Protein p53 is the "guardian" of our genome. When DNA damage is present, p53 stops cell division and gives the cell enough time to repair it. If the damage is irreparable, the protein sets off programmed cell death and protects the cells from degeneration.
In 50% of all human tumors, p53 is not functional. In cases of Li-Fraumeni syndrome, a genetic disease that leads to tumors at an early age, p53 is mutated. The mutations affect the protein’s DNA-binding site or destabilize the protein. In addition, there are mutations in a short, helical section which don’t fall into either of these categories. German researchers have now explained why these also inhibit the function of p53.
A team of researchers from the Chemistry Department at the Technical University of Munich, the Bavarian NMR Center, and Penzberg Pharma Research Center of Roche Diagnostics Inc. demonstrated that the DNA-binding site on p53 binds to special DNA sites when it is in the form of a dimer. The helical segment seems to be responsible for this dimerization. The group, headed by Horst Kessler, then generated different specific mutations in this segment and studied the mutants’ ability to bind DNA. Amino acid positions 180 and 181 proved to be particularly interesting. In wild-type p53, position 180 is occupied by glutamic acid (Glu) and 181 by arginine (Arg). Proteins with single mutations in which Glu-180 was replaced with Arg or Arg-181 was replaced with Glu (putting two of the same amino acids next to each other) could no longer dimerize and did not bind DNA as well. A mixture of both mutants, in contrast, binds as well as the natural protein. If the positions of Arg and Glu are exchanged in a double mutation, the mutant binds as well as wild-type p53.
How can these results be explained? The secret lies in the negative charge of Glu-180 and the positive charge of Arg-181. When two of the natural protein chains dimerize, these attract each other and form two salt bridges. If the two amino acids are exchanged, it makes no difference for the salt bridges; each position still involves both a positive and a negative charge. However, in the single mutations, similar charges are matched up, and repel each other. If the two single mutations are mixed, positive–positive meets negative– negative and all is well.
“Our results prove that the proposed dimerization, which stabilizes the selective binding of DNA, is largely held together by two salt bridges,” says Kessler. “In addition, the tumor-causing mutations of the 180 and 181 positions in Li-Fraumeni Syndrome also become comprehensible. The DNA is no longer bound tightly enough because of the failure to dimerize.”
Angewandte Chemie International Edition, doi: 10.1002/anie.200501887