The structural mechanism of allosteric communication between retinoid X receptor (RXR) and its heterodimer partners remains controversial. As a first step towards addressing this question, we report a nuclear magnetic resonance (NMR) study on the GW1929-bound peroxisome proliferator-activated receptor gamma (PPAR gamma) ligand-binding domain (LBD) with and without the 9-cis-retinoic acid (9cRA)-bound RXR alpha LBD. Sequence-specific C-13(alpha), C-13(beta), and (CO)-C-13 resonance assignments have been established for over 95% of the 275 residues in the PPAR gamma LBD monomer. The (HN)-H-1, N-15, and (CO)-C-13 chemical shift perturbations induced by the RXR alpha LBD binding are located at not only the heterodimer interface that includes the C-terminal residue Y477 but also residues Y473 and K474 in the activation function-2 (AF-2) helix. This result suggests that 9cRA-bound RXR alpha can affect the PPAR gamma AF-2 helix in solution and demonstrates that NMR is a powerful new tool for studying the mechanism of allosteric ligand activation in RXR heterodimers. Published by Elsevier Inc.