p53, the Cellular Gatekeeper for Growth and Division

These results clearly point out that p53 uses a hyThe p53 gene and its protein product have become the drophobic interface in its N-terminal domain to interact center of intensive study ever since it became clear with the transcriptional machinery of the cell and its that slightly more than 50% of human cancers contain negative regulators. Recently, the N-terminal domain of mutations in this gene. An extensive database (Hollstein MDM2 was cocrystalized with a peptide containing p53 et al., 1994) catalogs these mutations in more than 50 amino acid residues 13–29, covering the portion of p53 different cell and tissue types, although some types of that was shown, by mutational analysis, to interact with cancers never appear to select for p53 mutations (LutzMDM2. The MDM2 domain forms a deep hydrophobic ker and Levine, 1996). The nature of these genetic pocket, and the p53 peptide forms an amphipathic helix changes in cancer cells is most commonly a missense with its hydrophobic surface, pointing into and filling mutation in one allele, producing a faulty protein that is the hydrophobic pocket. F19, W23, and L26 all stabilize then observed at high concentrations in these cells, these hydrophobic interactions in this pocket between followed by a reduction to homozygosity. More rarely, p53 and MDM2 (Kussie et al., 1996). deletions or chain-termination mutations in the p53 gene The sequence-specific DNA-binding domain of p53 is indicate that the null phenotype predisposes to cancer, localized between amino acid residues 102 and 292. It is as has been observed in mice with a homozygous p53 a protease-resistant and independently folded domain null mutation (Donehower et al., 1992). There have been containing a Zn21 ion that is required for its sequencesome suggestions that the missense mutant producing specific DNA-binding activity. This domain folds into a a faulty p53 protein could contribute a “gain of function” four-stranded and five-stranded antiparallel b sheet that phenotype (Dittmer et al., 1993), but this remains to be in turn is a scaffold for two a-helical loops that interact substantiated by additional experimentation. directly with the DNA (Cho et al., 1994). The tetrameric A study of the mutational spectra at the p53 locus in p53 protein (which is a dimer of a dimer) binds to four different tissue types indicates a strong role for diverse repeats of a consensus DNA sequence 59-PuPuPuC(A/ environmental mutagens, with a set of tissue preferT)-39, and this sequence is repeated in two pairs, each ences. In addition, there is strong selection for a subset arranged as inverted repeats such as →←→←, where of mutations localized predominantly in the DNA-bind→ is the sequence given above. Residues K120, S241, ing domain of the protein. Several codons in this domain R273, A276, and R283 make contacts with the phosthat are never observed in the p53 mutational spectra phate backbone in the major groove, while K120, C277, have been altered by site-specific mutagenesis, and in and R280 interact via hydrogen bonds to the DNA bases. all cases these mutations have no phenotype and beR248 then makes multiple hydrogen bond contacts in have like the wild-type (Lin et al., 1995). Thus, both the minor groove of the DNA helix (Cho et al., 1994). selection and a strong set of environmental mutagens More than 90% of the missense mutations in p53 reside combine to produce mutations in the p53 gene in human in this sequence-specific DNA-binding domain, and cancers. these mutations fall into two classes. Mutations in amino