Structure, function, and aggregation of the zinc-free form of the p53 DNA binding domain

Exposure to ELF magnetic field tuned to Zn inhibits growth of cancer cells

The effects of ELF alternating magnetic fields tuned to Zn(2+) on the growth of cancer cells with different status of p53 were investigated using a cell proliferation assay. Human cancer cells HeLa (cervix cancer, p53(+/+)), Saos-2 and Saos-2-His-273 (osteosarcoma, p53(-/-) and p53 His-273 mutant, respectively), H1299tTA and H1299tTA-His175 (lung carcinoma, p53(-/-) and p53 His-175 mutant), and normal human fibroblasts VH-10 (p53(+/+)) were used. Exposure parameters were calculated for the first harmonic of Zn(2+) based either on the magnetic parametric resonance (MPR) model of Lednev or the ion parametric resonance (IPR) model of Blanchard and Blackman. ELF exposure was for 72 and 96 h. The vertical alternating field was 20 Hz at amplitudes of either 38.7 or 77.4 microT (peaks, IPR or MPR, respectively). The vertical static magnetic field was 43 microT, and the horizontal static magnetic field was zeroed. Treatments of cells with PRIMA-1 and gamma-rays were used as positive controls. Growth inhibition was observed in cells after exposure to ELF at 38.7 microT. Inhibition of HeLa, VH-10, and Saos-2-His-273 cells was statistically significant, P=0.0003, 0.02, and 0.006, respectively. No consistent ELF effects following exposure 77.4 microT were seen. PRIMA-1 inhibited the growth of all cell lines with the strongest effect in mutant p53-carrying cell line H1299tTA-His175. The effects of gamma-rays were relatively weak, suggesting that the cell proliferation assay under conditions employed in this study is not very sensitive to apoptosis. In conclusion, ELF under conditions of exposure tuned to Zn(2+) according to the IPR model inhibited the growth of cancer and normal cells. No clear relationship of the observed growth inhibition to p53 status was found. Further experiments, using complementary techniques, are required to test whether p53 reactivation by ELF is feasible.

In vitro folding and characterization of the p53 DNA binding domain

The transcription factor p53 acts as major tumor suppressor and is inactivated by mutation in more than 50% of all human tumors. We have established an efficient procedure for the in vitro folding and purification of the p53 DNA binding domain (p53DBD) using a modified factorial matrix approach that supplies large amounts of homogeneous (isotope-labeled) p53DBD for application in biochemical, crystallographic and NMR spectroscopic studies. We further show with biophysical methods that in vitro folded p53DBD is fully functional and that its conformation is identical to that obtained from the soluble fraction.

The N-terminal domain of p53 is natively unfolded

p53 is one of the key molecules regulating cell proliferation, apoptosis and tumor suppression by integrating a wide variety of signals. The structural basis for this function is still poorly understood. p53 appears to exercise its function as a modular protein in which different functions are associated with distinct domains. Presumably, p53 contains both folded and partially structured parts. Here, we have investigated the structure of the isolated N-terminal part of p53 (amino acid residues 1-93) using biophysical techniques. We demonstrate that this domain is devoid of tertiary structure and largely missing secondary structure elements. It exhibits a large hydrodynamic radius, typical for unfolded proteins. These findings suggest strongly that the entire N-terminal part of p53 is natively unfolded under physiological conditions. Furthermore, the binding affinity to its functional antagonist Mdm2 was investigated. A comparison of the binding of human Mdm2 to the N-terminal part of p53 and full-length p53 suggests that unfolded and folded parts of p53 function synergistically.

Kinetic instability of p53 core domain mutants: implications for rescue by small molecules

Oncogenic mutations in the tumor suppressor protein p53 are found mainly in its DNA-binding core domain. Many of these mutants are thermodynamically unstable at body temperature. Here we show that these mutants also denature within minutes at 37 degrees C. The half-life (t(1/2)) of the unfolding of wild-type p53 core domain was 9 min. Hot spot mutants denatured more rapidly with increasing thermodynamic instability. The highly destabilized mutant I195T had a t(1/2) of less than 1 min. The wild-type p53-(94-360) construct, containing the core and tetramerization domains, was more stable, with t(1/2) = 37 min at 37 degrees C, similar to full-length p53. After unfolding, the denatured proteins aggregated, the rate increasing with higher concentrations of protein. A derivative of the p53-stabilizing peptide CDB3 significantly slowed down the unfolding rate of the p53 core domain. Drugs such as CDB3, which rescue the conformation of unstable mutants of p53, have to act during or immediately after biosynthesis. They should maintain the mutant protein in a folded conformation and prevent its aggregation, allowing it enough time to reach the nucleus and bind its sequence-specific target DNA or the p53 binding proteins that will stabilize it.