ChBE Seminar Series: Prof. Peter Vekilov, U. of Houston
Speaker: Peter Vekilov, Professor of Chemical and Biomolecular Engineering and Chemistry at the Univesrity of Houston
Title: Reversible fibrils of p53 nucleate within mesoscopic protein-rich clusters
The protein p53 is expressed in all cells and plays a central role in tumor suppression. Mutations in p53 are associated with about 50% of human cancers. Mutated p53 exhibits a dominant-negative (DN) activity, i.e., in the mixture of normal and mutated p53 proteins the anti-cancer activity of the wild-type species is inhibited. Furthermore, some of the mutants attain new functions, not present in the wild-type protein; this activity is referred to as gain of function (GoF) and it strongly contributes to cancer development. The mechanisms that define the DN and GoF activity of mutated p53 are not understood. We hypothesized that they may be intrinsically related with the aggregation propensity of wild-type and mutant p53 and explored the aggregation of the wild-type protein. We developed a procedure to express and purify p53 in milligram amounts by combined Ni-Sepharose and heparin sepharose chromatography. To detect and characterize p53 aggregates, we combined biophysical and molecular biology methods: oblique illumination microscopy, dynamic light scattering, sedimentation, spectroscopy, ThT and ANS fluorescence, and immunoblotting. We found that p53 aggregates at concentrations ca. 1000-fold lower than those observed with other proteins, but only at temperatures close to physiological and in crowded solutions. p53 forms both amyloid fibrils and mesoscopic clusters; clusters are preferred at low temperature, whereas fibrils dominate at near-physiological temperature. Aggregates of both classes are reversible, but the cluster dynamics are significantly faster, suggesting that the enhanced cluster formation might be a result of evolutionary pressure aimed at fast storage and release of this protein. Importantly, the nucleation of p53 fibrils is hosted by the protein-rich clusters. The ability to form amyloid fibrils may correlate with the DN and GoF activity of p53 in the transformation of normal cells to cancerous. Fibril nucleation hosted by precursors opens novel pathways for fibril suppression by controlling the cluster properties.