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

Orphan nuclear receptors share sequence homology with members of the nuclear receptor superfamily, but ligands are unknown or unnecessary. A novel orphan receptor, estrogen receptor-related protein 3 (ERR3), was identified by yeast two-hybrid screening, using the transcriptional coactivator glucocorticoid receptor interacting protein 1 (GRIP1) as bait, The putative full-length mouse ERRS contains 458 amino acids and is closely related to two known orphan receptors ERR1 and ERR2, All the ERR family members share an almost identical DNA-binding domain, which has 680/0 amino acid identity with that of estrogen receptor. ERRS bound specifically to an estrogen response element 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 of ERRS was primarily responsible for transcriptional activation, The ERRS AF-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 transcriptional coactivator for ERRS in both yeast and mammalian cells. Expression of ERRS in adult mouse was restricted; highest expression was observed in heart, kidney, and brain. In the mouse embryo no expression was observed at day 7, and highest expression occurred around the 11-15 day stages, Although ERR3 is much more closely related to ERR2 than to ERR1, the expression pattern for ERRS was similar to that of ERR1 and distinct from that for ERRB, suggesting a unique role for ERRS in development.