Hormone-independent transcriptional activation and coactivator binding by novel orphan nuclear receptor ERR3

Orphan nuclear receptors share sequence homology with members of the nuclea r receptor superfamily, but ligands are unknown or unnecessary. A novel orp han receptor, estrogen receptor-related protein 3 (ERR3), was identified by yeast two-hybrid screening, using the transcriptional coactivator glucocor ticoid receptor interacting protein 1 (GRIP1) as bait, The putative full-le ngth mouse ERRS contains 458 amino acids and is closely related to two know n orphan receptors ERR1 and ERR2, All the ERR family members share an almos t identical DNA-binding domain, which has 680/0 amino acid identity with th at of estrogen receptor. ERRS bound specifically to an estrogen response el ement and activated reporter genes controlled by estrogen response elements , both in yeast and in mammalian cells, in the absence of any added ligand. A conserved AF-2 activation domain located in the hormone-binding domain o f ERRS was primarily responsible for transcriptional activation, The ERRS A F-2 domain bound GRIP1 in a Ligand-independent manner both in vitro and in vivo, through the LXXLL motifs of GRTP1, and GRIP1 functioned as a transcri ptional coactivator for ERRS in both yeast and mammalian cells. Expression of ERRS in adult mouse was restricted; highest expression was observed in h eart, kidney, and brain. In the mouse embryo no expression was observed at day 7, and highest expression occurred around the 11-15 day stages, Althoug h ERR3 is much more closely related to ERR2 than to ERR1, the expression pa ttern for ERRS was similar to that of ERR1 and distinct from that for ERRB, suggesting a unique role for ERRS in development.