiol or raloxifene and will help to design new selective antagonists.The estrogen-related receptors ERR␣, 1 ERR␤, and ERR␥ (NR3B1, -2, and -3) (1) form a subfamily of orphan nuclear receptors that share significant amino acid homology with the estrogen receptors ER␣ and ER␤ (NR3A1 and -2) (2, 3). Because of the high conservation in the DNA binding domain, ERRs and ERs have overlapping DNA binding selectivity (4 -6) and, accordingly, may co-regulate target genes in tissues in which they are co-expressed. ERR subfamily members have for example been shown to modulate the expression of ER target genes in bone (7,8) or breast tissue (9, 10). Importantly, overexpression of ERR␣ and ERR␥ in samples from breast cancer patients correlates with unfavorable and favorable biomarkers, respectively (11). Therefore, these receptors might serve as prognostic markers themselves or even be targets for endocrine therapy in human breast cancer.Despite their significant homology with ERs in the ligand binding domain (LBD), ERRs do not (or only very weakly) respond to estradiol (E2) (2, 12). Furthermore, whereas ERs are ligand-activated receptors, ERRs are constitutively active (13-16), and a structural study confirmed that the ERR␥ LBD can adopt a transcriptionally active conformation and interact with the steroid receptor coactivator 1 (SRC-1) in the absence of any ligand (12). Together, these observations suggest that ERRs are ligand-independent activators of transcription whose activation potential may rather be determined by the presence of transcriptional coactivators (17)(18)(1...

High-resolution X-ray diffraction data collected at 20 K are interpreted in terms of the rigid-pseudoatom formalism to derive the electron density and related properties, such as the electrostatic potential and electric moments, of the crystalline d,l-aspartic acid. The refinement models applied are restricted via rigid-bond type constraints to reduce possible bias in the mean-square displacement amplitudes due to inadequacies in the thermal deconvolution. The density and its Laplacian extracted from the data is analyzed in terms of the topological properties of covalent bonds and nonbonded interactions. The results are compared to those calculated at the Hartree−Fock level of theory and to those obtained experimentally for analogous molecules. The comparison must consider the differences in the locations of the bond critical points of the densities in question, that is, how the bond polarity manifests itself in the distribution of charge obtained by different methods. One of the key questions to the reliability of experimental pseudoatomic densities seems to be whether the treatment of the X-ray data can be standardized so as to reduce model inadequacies, especially those related to the derivation of monopole populations.

NGFI-B is a ligand-independent orphan nuclear receptor of the NR4A subfamily that displays important functional differences with its homolog Nurr1. In particular, the NGFI-B ligand-binding domain (LBD) exhibits only modest activity in cell lines in which the Nurr1 LBD strongly activates transcription. To gain insight into the structural basis for the distinct activation potentials, we determined the crystal structure of the NGFI-B LBD at 2.4-Å resolution. Superimposition with the Nurr1 LBD revealed a significant shift of the position of helix 12, potentially caused by conservative amino acids exchanges in helix 3 or helix 12. Replacement of the helix 11-12 region of Nurr1 with that of NGFI-B dramatically reduces the transcriptional activity of the Nurr1 LBD. Similarly, mutation of Met 414 in helix 3 to leucine or of Leu 591 in helix 12 to isoleucine (the corresponding residues found in NGFI-B) significantly affects Nurr1 transactivation. In comparison, swapping the helix 11-12 region of Nurr1 into NGFI-B results in a modest increase of activity. These observations reveal a high sensitivity of LBD activity to changes that influence helix 12 positioning. Furthermore, mutation of hydrophobic surface residues in the helix 11-12 region (outside the canonical co-activator surface constituted by helices 3, 4, and 12) severely affects Nurr1 transactivation. Together, our data suggest that a novel co-regulator surface that includes helix 11 and a specifically positioned helix 12 determine the cell type-dependent activities of the NGFI-B and the Nurr1 LBD.