to why the availability of a repressive mechanism such as the one outlined in the new study might be crucial for cellular survival.
"You can imagine that if the p53 protein were present at all times and able to bind to DNA, cells would be in big trouble," Berger explains. "They wouldn't be able to divide, and they'd die. We think this new mechanism may be a way for the cell to keep p53 turned off but present, ready to be activated if the DNA should be damaged."
In their study, the scientists identified an enzyme called Smyd2 that adds a methyl group to the p53 protein at a specific site, with the result being that p53 cannot bind to DNA and, therefore, cannot act.
"The ability to bind to DNA is critical for p53's function," says Jing Huang, Ph.D., one of the study's two lead authors. "What we found was that methylation at the site we identified prevents p53 from binding to DNA, which also explains why it's a repressive modification."
Berger and Huang note that this is one of only a small number of studies to identify methylation as playing a role in regulating the activity of proteins that are not histones. Histones are relatively small proteins around which DNA is coiled to create structures called nucleosomes. Compact strings of nucleosomes, then, form into chromatin, the substructure of chromosomes.
With histones, methylation is well recognized as a regulatory mechanism, but the fact that other proteins are also be modified in the same way is a relatively new observation. Berger believes that scientists will likely find this type of regulatory mechanism at work in many other protein systems over the next few years.
Interestingly, only one other study has shown a role for methylation in regulating p53. In that study, a methyl group added to a specific site on p53 called K372 was shown to activate the tumor-suppressor molecule rather than repress it.
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