Identification of Amino Acids Critical for the DNA Binding and Dimerization Properties of the Human Retinoic Acid Receptor Î

Retinoic acid receptors (RARs) and retinoid X receptors (RXRs) activate target genes by binding to retinoic acid response elements (RAREs) as heterodimeric, asymmetrical complexes, and display a high degree of cooperativity in binding to RAREs. We have examined here the effect of lysine, cysteine, arginine, histidine, and tyrosine side chain chemical modification on the DNA binding, homo- and heterodimerization properties of the full-length human retinoic acid receptor α (hRARα). Lysines are the only residues to be engaged in the dimerization with human retinoid X receptor α (hRXRα) in the absence of DNA, whereas histidines are selectively involved in the homodimerization of hRARα in the presence of a RARE. Arginine modification affected the DNA binding activity of each type of dimer, whereas cysteines and tyrosines were primarily involved in the homo- or heterodimerization process in the presence of the same RARE. Modified lysines, interfering with the dimerization with hRXRα, were identified by receptor labeling and peptide mapping. They are located in the hormone binding domain eighth heptad repeat, at positions 360 and 365. In keeping with these results, mutation of Lys 360 , Val 361 , and Lys 365 diminished strongly the DNA binding activity of hRARα as a homodimer or a heterodimer. Our results thus provide direct evidence for the differential involvement of basic, polar, or aromatic amino acids in the DNA binding, homodimerization, and heterodimerization properties of hRARα. Furthermore, they demonstrate the use of distinct dimerization interfaces and identify the type of amino acids involved in these protein-protein interactions.