Human oncoprotein MDM2 is the E3 ubiquitin ligase for the tumor suppressor protein p53. Since p53 prevents tumorigenesis through cell cycle arrest or apoptosis of cells in response to cellular stress such as DNA damage, inactivation of p53 by MDM2 leads to the development of several human cancers.
MDM2 directly binds to the small region in the transactivation domain of p53. Importantly, peptides that mimic MDM2-binding region of p53 reportedly restore the anti-cancer activity by p53. We had performed rigorous selection of MDM2-binding peptides by means of mRNA display (named in vitro virus method), which can handle larger random peptide libraries than phage-display.1,2 As a result, we identified an optimal 12-mer peptide, MIP, which shows higher activities for MDM2-binding and tumor cell proliferation suppression than the previously reported peptides.3
Here, we have performed an NMR structural analysis of MIP bound to MDM2. The obtained structure shows that a region spanning from F3 to E12 of MIP forms a single α-helix, which is similar to that in the previously reported MDM2-binding peptides, but is elongated by M11 and E12. MIP shares conserved F3-W7-L10 triad. Side chains of these residues are oriented towards the binding cleft of MDM2 and form a hydrophobic core. In addition, two hydrophobic patches were formed by solvent exposed residues: (i) W4, L8 and M11 of MIP and three residues of MDM2; and (ii) Y6 of MIP and two residues of MDM2. All of these hydrophobic interactions reside at the interface of the two molecules and contribute to the strong binding. Thus higher anti-MDM2 activity observed for MIP turned out to originate from its ability to enlarge the binding surface. The structural information obtained in the present study provides a road map for the rational design of an inhibitor against the MDM2:p53 binding.