Crystal structure of a superstable mutant of human p53 core domain. Insights into the mechanism of rescuing oncogenic mutations.

Most of the cancer-associated mutations in the tumor suppressor p53 map to its DNA-binding core domain. Many of them inactivate p53 by decreasing its thermodynamic stability. We have previously designed the superstable quadruple mutant M133L/V203A/N239Y/N268D containing the second-site suppressor mutations N239Y and N268D, which specifically restore activity and stability in several oncogenic mutants. Here we present the x-ray structure of this quadruple mutant at 1.9 A resolution, which was solved in a new crystal form in the absence of DNA. This structure reveals that the four point mutations cause only small local structural changes, whereas the overall structure of the central beta-sandwich and the DNA-binding surface is conserved. The suppressor mutation N268D results in an altered hydrogen bond pattern connecting strands S1 and S10, thus bridging the two sheets of the beta-sandwich scaffold in an energetically more favorable way. The second suppressor mutation N239Y, which is located in close proximity to the DNA-binding surface in loop L3, seems to reduce the plasticity of the structure in large parts of loop L3 as indicated by decreased crystallographic temperature factors. The same is observed for residues in the vicinity of the N268D substitution. This increase in rigidity provides the structural basis for the increase in thermostability and an understanding how N268D and N239Y rescue some of the common cancer mutants.