Mammalian RXR - protein interaction and binding of RXR-RAR heterodimers to DNA

Retinoic acid, a pleiotropic regulator of development and homeostasis, controls the expression of specific gene networks via direct interactions with nuclear receptors. The retinoic acid receptor (RAR), as a heterodimer with the retinoid-x receptor (RXR), binds to DNA recognition sites, referred to as retinoic acid response elements (RAREs), that are generally composed of a direct repeat of the half-site core motif PuGGTCA spaced by 2 (DR-2) or 5 (DR-5) basepairs. The asymmetric nature of direct repeat RAREs suggests that RAR and RXR bind preferentially to one of the two half-site core motifs. RXR occupies the 5'-up-stream half-site, and RAR the 3'-down-stream half-site of the direct repeat in both DR-2 and DR-5 RAREs. A region adjacent to the zinc finger region of RAR and RXR is essential for specific and cooperative binding of DNA-binding domain peptides to RAREs. However, differential utilization of these determinants mediates RAR-RXR heterodimer binding to DR-2 and DR-5 RAREs. The demonstration of ordered but nonequivalent binding of RAR-RXR complexes to DR-2 and DR-5 RAREs sets a precedent for the generation of sequence specificities in heterodimeric DNA-binding proteins (Predki, 1994).

The biological activities of the retinoids are mediated by two nuclear hormone receptors: the retinoic acid receptor (RAR) and the retinoid-X receptor (RXR). RXR (and its insect homolog Ultraspiracle) is a common heterodimeric partner for many other nuclear receptors, including the insect ecdysone receptor. Whereas RXR can be readily expressed in Escherichia coli to produce soluble protein, this is not the case for many of its heterodimeric partners. For example, overexpression of RAR results mostly in inclusion bodies with the residual soluble component unable to interact with RXR or ligand efficiently. Similar results are seen with other RXR/ultraspiracle partners. To overcome these problems, a novel double cistronic vector was designed to coexpress RXR and its partner ligand-binding domains in the same bacterial cell. This results in a dramatic increase in production of soluble and apparently stable heterodimer. Hormone-binding studies using the purified RXR-RAR heterodimer reveal increased ligand-binding capacity of both components of 5- to 10-fold, resulting in virtually complete functionality. Bacterially expressed receptors can exist in one of three distinct states: insoluble, soluble but unable to bind ligand, or soluble with full ligand-binding capacity. These results suggest that coexpression may represent a general strategy for biophysical and structural analysis of receptor complexes (Li, 1997).

Several nuclear receptors including the all-trans retinoic acid receptor RAR, form heterodimers with the 9-cis retinoic acid receptor, RXR. RXR-RAR heterodimers show an impressive flexibility in DNA binding and can recognize palindromic, inverted palindromes and direct repeats of the core half-site sequence AGGTCA. Dimerization interfaces in the DNA-binding domains of RXR, RAR, and thyroid hormone receptor (TR) that promote selective binding to strictly spaced direct repeats have previously been identified. However, an additional dimerization domain is present within the ligand-binding domains (LBDs) of these receptors. A transferable 40-amino acid region is located within the LBDs of RXR, RAR, TR, and chicken ovalbumin upstream promoter transcription factor that is critical for determining identity in the heterodimeric interaction and for high-affinity DNA binding. This region overlaps almost perfectly with a helical segment in the RXR LBD crystal structure that is part of the dimer interface. These data suggest a sequential pathway for nuclear receptor dimerization whereby the LBD dimerization interface initiates the formation of solution heterodimers that, in turn, acquire the capacity to bind to a number of differently organized repeats. Formation of a second dimer interface within the DNA-binding domain (DBD) restricts receptors to direct repeat targets. Accordingly, the combination of an obligatory (LBD) and an optional (DBD) dimerization domain imparts a dynamic DNA-binding potential to the heterodimerizing receptors that both increases the diversity of the hormonal response as well as providing a restricted set of target sequences in direct repeat elements that ensures physiological specificity (Perlmann, 1996).

The 9-cis retinoic acid receptor (retinoid X receptor, RXR) forms heterodimers with the all-trans retinoic acid receptor (RAR) and other nuclear receptors on DNA regulatory sites composed of tandem binding elements. The 1.70Å resolution structure of the ternary complex of RXR and RAR DNA-binding regions in complex with the retinoic acid response element DR1 is described. The receptors recognize identical half-sites through extensive base-specific contacts; however, RXR binds exclusively to the 3' site to form an asymmetric complex with the reverse polarity of other RXR heterodimers. The subunits associate in a strictly DNA-dependent manner using the T-box of RXR and the Zn-II region of RAR, both of which are reshaped in forming the complex. The protein-DNA contacts as well as the dimerization interface and the DNA curvature in the RXR-RAR complex are all distinct from those of the RXR homodimer, which also binds DR1. Together, these structures illustrate how the nuclear receptor superfamily exploits conformational flexibility and locally induced structures to generate combinatorial transcription factors (Rastinejad, 2000).

The malic enzyme (ME) gene is a target for both thyroid hormone receptors and peroxisome proliferator-activated receptors (PPAR). Within the ME promoter, two direct repeat (DR)-1-like elements, MEp and MEd, have been identified as putative PPAR response elements (PPRE). Only MEp and not MEd is able to bind PPAR/retinoid X receptor (RXR) heterodimers and mediate peroxisome proliferator signaling. Taking advantage of the close sequence resemblance of MEp and MEd, crucial determinants of a PPRE have been identified. Using reciprocal mutation analyses of these two elements, the preference for adenine as the spacing nucleotide between the two half-sites of the PPRE is shown and the importance of the two first bases flanking the core DR1 in 5' has been demonstrated. This latter feature of the PPRE led to a consideration of the polarity of the PPAR/RXR heterodimer bound to its cognate element. In contrast to the polarity of RXR/TR and RXR/RAR bound to DR4 and DR5 elements respectively, PPAR binds to the 5' extended half-site of the response element, while RXR occupies the 3' half-site. Consistent with this polarity is the finding that formation and binding of the PPAR/RXR heterodimer requires an intact hinge T region in RXR while its integrity is not required for binding of the RXR/TR heterodimer to a DR4 (Ijpenberg, 1997).

The crystal structure of a heterodimer between the ligand-binding domains (LBDs) of the human RARalpha bound to a selective antagonist and the constitutively active mouse RXRalphaF318A mutant shows that, pushed by a bulky extension of the ligand, RAR alpha helix H12 adopts an antagonist position. The unexpected presence of a fatty acid in the ligand-binding pocket of RXRalphaF318A is likely to account for its apparent 'constitutivity'. Specific conformational changes suggest the structural basis of pure and partial antagonism. The RAR-RXR heterodimer interface is similar to that observed in most nuclear receptor (NR) homodimers. A correlative analysis of 3D structures and sequences provides a novel view on dimerization among members of the nuclear receptor superfamily (Bourguet, 2000).

The nuclear receptor PPARgamma/RXRalpha heterodimer regulates glucose and lipid homeostasis and is the target for the antidiabetic drugs GI262570 and the thiazolidinediones (TZDs). Peroxisome proliferator-activated receptor gamma (PPARgamma) is a member of the nuclear receptor superfamily of ligand-activated transcription factors that serves as a key regulator of adipocyte differentiation and glucose homeostasis. In addition to its role in hormone binding, the ligand binding domain (LBD) also contains dimerization and transactivation functions, including the transcriptional activation function 2 (AF-2). Upon hormone binding, the LBD undergoes a conformational change, most notably in the AF-2 domain. These conformational changes result in the displacement of corepressor proteins, such as NCoR and SMRT, that inhibit transcription and the recruitment of coactivator proteins, such as p160 and DRIP/TRAP, family members that are involved in transcriptional activation. Reported here are the crystal structures of the PPARgamma and RXRalpha LBDs complexed to the RXR ligand 9-cis-retinoic acid (9cRA), the PPARgamma agonist and coactivator peptides. The PPARgamma/RXRalpha heterodimer is asymmetric, with each LBD deviated ~10° 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/RXRalpha heterodimer by 9cRA (Gampe, 2000).

Circadian clock genes are expressed in the suprachiasmatic nucleus and in peripheral tissues to regulate cyclically physiological processes. Synchronization of peripheral oscillators is thought to involve humoral signals, but the mechanisms by which these are mediated and integrated are poorly understood. A hormone-dependent physical interaction of the nuclear receptors, RARalpha and RXRalpha, with CLOCK and the Cycle homolog MOP4 is reported. These interactions negatively regulate CLOCK/MOP4:BMAL1-mediated transcriptional activation of clock gene expression in vascular cells. MOP4 exhibits a robust rhythm in the vasculature, and retinoic acid can phase shift Per2 mRNA rhythmicity in vivo and in serum-induced smooth muscle cells in vitro, providing a molecular mechanism for hormonal control of clock gene expression. It is proposed that circadian or periodic availability of nuclear hormones may play a critical role in resetting a peripheral vascular clock (McNamara, 2001).

Genome-wide profiling of PPARgamma:RXR and RNA polymerase II occupancy reveals temporal activation of distinct metabolic pathways and changes in RXR dimer composition during adipogenesis

The nuclear receptor peroxisome proliferator-activated receptor gamma (PPARgamma) is a key regulator of adipocyte differentiation in vivo and ex vivo and has been shown to control the expression of several adipocyte-specific genes. This study used chromatin immunoprecipitation combined with deep sequencing to generate genome-wide maps of PPARgamma and retinoid X receptor (RXR)-binding sites, and RNA polymerase II (RNAPII) occupancy at very high resolution throughout adipocyte differentiation of 3T3-L1 cells. Greater than 5000 high-confidence shared PPARgamma:RXR-binding sites were identified in adipocytes; during early stages of differentiation, many of these are preoccupied by non-PPARgamma RXR-heterodimers. Different temporal and compositional patterns of occupancy are observed. In addition, co-occupancy was detected with members of the C/EBP family. Analysis of RNAPII occupancy uncovers distinct clusters of similarly regulated genes of different biological processes. PPARgamma:RXR binding is associated with the majority of induced genes, and sites are particularly abundant in the vicinity of genes involved in lipid and glucose metabolism. These analyses represent the first genome-wide map of PPARgamma:RXR target sites and changes in RNAPII occupancy throughout adipocyte differentiation and indicate that a hitherto unrecognized high number of adipocyte genes of distinctly regulated pathways are directly activated by PPARgamma:RXR (Nielsen, 2008).

The active enhancer network operated by liganded RXR supports angiogenic activity in macrophages

RXR signaling is predicted to have a major impact in macrophages, but neither the biological consequence nor the genomic basis of its ligand activation is known. Comprehensive genome-wide studies were carried out to map liganded RXR-mediated transcriptional changes, active binding sites, and cistromic interactions in the context of the macrophage genome architecture. The macrophage RXR cistrome has 5200 genomic binding sites, which are not impacted by ligand. Active enhancers are characterized by PU.1 binding, an increase of enhancer RNA, and P300 recruitment. Using these features, 387 liganded RXR-bound enhancers were linked to 226 genes, which predominantly reside in CTCF/cohesin-limited functional domains. These findings were molecularly validated using chromosome conformation capture (3C) and 3C combined with sequencing (3C-seq), and it was also shown that selected long-range enhancers communicate with promoters via stable or RXR-induced loops and that some of the enhancers interact with each other, forming an interchromosomal network. A set of angiogenic genes, including Vegfa, has liganded RXR-controlled enhancers and provides the macrophage with a novel inducible program (Daniel, 2014).

De-novo identification of PPARgamma/RXR binding sites and direct targets during adipogenesis

The pathophysiology of obesity and type 2 diabetes mellitus is associated with abnormalities in endocrine signaling in adipose tissue and one of the key signaling affectors operative in these disorders is the nuclear hormone transcription factor peroxisome proliferator-activated receptor-gamma (PPARgamma). PPARgamma has pleiotropic functions affecting a wide range of fundamental biological processes including the regulation of genes that modulate insulin sensitivity, adipocyte differentiation, inflammation and atherosclerosis. To date, only a limited number of direct targets for PPARgamma have been identified through research using the well established pre-adipogenic cell line, 3T3-L1. In order to obtain a genome-wide view of PPARgamma binding sites, the pair end-tagging technology (ChIP-PET) was employed to map PPARgamma binding sites in 3T3-L1 preadipocyte cells. Coupling gene expression profile analysis with ChIP-PET, in a genome-wide manner, over 7700 DNA binding sites of the transcription factor PPARgamma and its heterodimeric partner RXR were identified during the course of adipocyte differentiation. The validation studies prove that the identified sites are bona fide binding sites for both PPARgamma and RXR and that they are functionally capable of driving PPARgamma specific transcription. The results strongly indicate that PPARgamma is the predominant heterodimerization partner for RXR during late stages of adipocyte differentiation. Additionally, PPARgamma/RXR association is enriched within the proximity of the 5' region of the transcription start site and this association is significantly associated with transcriptional up-regulation of genes involved in fatty acid and lipid metabolism confirming the role of PPARgamma as the master transcriptional regulator of adipogenesis. Evolutionary conservation analysis of these binding sites is greater when adjacent to up-regulated genes than down-regulated genes, suggesting the primordial function of PPARgamma/RXR is in the induction of genes. Functional validations resulted in identifying novel PPARgamma direct targets that have not been previously reported to promote adipogenic differentiation. In conclusion, the binding sites of PPARgamma and RXR during the course of adipogenic differentiation were identifed in 3T3L1 cells, and an important resource is provided for the study of PPARgamma function in the context of adipocyte differentiation (Hamza, 2009).

Mammalian RXR - Domain structure and function

A subset of nuclear receptors, including those for thyroid hormone (TR), retinoic acid, vitamin D3, and eicosanoids, can form heterodimers with the retinoid X receptor (RXR) on DNA regulatory elements in the absence of their cognate ligands. In a mammalian two-hybrid assay, recruitment of a VP16-RXR chimera by a Gal4-TRbeta ligand-binding domain fusion is enhanced up to 50-fold by thyroid hormone (T3). This is also observed with a mutant fusion, Gal4-TR(L454A), lacking ligand-inducible activation function (AF-2) and unable to interact with putative coactivators, suggesting that the AF-2 activity of TR or intermediary cofactors is not involved in this effect. The wild-type and mutant Gal4-TR fusions also exhibit hormone-dependent recruitment of RXR in yeast. Hormone-dependent recruitment of RXR is also evident with another Gal4-TR mutant, AHTm, which does not interact with the nuclear receptor corepressor N-CoR, suggesting that ligand-enhanced dimerization is not a result of T3-induced corepressor release. The interaction between RXR and TR is augmented by T3 in vitro, arguing against altered expression of either partner in vivo mediating this effect. It is proposed that ligand-dependent heterodimerization of TR and RXR in solution may provide a further level of control in nuclear receptor signaling (Collingwood, 1997).

The receptor for 9-cis-retinoic acid, retinoid X receptor (RXR), forms heterodimers with several nuclear receptors, including the receptor for all-trans-retinoic acid, RAR. Previous studies have shown that retinoic acid receptor can be activated in RAR/RXR heterodimers, whereas RXR is believed to be a silent co-factor. Efficient growth arrest and differentiation of the human monocytic cell line U-937 require activation of both RAR and RXR. The allosteric inhibition of RXR is not obligatory; RXR can be activated in the RAR/RXR heterodimer in the presence of RAR ligands. Remarkably, RXR inhibition by RAR can also be relieved by an RAR antagonist. Moreover, the dose response of RXR agonists differs between RXR homodimers and RAR/RXR heterodimers, indicating that these complexes are pharmacologically distinct. The AF2 activation domain of both subunits contributes to activation even if only one of the receptors is associated with ligand. These data emphasize the importance of signaling through both subunits of a heterodimer in the physiological response to retinoids and show that the activity of RXR is dependent on both the identity and the ligand binding state of its partner (Botling, 1997).

Thyroid hormone receptors (TR) function as part of multiprotein complexes that also include retinoid X receptor (RXR) and transcriptional coregulators. Both the Thyroid receptor CoR box and ninth heptad domains are required for interaction with RXR, and in turn, both domains are required for interaction with corepressor proteins N-CoR and SMRT. Remarkably, the recruitment of RXR to the repression-defective CoR box and ninth-heptad mutants via a heterologous dimerization interface restores both corepressor interaction and repression. The addition of thyroid hormone obviates the CoR box requirement for RXR interaction, provided that the AF2 activation helix at the C terminus of TR is intact. These results indicate that RXR differentially recognizes the unliganded and liganded conformations of TR and that these differences appear to play a major role in the recruitment of corepressors to TR-RXR heterodimers (Zhang, 1997).

A mouse mutation has been engineered that specifically deletes the C-terminal 18 amino acid sequence of the RXRalpha protein. This deletion (RXRalphaaf2o) corresponds to the last helical alpha structure (H12) of the ligand-binding domain (LBD), and includes the core of the Activating Domain of the Activation Function 2 (AF-2 AD core), which is thought to be crucial in mediating ligand-dependent transactivation by RXRalpha. The homozygous mutants, which die during the late fetal period or at birth, exhibit a subset of the abnormalities previously observed in RXRalpha-/- mutants, often with incomplete penetrance. In marked contrast, compound mutants bearing mutations in RXRalpha and RXRbeta or RXRalpha, RXRbeta and RXR gamma display a large array of malformations, which nearly recapitulate the full spectrum of the defects that characterize the fetal vitamin A-deficiency (VAD) syndrome and were previously found in RAR single and compound mutants, as well as in RXRalpha/RAR(alpha, beta or gamma) compound mutants. Analysis of RXRalphaaf2o/RAR(alpha, beta or gamma) compound mutants also reveals that they exhibit many of the defects observed in the corresponding RXRalpha/RAR compound mutants. Together, these results demonstrate the importance of the integrity of RXR AF-2 for the developmental functions mediated by RAR/RXR heterodimers, and hence suggest that RXR ligand-dependent transactivation is instrumental in retinoid signaling during development. In the presence of both RXR and RAR ligands, RXRalphaAF2o/RAR heterodimers are less efficient than wild-type heterodimers at providing stable occupancy of RA response elements. Since RXRalphaAF2o/RAR heterodimers bind RARE in vitro as efficiently as WT heterodimers, irrespective of ligand presence, this RXR AF2- and ligand-dependent enhancement of promoter occupancy most probably involves events occurring at the chromatin level (Mascrez, 1998).

The N-terminal A/B domain of RXRs contains an autonomous ligand-independent transcriptional activation function called AF-1, whereas the C-terminal, ligand-binding domain E, contains a ligand-dependent transcriptional activation function, AF-2. For each RXR, at least two isoforms exist, which differ in their N-terminal region. As to RXRalpha, the major isoform RXRalpha1 is widely expressed in embryos and adults, whereas RXRalpha2 and alpha3 are restricted to the adult testis. Furthermore, RXRalpha can be phosphorylated at several serine and threonine residues in its A/B domain. The AF-2 activity crucially depends upon a conserved amphipathic alpha helix (the AF-2 AD core), whose deletion in the mouse has revealed its requirement for a number of RA-dependent developmental events. However, little is known about the mechanisms through which AF-1 activates transcription or about the relevance of the A/B domain in the global activity of the receptor under physiological conditions in vivo. Depending on the promoter context, the AF-1 of a given RXR modulates the AF-2 activity in cultured cells. Thus, the transcriptional activity of a given RXR isoform may ultimately be determined, not only by its AF-2 activity, but also by its isoform-specific A/B domain (Mascrez, 2001 and references therein).

A mouse mutation has been engineered that specifically deletes most of the RXRalpha N-terminal A/B region, which includes the activation function AF-1 and several phosphorylation sites. The homozygous mutants (RXRalphaaf1o), as well as compound mutants that further lack RXRß and RXRgamma, are viable and display a subset of the abnormalities previously described in RXRalpha-null mutants. In contrast, RXRalphaaf1o/RAR-/-(alpha, ß or gamma) compound mutants die in utero and exhibit a large array of malformations that nearly recapitulate the full spectrum of the defects that characterize the fetal vitamin A-deficiency (VAD) syndrome. Altogether, these observations indicate that the RXRalpha AF-1 region A/B is functionally important, although less so than the ligand-dependent activation function AF-2, for efficiently transducing the retinoid signal through RAR/RXRalpha heterodimers during embryonic development. Moreover, it has a unique role in retinoic acid-dependent involution of the interdigital mesenchyme. During early placentogenesis, both the AF-1 and AF-2 activities of RXRalpha, ß and gamma appear to be dispensable, suggesting that RXRs act as silent heterodimeric partners in this process. However, AF-2 of RXRalpha, but not AF-1, is required for differentiation of labyrinthine trophoblast cells, a late step in the formation of the placental barrier (Mascrez, 2001).

Mammalian RXR - allostery

Regulation of gene expression via allosteric control of transcription is one of the fundamental concepts of molecular biology. Studies in prokaryotes have illustrated that binding of small molecules or ligands to sequence-specific transcription factors can produce conformational changes at a distance from the binding site. These ligand-induced changes can dramatically alter the DNA binding and/or trans-activation abilities of the target transcription factors. In this work, analysis of trans-activation by members of the steroid and thyroid hormone receptor superfamily identifies a unique form of allosteric control, the phantom ligand effect. Binding of a novel ligand (LG100754) to one subunit (RXR) of a heterodimeric transcription factor results in a linked conformational change in the second noncovalently bound subunit of the heterodimer (RAR). This conformational change results in both the dissociation of corepressors and association of coactivators in a fashion mediated by the activation function of the non-liganded subunit. Without occupying the RAR hormone binding pocket, binding of LG100754 to RXR mimics exactly the effects observed when hormone is bound to RAR. Thus, LG100754 behaves as a phantom ligand (Schulman, 1997).

Heterodimerization is a common paradigm among eukaryotic transcription factors. The 9-cis retinoic acid receptor (RXR) serves as a common heterodimerization partner for several nuclear receptors, including the thyroid hormone receptor (T3R) and retinoic acid receptor (RAR). This raises the question as to whether these complexes possess dual hormonal responsiveness. A strategy was devised to examine the transcriptional properties of each receptor, either individually or when tethered to a heterodimeric partner. The intrinsic binding properties of RXR are masked in T3R-RXR and RAR-RXR heterodimers. In contrast, RXR is active as a non-DNA-binding cofactor with the NGFI-B/Nurr1 orphan receptors. Heterodimerization of RXR with constitutively active NGFI-B/Nurr1 creates a novel hormone-dependent complex. These findings suggest that allosteric interactions among heterodimers create complexes with unique properties. It is suggested that allostery is a critical feature underlying the generation of diversity in hormone response networks (Forman, 1995).

OR1 is a member of the steroid/thyroid hormone nuclear receptor superfamily that mediates transcriptional responses to retinoids and oxysterols. On a DR4 response element, an OR1 heterodimer with the nuclear receptor retinoid X receptor alpha (RXR alpha) conveys transcriptional activation in both the absence and presence of the RXR ligand 9-cis retinoic acid, the underlying mechanisms remaining unclear. The role of the respective carboxy-terminal activation domains (AF-2s) in the absence and presence of the RXR ligand has also been analyzed. The interaction of the RXR and OR1 ligand-binding domains unleashes a transcription activation potential that is mainly dependent on the AF-2 of OR1, indicating that interaction with RXR activates OR1. This defines dimerization-induced activation as a novel function of heterodimeric interaction and appears to be a mechanism for receptor activation not previously described for nuclear receptors. Activation of OR1 occurs by a conformational change induced upon heterodimerization with RXR (Wiebel, 1997).

Mutations of a single residue in the retinoid X receptor alpha (RXRalpha) ligand-binding pocket (LBP) generate constitutive, ligand-binding-competent mutants with structural and functional characteristics similar to those of agonist-bound wild-type RXR. Modelling of the mouse RXRalphaF318A LBP suggests that, like agonist binding, the mutation disrupts a cluster of van der Waals interactions that maintain helix H11 in the apo-receptor location, thereby shifting the thermodynamic equilibrium to the holo form. Heterodimerization with some apo-receptors (retinoic acid, thyroid hormone and vitamin D3 receptors) results in "silencing" of RXRalphaF318A constitutive activity, however, this efficiently contributes to synergistic transactivation within NGFI-B-RXR heterodimers. RAR mutants disabled for corepressor binding and/or lacking a functional AF-2 activation domain, do not relieve RXR "silencing." Not only RAR agonists, but also the RAR antagonist BMS614 induce conformational changes allowing RXR to exert constitutive (RXRalphaF318A) or agonist-induced (wild-type RXR) activity in heterodimers. Interestingly, the RXRalphaF318A constitutive activity generated within heterodimers in the presence of BMS614 requires the integrity of both the RXR and RAR AF-2 domains. These observations suggest that within RXR-RAR heterodimers, RAR can adopt a structure distinct from that of the active holo-RAR, thus allowing RXR to become transcriptionally responsive to agonists (Vivat, 1997).

Transactivation-defective retinoid X and thyroid hormone receptors have been used to examine mechanisms for hormonal activation. Activation and repression of transcription by retinoid X and thyroid hormone receptors are shown to be mediated by physically distinct and functionally independent regions of the hormone binding domain. Nevertheless, the ability of receptors to respond to hormone requires communication between both functional domains. Deletion of the hormone-dependent transactivation function of the retinoid X receptor, the common subunit of heterodimeric nuclear receptors, significantly impairs hormone-dependent transcription by retinoic acid, thyroid hormone, and vitamin D receptors. The results indicate that receptors do not exist in static off and on conformations but that hormone alters an equilibrium between inactive and active states (Schulman, 1996).

Mammalian RXR - interactions during transcriptional activation

Continued: see Evolutionary Homologs: part 3/4 | part 4/4 | Return part 1/4

ultraspiracle: Biological Overview | Regulation | Developmental Biology | Effects of Mutation | References

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