RXR heterodimerization allosterically activates LXR binding to the second NR box of activating signal co-integrator-2.
ASC-2 (activating signal co-integrator-2) is a transcriptional co-activator that mediates the transactivation of NRs (nuclear receptors) via direct interactions with these receptors. ASC-2 contains two separate NR-interaction domains harbouring a core signature motif, LXXLL (where X is any amino acid), named the NR box. Although the first NR box (NR box-1) of ASC-2 interacts with many different NRs, the second NR box (NR box-2) specifically interacts with only LXR (liver X receptor), whose transactivation in vivo requires heterodimerization with RXR (retinoid X receptor). Interestingly, RXR has been shown to enhance the LXR transactivation, even in the absence of LXR ligand via a unique mechanism of allosteric regulation. In the present study we demonstrate that LXR binding to an ASC-2 fragment containing NR box-2 (Co4aN) is enhanced by RXR and even further by liganded RXR. We also identified specific residues in Co4aN involved in its interaction with LXR that were also required for the ASC-2-mediated transactivation of LXR in mammalian cells. Using these mutants, we found that the Co4aN-LXR interaction surface is not altered by the presence of RXR and RXR ligand and that the Ser(1490) residue is the critical determinant for the LXR-specific interaction of Co4aN. Notably the NR box-2, but not the NR box-1, is essential for ASC-2-mediated transactivation of LXR in vivo and for the interaction between LXR-RXR and ASC-2 in vitro. These results indicate that RXR does not interact directly with NR box-1 of ASC-2, but functions as an allosteric activator of LXR binding to NR box-2 of ASC-2.
Son
You Lee
YL
Hormone Research Center, School of Biological Sciences and Technology, Chonnam National University, Gwangju 500-757, South Korea.
Park
Ok Gu
OG
Kim
Gwang Sik
GS
Lee
Jae Woon
JW
Lee
Young Chul
YC
D000494:Allosteric Regulation; D051017:Apoptosis Regulatory Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D001665:Binding Sites; D004268:DNA-Binding Proteins / Q000737:chemistry / Q000378:metabolism*; D019281:Dimerization; D006801:Humans; D000071518:Liver X Receptors; D016297:Mutagenesis, Site-Directed; D057093:Orphan Nuclear Receptors; D010446:Peptide Fragments / Q000737:chemistry / Q000378:metabolism; D010957:Plasmids; D011485:Protein Binding; D018160:Receptors, Cytoplasmic and Nuclear / Q000737:chemistry / Q000378:metabolism*; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D012261:Ribonucleoproteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D015533:Transcriptional Activation
D051017:Apoptosis Regulatory Proteins; D004268:DNA-Binding Proteins; D000071518:Liver X Receptors; D057093:Orphan Nuclear Receptors; C490350:POP1 protein, human; D010446:Peptide Fragments; D018160:Receptors, Cytoplasmic and Nuclear; D011994:Recombinant Proteins; D012261:Ribonucleoproteins