FOR, a Novel Orphan Nuclear Receptor Related to Farnesoid X Receptor*

We have identified and characterized a new amphibian orphan member of the nuclear receptor superfamily and termed it FOR1 (farnesoid X receptor (FXR)-like OrphanReceptor) because it shares the highest amino acid identity with the mammalian FXR. We also identified a variant of FOR1, called FOR2, which has 15 additional C-terminal amino acids. Both variants include an unusual insertion of 33 amino acids in the helix 7 region of the canonical ligand binding domain sequence, suggesting a unique structure for FOR. Northern blot analysis demonstrates that the FORgene is highly expressed in adult and tadpole liver, kidney, and tail bud stage of the embryo. Detailed expression analysis using in situhybridization indicates that FOR expression is first detectable at stage 30/31 in the presumptive liver region lasting until stage 41 with a peak level evident at stage 35/36. FOR forms heterodimeric complexes with retinoid X receptor (RXR) as demonstrated by biochemical and mammalian two-hybrid approaches. Gel mobility shift assays demonstrate that FORs form specific DNA-protein complexes on an FXR binding element consisting of an inverted repeat DNA element with 1 nucleotide spacing (IR1) from the phospholipid transfer proteingene promoter. Finally, although FORs do not exhibit constitutive transcriptional activity, frog gallbladder extract significantly augments the transcriptional activities of FORs.