Credit: A.T. Tikhonenko
A new study reveals how the protein p53 attaches to its regulatory molecule, BCL-xL.
Understanding how these molecular puzzle pieces fit together could help scientists design drugs that would unleash p53 to battle a range of cancers, according to study author Richard Kriwacki, PhD, of St Jude Children’s Research Hospital in Memphis, Tennessee.
He and his colleagues described this research in Nature Structural & Molecular Biology.
In guarding the cell against genetic damage, the p53 machinery functions both in the nucleus of the cell and in the cytosol. When this machinery detects irreparable damage to the cell, p53 is unleashed to trigger apoptosis.
In about half of all cancers, this machinery is rendered inoperable by mutation, enabling cancer cells to proliferate despite their genetic malfunctions. The protein BCL-xL is a central inhibitor of the p53 machinery, binding both p53 and BH3 proteins, which also drive apoptosis.
“The molecular details of how BCL-xl performs this dual inhibitory function were not understood,” Dr Kriwacki said. “Having those details has enabled us to determine exactly how BCL-xL can restrain or inhibit apoptosis through interactions with BH3-domain-containing proteins, as well as p53.”
He and his colleagues used a structural analysis technique called NMR spectroscopy to map the 3-D structure of p53 binding to BCL-xL.
Their experiments revealed how the DNA-binding domain of the p53 protein serves double duty in the machinery. It enables p53 to attach to DNA in the cell’s nucleus, helping the cell repair genetic damage. The same domain also acts as an attachment point for BCL-xL in the cytosol.
“The structural details that we report are novel,” Dr Kriwacki said. “And they provide the key insights for really dissecting the dual roles of BCL-xL in inhibiting apoptosis . . ., inhibiting the BH3-containing proteins on the one side and p53 on the other. Also, through these studies, we solidified the mechanistic understanding for how p53 functions in the cytosol, which complements its pro-apoptotic role in the nucleus.”
Dr Kriwacki added that these findings could help scientists design better anticancer agents. In many cancers, p53 is prevented from triggering apoptosis by its attachment to BCL-xL.
Drugs are currently being tested that bind to BCL-xL to free BH3 proteins to trigger apoptosis. However, Dr Kriwacki said new drugs could be developed that also block BCL-xL from binding p53.
“Our hypothesis is that many cancers have normal p53, but it is being tied up by BCL-xL,” he said. “If it could be released, it could play its role in triggering apoptosis. A drug that could block both of BCL-xL’s anti-apoptotic functions could potentially more profoundly induce apoptosis in cancer cells.”
