In the past several decades there have been significant advances in understanding protein structure and function, which in turn have aided the engineering of proteins for improved function and also the design of new proteins. However, these achievements have been limited to polypeptides which have specific 3-dimensional structures. Intrinsically disorder proteins (IDPs), which fulfill their functions without well-defined secondary or tertiary structures, have been largely ignored. IDPs are actually abundant in various proteomes and are involved in many cellular processes such as signaling and regulation. IDPs show unique features in interacting with other molecules: the interactions are highly specific yet with low affinity, and one IDP can have multiple-binding partners. These properties are advantageous to dynamic processes requiring both reversibility and specificity, and in complex networks involving hub proteins interconnecting interactions. Not surprisingly, mutations that interfere with the function of IDPs have been predicted to be related to several human diseases such as cancer and cardiovascular disease. Despite their abundance and important roles, the structure-function properties of IDPs are not well understood.

We have investigated the relationship between ligand binding affinity and disorder in engineered variants of the unstructured N-terminal region (transactivation domain (TAD)) of the tumor suppressor p53. MDM2 and MDMX are negative regulators of the p53 activity. Inhibitors of the binding of MDM2 or MDX to TAD have been shown to induce the p53 pathway in tumor cell lines. The effects of mutations on the structural disorder of TAD and on the interactions with its known binding partners (MDM2, MDMX, TAFII31, and p300) will be discussed. The evaluation of the TAD variants on the activation of the p53 pathways in tumor cell lines is also under investigation. The results will not only provide new information on the evolutionary trajectories and the relationship between binding affinity and structural states for IDPs, but may also result in the development of novel proteins of potential use in cancer therapy.

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