Expanding on the possible protein interaction partners in a biochemical pathway is one key molecular goal in the post-genomic era. Phage peptide display is a versatile in vitro tool for mapping novel protein-protein interfaces and the advantage of this technique in expanding protein interaction maps is that in vitro manipulation of the bait protein conformational integrity can be controlled carefully. Phage peptide display was used to expand on the possible types of binding proteins for the conformationally responsive protein MDM2. Peptides enriched differ depending upon whether MDM2 is ligand-free, zinc-bound, or RNA-bound, suggesting that MDM2 conformational changes alter the type of peptide ligands enriched. Classes of putative/established MDM2-binding proteins identified by this technique included ubiquitin-modifying enzymes (F-box proteins, UB-ligases, UBC-E1) and apoptotic modifiers (HSP90, GAS1, APAF1, p53). Of the many putative MDM2 proteins that could be examined, the impact of HSP90 on MDM2 activity was studied, since HSP90 has been linked with p53 protein unfolding in human cancers. Zinc ions were required to reconstitute a stable MDM2-HSP90 protein complex. Zinc binding converted MDM2 from a monomer to an oligomer, and activated MDM2 binding to its internal RING finger domain, providing evidence for a conformational change in MDM2 protein when it binds zinc. Reconstitution of an HSP90-MDM2 protein complex in vitro stimulated the unfolding of the p53 tetramer. A p53 DNA-binding inhibitor purified from human cells that is capable of unfolding p53 at ambient temperature in vitro contains co-purifying pools of HSP90 and MDM2. These data highlight the utility of phage peptide display as a powerful in vitro method to identify regulatory proteins that bind to a conformationally flexible protein like MDM2.