[Structure and function of TRAP].

Thyroid hormone receptor (THR) binds to specific thyroid hormone response element (TRE). Using an electrophoretic mobility shift assay (gel-shift), Murray and Towel recently found that cell nuclear extracts enhance the binding of THR to TRE. This protein has been designated T3 receptor auxiliary protein (TRAP). Retinoid X receptors (RXR alpha, beta) can function as TRAP. TRAP forms a heterodimer with THR through the ligand binding domain of both receptors. This area contains leucine zipper like heptad repeats which can form amphipathic alpha-helixes. The ligand (9-cis retinoic acid) of RXR may or may not synergistically increase T3/THR-mediated transactivation. The effects of 9-cis retinoic acid depends on the nature of the TRE. One of the functions of TRAP may be to alter the expression of T3-regulated genes.