Identification and Characterization of a Tissue-Specific Coactivator, GT198, That Interacts with the DNA-Binding Domains of Nuclear Receptors

Abstract

Gene activation mediated by nuclear receptors is regulated in a tissue-specific manner and requires interactions between nuclear receptors and their cofactors. Here, we identified and characterized a tissue-specific coactivator, GT198, that interacts with the DNA-binding domains of nuclear receptors. GT198 was originally described as a genomic transcript that mapped to the human breast cancer susceptibility locus 17q12-q21 with unknown function. We show that GT198 exhibits a tissue-specific expression pattern in which its mRNA is elevated in testis, spleen, thymus, pituitary cells, and several cancer cell lines. GT198 is a 217-amino-acid nuclear protein that contains a leucine zipper required for its dimerization. In vitro binding and yeast two-hybrid assays indicated that GT198 interacted with nuclear receptors through their DNA-binding domains. GT198 potently stimulated transcription mediated by estrogen receptor α and β, thyroid hormone receptor β1, androgen receptor, glucocorticoid receptor, and progesterone receptor. However, the action of GT198 was distinguishable from that of the ligand-binding domain-interacting nuclear receptor coactivators, such as TRBP, CBP, and SRC-1, with respect to basal activation and hormone sensitivity. Furthermore, protein kinase A, protein kinase C, and mitogen-activated protein kinase can phosphorylate GT198 in vitro, and cotransfection of these kinases regulated the transcriptional activity of GT198. These data suggest that GT198 is a tissue-specific, kinase-regulated nuclear receptor coactivator that interacts with the DNA-binding domains of nuclear receptors.

We thank Marc Montminy for CREB-related studies; Ami Aronheim for the yeast two-hybrid system; Elizabeth Galbreath and Linda Schirtzinger for immunohistochemistry; Jacques Simard for the human GT198 cDNA; and Mary Makowske for the MMTV and PKC plasmids. We also acknowledge Thomas Burris for helpful discussions and Toshiharu Iwasaki, Christine Reyes, Keith Stayrook, and Xin Bu for their technical assistance.

This work was supported by the Lilly Postdoctoral Research Fellowship Program.