Identification of a Novel Testicular Orphan Receptor-4 (TR4)-associated Protein as Repressor for the Selective

Although many co-activators have been identified for various nuclear receptors, relatively fewer co-repres-sors have been isolated and characterized. Here we re-port the identification of a novel testicular orphan nu-clear receptor-4 (TR4)-associated protein (TRA16) that is mainly localized in the nucleus of cells as a repressor to suppress TR4-mediated transactivation. The sup-pression of TR4-mediated transactivation is selective because TRA16 shows only a slight influence on the transactivation of androgen receptor, glucocorticoid receptor, and progesterone receptor. Sequence analysis shows that TRA16 is a novel gene with 139 amino acids in an open reading frame with a molecular mass of 16 kDa, which did not match any published gene se-quences. Mammalian two-hybrid system and co-immu