p53 tumour suppressor

Structure of p53 tumour suppressor by cryo-electron microscopy

A key factor in preventing us from developing cancer is the protein p53. This protein has been called the “guardian of the genome” as it prevents potentially destructive mutations from building up in eukaryotic DNA. Mutations in p53 have been implicated in numerous cancer types. A multi-domain protein, p53, contains two separate DNA-binding domains, a large “core domain” in the centre of the protein and a much smaller one towards its C-terminus. In solution, four p53 molecules associate to form a tetramer, which is known to be the active form of the protein. The first structure of the complete p53 tetramer was determined using cryo-electron microscopy (Okorokov et al, 2006). The structure of mouse p53, stabilised with ATP, was found to resemble a hollow, skewed cube with symmetry D2. The beta-sheet structure of this domain fitted well into the electron density associated with a large node. Automatic “docking” allowed the atomic structure of p53’s core domain and helices of the tetramerisation domain to be located within the EM map (Okorokov et al, 2006). Determining the structure of the p53 tetramer by single-particle electron microscopy is a major technological achievement, as this is one of the smallest proteins to have its structure resolved using this technique. This model of p53 is compatible with the biochemical evidence and provides crucial insights into the mechanism of action of this protein, which is important as a cancer drug target.

This novel structure provides a model for the coordinated conformational re-adjustment of the p53 DNA binding domains. This structural plasticity required for p53’s activity is enabled by the D2 symmetry of p53 tetramer. The steric hindrance theory is now supported by the proximity of p53 C-terminal and core domains. The structure suggests a step-wise mechanism where the initial contact with DNA is made by a pair of C-terminal domains that (together with the core domains) are involved in DNA scanning via a linear diffusion mechanism (McKinney et al., 2004). Whenever the sequence-specific DNA element is encountered, a coordinated re-adjustment of DNA binding domains occurs, placing them in a favourable position for a stable complex with the sequence-specific DNA. In addition, the organisation of p53 bound to DNA would place N-terminal transcription activation domains in close proximity to the site of binding on either side of the tetramer marking the site for assembly of the transcription complex. The new 3D structure of the complete p53 tetramer is consistent with existing biochemical and physiological data, provides a comprehensive model of p53 architecture and opens the way for better understanding of this crucial tumour suppressor protein.