Coactivators for the Orphan Nuclear Receptor RORa

A mutation in the nuclear orphan receptor RORa results in a severe impairment of cerebellar development by unknown mechanisms. We have shown previously that RORa contains a strong constitutive activation domain in its C terminus. We therefore searched for mammalian RORa 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 RORa although different requirements for interaction with GRIP-1 and PBP were detected. Even in the absence of exogenous ligand, RORa 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 RORa coactivators that, in contrast to the interactions with hormone receptors, interact with RORa 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 RORa both in yeast and in mammalian cells. Thus, GRIP-1 is the first proven coactivator for RORa. (Molecular Endocrinology 13: 1550–1557, 1999) INTRODUCTION