Coactivators for the orphan nuclear receptor RORα

A mutation in the nuclear orphan receptor ROR alpha results in a severe impairment of cerebellar development by unknown mechanisms. We have shown previously that ROR alpha contains a strong constitutive activation domain in its C terminus, We therefore searched for mammalian ROR alpha coactivators using the minimal activation domain as bait in a two-hybrid screen. Several known and putative coactivators were isolated, including glucocorticoid receptor-interacting protein-1 (GRIP-1) and peroxisome proliferator-activated receptor (PPAR)-binding protein (PBP/TRAP220/DRIP205). These interactions were confirmed in vitro and require the intact activation domain of ROR alpha although different requirements for interaction with GRIP-1 and PBP were detected. Even in the absence of exogenous ligand, ROR alpha interacts with a complex or complexes of endogenous proteins, similar to those that bind to ligand-occupied thyroid hormone and vitamin D receptors, Both PBP and GRIP-1 were shown to be present in these complexes. Thus we have identified several potential ROR alpha coactivators that, in contrast to the interactions with hormone receptors, interact with ROR alpha in yeast, in bacterial extracts, and in mammalian cells in vivo and in vitro in the absence of exogenous ligand. GRIP-1 functioned as a coactivator for the ROR alpha both in yeast and in mammalian cells. Thus, GRIP-1 is the first proven coactivator for ROR alpha.