Epigallocatechin gallate (EGCG) from green tea can induce apoptosis in cancerous cells, but the underlying molecular mechanisms remain poorly understood. Using SPR and NMR, here we report a direct, μM interaction between EGCG and the tumor suppressor p53 (KD = 1.6 ± 1.4 μM), with the disordered N-terminal domain (NTD) identified as the major binding site (KD = 4 ± 2 μM). Large scale atomistic simulations (>100 μs), SAXS and AUC demonstrate that EGCG-NTD interaction is dynamic and EGCG causes the emergence of a subpopulation of compact bound conformations. The EGCG-p53 interaction disrupts p53 interaction with its regulatory E3 ligase MDM2 and inhibits ubiquitination of p53 by MDM2 in an in vitro ubiquitination assay, likely stabilizing p53 for anti-tumor activity. Our work provides insights into the mechanisms for EGCG’s anticancer activity and identifies p53 NTD as a target for cancer drug discovery through dynamic interactions with small molecules.

Missense mutations in the p53 DNA binding domain (DBD) contribute to half of new cancer cases annually. Here we present a thermodynamic model that quantifies and links the major pathways by which mutations inactivate p53. We find that DBD possesses two unusual properties-one of the highest zinc affinities of any eukaryotic protein and extreme instability in the absence of zinc-which are predicted to poise p53 on the cusp of folding/unfolding in the cell, with a major determinant being available zinc concentration. We analyze the 20 most common tumorigenic p53 mutations and find that 80% impair zinc affinity, thermodynamic stability, or both. Biophysical, cell-based, and murine xenograft experiments demonstrate that a synthetic zinc metallochaperone rescues not only mutations that decrease zinc affinity, but also mutations that destabilize DBD without impairing zinc binding. The results suggest that zinc metallochaperones have the capability to treat 120,500 patients annually in the U.S.

Domain swapping occurs when identical proteins exchange segments in reciprocal fashion. Natural swapping mechanisms remain poorly understood and engineered swapping has the potential for creating self-assembling biomaterials that encode for emergent functions. We demonstrate that induced swapping can be used to regulate function of a target protein. Swapping is triggered by inserting a ‘lever’ protein (ubiquitin) into one of four loops of the ribose binding protein (RBP) target. The lever splits the target, forcing RBP to refold in trans to generate swapped oligomers. Identical RBP-ubiquitin fusions form homo-swapped complexes with the ubiquitin domain acting as the hinge. Surprisingly, some pairs of non-identical fusions swap more efficiently with each other than they do with themselves. NMR experiments reveal that the hinge of these hetero-swapped complexes maps to a region of RBP distant from both ubiquitins. This design is expected to be applicable to other proteins to convert them into functional switches.

Angiogenin (ANG) promotes the formation of blood vessels in animals. This hormone is a small, monomeric protein that is homologous to bovine pancreatic ribonuclease A (RNase). ANG is a poor ribonuclease but its ribonucleolytic activity is essential for its angiogenic activity. RNase is not angiogenic. A hybrid protein was produced in which 13 residues of a divergent surface loop of ANG were substituted for the analogous 15 residues of RNase. The value of kcat/Km for the cleavage of uridylyl(3'-->5')adenosine by this hybrid protein was 20-fold less than that of RNase but 10(5)-fold greater than that of ANG. The thermal stability of the hybrid protein was also less than that of RNase. Nevertheless, the RNase/ANG hybrid protein promotes angiogenesis in mice at least as extensively as does authentic ANG. Thus we present a protein endowed with a noncognate biological activity simply by replacing a single element of secondary structure. In addition, a 13-residue peptide corresponding to the surface loop of ANG inhibits endogenous angiogenesis in mice. These results support a model in which both a surface loop and a catalytic site are necessary for the promotion of blood vessel formation by ANG or RNase. The dissection of structure/function elements in ANG reveals a unique opportunity to develop new molecules that modulate neovascularization.

Structural characterization of protein unfolding intermediates [Kiefhaber et al. (1995) Nature 375, 513; Hoeltzli et al.(1995) Proc. Natl. Acad. Sci. U.S.A. 92, 9318], which until recently were thought to be nonexistent, is beginning to give information on the mechanism of unfolding. To test for apomyoglobin unfolding intermediates, we monitored kinetics of urea-induced denaturation by stop-flow tryptophan fluorescence and quench-flow amide hydrogen exchange. Both measurements yield a single, measurable kinetic phase of identical rate, indicating that the reaction is highly cooperative. A burst phase in fluorescence, however, suggests that an intermediate is rapidly formed. To structurally characterize it, we carried out stop-flow thiol-disulfide exchange studies of 10 single cysteine-containing mutants. Cysteine probes buried at major sites of helix-helix pairing revealed that side chains throughout the protein unpack and become accessible to the labeling reagent [5, 5'-dithiobis (2-nitrobenzoic acid)] with one of two rates. Probes located at all helical-packing interfaces-except for one-become exposed at the rate of global unfolding as determined by fluorescence and hydrogen exchange measurements. In contrast, probes located at the A-E helical interface undergo complete thiol-disulfide exchange within the mixing dead time of 6 ms. These results point to the existence of a burst-phase unfolding intermediate that contains globally intact hydrogen bonds but locally disrupted side-chain packing interactions. Dissolution of secondary and tertiary structure are therefore not tightly coupled processes. We suggest that disruption of tertiary structure may be a stepwise process that begins at the weakest point of the native fold, as determined by native-state hydrogen-exchange parameters.