Biomedical subjects

S Kersten

Publications and source records attributed to S Kersten.

24 records · Page 2Linked to original sources

PubMed1997

The tetramerization region of the retinoid X receptor is important for transcriptional activation by the receptor.

The retinoid X receptor (RXR), a member of the superfamily of hormone nuclear receptors, is a ligand-inducible transcription factor that is activated by the vitamin A derivative 9-cis-retinoic acid. We previously showed that RXR self-associates into tetramers with a high affinity and that ligand binding induces rapid dissociation of receptor tetramers to smaller species. Here, the RXR region that is responsible for mediating tetramer formation is identified. It is shown that this interface, which we term the "tetramerization domain," critically contains two consecutive phenylalanine residues located at the C-terminal region of the receptor. Mutation of these residues is sufficient to disrupt RXR tetramers without affecting the overall fold of the protein or interfering with ligand binding, dimer formation, or DNA binding by the receptor. Nevertheless, the tetramer-impaired mutant was found to be transcriptionally defective. The newly characterized tetramerization domain and the previously identified main dimerization interface of RXR act autonomously to affect separate intersubunit interactions that, overall, lead to formation of tetramers. Protein-protein interactions mediated by the tetramerization domain, but not those that involve the dimerization interface, are disrupted following ligand binding by RXR. Overall, these data attest to the specificity of the interaction and implicate the tetramerization interface in playing a direct role in regulating transcriptional activation by RXR.

Amino Acid Sequence↗

PubMed1997

The DNA binding pattern of the retinoid X receptor is regulated by ligand-dependent modulation of its oligomeric state.

The retinoid X receptor (RXR) regulates target gene transcription via its association with cognate DNA response elements either as a homodimer or as a heterodimer with a number of other nuclear receptors. We previously demonstrated that, in solution, RXR forms tetramers with a high affinity and that ligand binding leads to dissociation of receptor tetramers to smaller species. Here it is shown that RXR tetramers form stable complexes with direct repeats (DR-1 or DR-5) or palindromic (TREpal) response elements. Binding of RXR tetramers to cognate DNA occurs with a significantly higher affinity as compared with dimers. Ligand binding by DNA-bound RXR tetramers results in their dissociation to DNA-bound dimers, a process that is completely reversed upon removal of the ligand. Formation of stable tetramer-DNA complexes requires binding of two oligonucleotides/tetramer. It is proposed that ligand-dependent modulation of the oligomeric state of RXR is a regulatory feature of this nuclear receptor.

Animals↗

PubMed1996

Individual subunits of heterodimers comprised of retinoic acid and retinoid X receptors interact with their ligands independently.

The retinoid X receptor (RXR) is a member of a family of transcription factors, known as hormone nuclear receptors, that mediate the effects of hydrophobic hormones on gene transcription. RXR, which is activated by 9-cis-retinoic acid (9cRA), can modulate several signaling pathways by virtue of its ability to form heterodimers with other members of the receptor family, as, for example, the retinoic acid receptor (RAR). The roles of the individual receptors within heterodimers are not clear as yet. It was recently reported that heterodimerization inhibits transcriptional activation by RXR, an effect that was attributed to an inability of RXR within heterodimers to bind its ligand. This inhibition was reported to depend on the association of heterodimers with cognate DNA and on the level of saturation of the RAR subunit within the heterodimers. In the present work, the ligand-binding characteristics of RXR and RAR individually and within heterodimers were examined by fluorescence-based methods. The results indicate that heterodimerization with RAR does ot alter the ligand-binding capacity of RXR nor the rate of dissociation of the ligand from this receptor. The ligand-binding capacity of RXR also was not affected by association of heterodimers with cognate DNA nor by the level of saturation of the RAR subunit. The data indicate further that the affinity of RAR for 9cRA is considerably higher as compared to RXR and that this differential affinity is retained within RAR-RXR heterodimers. Thus, binding of ligands by subunits within RAR-RXR heterodimers proceeds independently.

Base Sequence↗

PubMed1995

On the role of ligand in retinoid signaling: positive cooperativity in the interactions of 9-cis retinoic acid with tetramers of the retinoid X receptor.

Previously, we have shown that the retinoid X receptor (RXR) forms tetramers with a high affinity and that interactions of the receptor with its ligand, 9-cis retinoic acid (9cRA), result in dissociation of protein tetramers. Here it is shown by fluorescence anisotropy studies that ligand-induced tetramer dissociation displays a pronounced positive cooperativity. The binding affinity of RXR for 9cRA at low saturation levels of the receptor with ligand was found to be significantly weaker than the affinity observed at higher levels of saturation. In addition, the rate of dissociation of 9cRA from RXR was found to be faster at low vs. high saturation levels of the receptor. These data suggest that the observed positive cooperativity of the ligand-induced dissociation of RXR tetramers stems from positive cooperativity in binding of 9cRA by the receptor. Kinetic studies showed that dissociation of RXR tetramers upon ligand binding is a rapid reaction characterized by a t1/2 of 80 ms, which is about 5 orders of magnitude faster than the rate of dissociation in the absence of ligand. The data indicate that the oligomeric state of RXR is tightly regulated by the precise concentrations of 9cRA and that it rapidly responds to changes in the ligand's concentrations. These findings further substantiate the hypothesis that modulation of the oligomeric state of RXR by 9cRA is an important regulatory step in the pathway by which retinoids affect gene transcription.

Fluorescence Polarization↗

PubMed1995

Role of ligand in retinoid signaling. 9-cis-retinoic acid modulates the oligomeric state of the retinoid X receptor.

Many of the effects of retinoids on cells are mediated by the transcription factors known as retinoid nuclear receptors, but the mechanisms by which retinoids regulate the activity of the receptors are not known. It was previously shown that the retinoid X receptor (RXR) forms tetramers with a high affinity. In the present work it is demonstrated that binding of 9-cis-retinoic acid to RXR leads to rapid dissociation of receptor tetramers. In addition, fluorescence anisotropy studies indicate that ligand-binding results in a significant conformational change such that holo-RXR is more compactly folded as compared to the apo-protein. These findings suggest that the initial event in signaling by 9-cis-retinoic acid is a change in the oligomeric state of RXR. The data also imply that tetramer formation is a regulatory feature of the pathway by which RXR mediates the effects of retinoids on gene transcription.

Binding Sites↗

PubMed1995

Retinoid X receptor alpha forms tetramers in solution.

Protein-protein interactions allow the retinoid X receptor (RXR) to bind to cognate DNA as a homo- or a heterodimer and to participate in mediating the effects of a variety of hormones on gene transcription. Here we report a systematic study of the oligomeric state of RXR in the absence of a DNA template. We have used electrophoresis under nondenaturing conditions and chemical crosslinking to show that in solution, RXR alpha forms homodimers as well as homotetramers. The dissociation constants governing dimer and tetramer formation were estimated by fluorescence anisotropy studies. The results indicate that RXR tetramers are formed with a high affinity and that at protein concentrations higher than about 70 nM, tetramers will constitute the predominant species. Tetramer formation may provide an additional level of the regulation of gene transcription mediated by RXRs.

Base Sequence↗