Retinoid X receptors (RXR) and retinoic acid receptors (RAR) act as heterodimers and as heterodimers with other nuclear hormone receptors to regulate gene expression. Among the genes regulated by receptors for retinoids are those that mediate the expression of xenobiotic-metabolizing enzymes and the oncogene, v-ErbA. Because of the important role these receptors play in growth and development, retinoids are of interest as chemopreventative and chemotherapeutic agents for treatment of cancer. Although the structures of several receptors for retinoids have been determined, information concerning the dynamics and stability of RAR and RXR, and the effects of ligands on their properties, is lacking. This proposal will utilize H/D exchange analyzed by mass spectrometry to examine the dynamic properties of RXRalpha and RARalpha in the presence and absence of ligands. Thermodynamic and kinetic studies using H/D exchange, circular dichroism and fluorescence are also proposed to examine the energetics and mechanism of RXR and RAR folding. Because of the central role of H/D exchange experiments in the proposed research, studies are included to explore and establish optimal conditions for reliable H/D exchange and provide a better understanding of the mechanisms of H/D exchange and label scrambling. In combination, these studies will develop the H/D exchange methodology and provide the foundation for future work to characterize RARalpha and RXRalpha interactions with other components of the signal transduction pathway. This dynamic information should complement the static, structural data and provide the basis for the design of more effective drugs that act on receptors for retinoids.