title: Distinct retinoid X receptor activation function-2 residues mediate transactivation in homodimeric and vitamin D receptor heterodimeric contexts.
abstract: The vitamin D receptor (VDR) stimulates transcription as a 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3))-activated heterodimer with retinoid X receptor (RXR). RXR also forms homodimers to mediate 9-cis retinoic acid (9-cis RA)-induced gene expression. Both receptors possess a C-terminal hormone-dependent activation function-2 (AF-2), a highly conserved region that binds coactivators to transduce the transcriptional signal. By replacing single amino acids within the AF-2 of human RXR alpha (hRXR alpha) or mouse RXR beta (mRXR beta), the contribution of these residues to transactivation by the RXR-VDR heterodimer and the RXR-RXR homodimer was evaluated. In 9-cis RA-responsive homodimers, the second and fourth positions of the AF-2 (leucine and glutamate respectively) are essential. However, in the context of an RXR-VDR heterodimer activated by 1,25(OH)(2)D(3), alteration of these two RXR residues has little effect. Instead, AF-2 residues located towards the C-terminus, such as the penultimate position (L455 in hRXR alpha or L441 in mRXR beta), are crucial for RXR-VDR heterodimers. Indeed, L455A mutant RXR exerts a dominant negative effect on RXR-VDR transcriptional responsiveness to 1,25(OH)(2)D(3). Further experiments with a mutant hRXR alpha (F313A) which elicits 9-cis RA-independent transactivation as a homodimer demonstrate that, when heterodimerized with VDR, this RXR mutant is incapable of activating the RXR-VDR heterocomplex in the absence of the VDR ligand. Taken together, these results indicate that RXR is a subordinate, yet essential transcriptional partner in RXR-VDR-mediated activation of gene expression. Furthermore, a functional switch in RXR AF-2 signaling occurs between RXR residues in the homodimeric versus the heterodimeric states, likely reflecting different interactions between subregions of the AF-2 and coactivator(s).
authors.affiliation: Department of Biochemistry and Molecular Biophysics, College of Medicine, University of Arizona, Tucson, Arizona 85724, USA.
authors.forename: P D
authors.initials: PD
authors.lastname: Thompson
authors.forename: L S
authors.initials: LS
authors.lastname: Remus
authors.forename: J C
authors.initials: JC
authors.lastname: Hsieh
authors.forename: P W
authors.initials: PW
authors.lastname: Jurutka
authors.forename: G K
authors.initials: GK
authors.lastname: Whitfield
authors.forename: M A
authors.initials: MA
authors.lastname: Galligan
authors.forename: C
authors.initials: C
authors.lastname: Encinas Dominguez
authors.forename: C A
authors.initials: CA
authors.lastname: Haussler
authors.forename: M R
authors.initials: MR
authors.lastname: Haussler
mesh_terms: D000077556:Alitretinoin; D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites / Q000235:genetics; D019556:COS Cells; D019281:Dimerization; D006801:Humans; D051379:Mice; D008954:Models, Biological; D009154:Mutation; D017434:Protein Structure, Tertiary; D051381:Rats; D018167:Receptors, Calcitriol / Q000737:chemistry / Q000235:genetics*; D018168:Receptors, Retinoic Acid / Q000737:chemistry / Q000235:genetics*; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics; D047488:Retinoid X Receptors; D015398:Signal Transduction; D014157:Transcription Factors / Q000737:chemistry / Q000235:genetics*; D015533:Transcriptional Activation / Q000187:drug effects; D014162:Transfection; D014212:Tretinoin / Q000494:pharmacology
chemical_list: D018167:Receptors, Calcitriol; D018168:Receptors, Retinoic Acid; D011994:Recombinant Proteins; D047488:Retinoid X Receptors; D014157:Transcription Factors; D000077556:Alitretinoin; D014212:Tretinoin