Cloning and characterization of a novel binding factor (GMEB-2) of the glucocorticoid modulatory element.

The 21-base pair glucocorticoid modulatory element (GME) of the rat tyrosine aminotransferase gene is the only cis-acting element known to modulate the transcriptional activity of receptors bound to glucocorticoid response elements. Specifically, the GME increases the activity of complexes bound both by physiological concentrations of glucocorticoids, due to a left shift in the dose-response curve, and by saturating concentrations of anti-glucocorticoids. For this reason, the nuclear protein(s) that has been demonstrated to bind to the GME is of major interest as a possible transcription factor with hitherto undescribed properties. Subsequent studies indicated that not one but two proteins of 88 and 67 kDa (= GMEB-1 and -2, respectively) formed a heteromeric complex with double-stranded GME oligonucleotides in gel shift assays and participated in the expression of GME activity (Oshima, H., Szapary, D., and Simons, S. S., Jr. (1995) J. Biol. Chem. 270, 21893-21910). Here, we report the use of polymerase chain reaction of degenerate oligonucleotides and 5'- and 3'-rapid amplification of cDNA ends to clone two cDNAs of 2. 0 and 1.9 kilobase pairs that probably result from alternative splicing. Both cDNAs encoded open reading frames containing all four previously sequenced peptides. The longer 2.0-kilobase pair cDNA encoded an open reading frame for an acidic, 529-amino acid protein and afforded a major 67-kDa and a minor 58-kDa protein after in vitro transcription/translation. Both proteins were recognized by a mono-epitopic antibody raised against a peptide of GMEB-2. The in vitro translated protein bound to GME DNA in gel shift assays. However, the binding to GME DNA increased markedly after mixing with authentic GMEB-1 to give a gel-shifted complex that was similar to that derived from HTC cell cytosol. GMEB-2 shares a unique domain (KDWKR) with proteins derived from diverse organisms as follows: Drosophila (DEAF-I), rat (Suppressin), and Caenorhabditis elegans (three unknown open reading frames). Collectively, these data suggest that the 67-kDa GMEB-2 not only is an important factor for the modulation of glucocorticoid receptor bound to glucocorticoid response elements but also may belong to a novel family of transcription factors.