LIGAND-INDUCED AND DNA-INDUCED DISSOCIATION OF RXR TETRAMERS

Unliganded bacterially expressed RXR alpha lacking the N-terminal regi on AB (apo-RXR alpha Delta AB) was found in solution as an apparent mi xture of 165 kDa tetramers and 42 kDa monomers which could be quantita tively separated by gel filtration and non-denaturing gel electrophore sis. Under identical conditions both liganded (holo-) and apo-RAR alph a Delta AB were present as single monomeric species. apo-RXR alpha Del ta AB tetramers, as well as dimers of the apo-RXR ligand binding domai n (apo-LBD), dissociated readily into monomers when exposed to their c ognate ligand 9-cis retinoic acid (9c-RA). The apo-RXR alpha Delta AB tetramer bound only transiently to a cognate DR1 response element, and was converted into DR1-apo-RXR alpha Delta AB homodimer complexes ind istinguishable from those generated by cooperative DNA binding of apo- RXR alpha Delta AB monomers. In the absence of DNA, the addition of 9c -RA greatly accelerated the formation of heterodimers with the apo-RAR alpha Delta AB heterodimerization partner. No RXR alpha Delta AB or R AR alpha Delta AB homodimers could be observed in solution, but upon m ixing of the two receptor monomers stable heterodimers could be isolat ed which bound to DR5 response elements in a highly cooperative manner . in these heterodimers, RXR alpha Delta AB interacted with its cognat e ligand as efficiently as in RXR alpha Delta AB homodimers. The prese nce of Ligand did not alter the stability of RXR alpha Delta AB homodi mer or RXR alpha Delta AB-RAR alpha Delta AB heterodimer complexes on DR1 and DR5 response elements, respectively. These in vitro data suppo rt a model in which RXR tetramers could serve as an inactive pool with the dual function of: (i) rapidly supplying large amounts of RXR hete rodimerization partners upon 9c-RA generation; and (ii) allowing RXR h omodimer formation on ''accessible'' cognate response elements in the absence of 9c-RA. These events may represent a ligand-dependent regula tory mechanism controlling the availability of the promiscuous RXR dim erization partner that is engaged in multiple nuclear receptor signall ing pathways. (C) 1998 Academic Press Limited.