Isolation and characterization of PBP, a protein that interacts with peroxisome proliferator-activated receptor

In an attempt to identify cofactors that could possibly influence the transcriptional activity of peroxisome proliferator-activated receptors (PPARs), we used a yeast two-hybrid system with Gal4-PPAR gamma as bait to screen a mouse liver cDNA library and have identified steroid receptor coactivator-l (SRC-1) as a PPAR transcriptional coactivator. We now report the isolation of a cDNA encoding a 165-kDa PPAR gamma-binding protein, designated PBP which also serves as a coactivator. PBP also binds to PPAR alpha, RAR alpha RXR, and TR beta 1, and this binding is increased in the presence of specific ligands. Deletion of the last 12 amino acids from the carboxyl terminus of PPAR gamma results in the abolition of interaction between PBP and PPAR gamma. PBP modestly increased the transcriptional activity of PPAR gamma, and a truncated form of PBP (amino acids 487-735) acted as a dominant-negative repressor, suggesting that PBP is a genuine coactivator for PPAR. In addition, PBP contains two LXXLL signature motifs considered necessary and sufficient for the binding of several coactivators to nuclear receptors. In situ hybridization and Northern analysis showed that PBP is expressed in many tissues of adult mice, including the germinal epithelium of testis, where it appeared most abundant, and during ontogeny, suggesting a possible role for this cofactor in cellular proliferation and differentiation.