Binding induced folding in p53-MDM2 complex.

The MDM2 N-terminal domain can bind to the transactivation domain of p53 and downregulate its ability to activate transcription. It was found that binding with p53 stabilizes the MDM2 N-terminal domain. Thus the coupling between binding and folding is essential in the normal functional interactions between p53 and MDM2. We have performed explicit-solvent molecular dynamics simulations (MD) for both bound MDM2(N) and apo-MDM2 to study the interdependence of binding and folding in the p53-MDM2 complex. Kinetic analysis of high-temperature MD simulations shows that both bound MDM2(N) and apo-MDM2 unfold via a two-state process. Both kinetics and free energy landscape analyses indicate that bound MDM2 unfolds in the order of p53 unbinding, tertiary unfolding, and finally secondary structure unfolding. Our data show that the unfolding pathways are different between bound MDM2(N) and apo-MDM2: the unfolding order of unstable helices and tertiary contacts is reversed. Transition state analysis shows that the transition state of bound MDM2 is more nativelike and more heterogeneous than that of apo-MDM2. The predicted Phi-values suggest that the stable helices are more nativelike than other regions in both bound MDM2(N) and apo-MDM2. Within the stable helices, helix II in bound MDM2 is more nativelike than that in apo-MDM2. However, helix I and IV in bound MDM2 are less nativelike than those in apo-MDM2.