Few family dramas have fascinated biologists like the one playing out among the BCL-2 group of proteins. Some family members, such as BAX, promote apoptosis while others prevent it, and they do so by trapping their destructive cousins in a groove located on their surface. Some researchers have assumed that it is only when BAX is released from this groove that it can be activated. Indeed, efforts to harness BAX for the purpose of killing cancer cells have focused on keeping or pulling it out of the anti-apoptotic proteins’ pocket.
Loren Walensky, Evripidis Gavathiotis, and colleagues have discovered a new binding site on BAX, one that can trigger its killer activity directly. The finding, reported in the Oct. 23 Nature, supports an expanded view about how BAX is activated.
The view was first put forth several years ago by the late Stanley Korsmeyer, who suggested that another set of BCL-2 family members might be activating BAX directly. These proteins, which include BIM and BID, are known to respond to various stresses on the cell by promoting apoptosis. Korsmeyer thought they might be doing so by touching and activating BAX, but it was not clear if proteins like BIM could actually bind BAX.
Part of the problem is that BIM’s key binding subunit, an alpha-helical peptide, loses its normal shape once separated from the rest of the protein. Employing a method developed by Harvard University’s Gregory Verdine, Walensky, HMS assistant professor of pediatrics at Dana–Farber Cancer Institute, and colleagues stapled the peptide into its natural helical shape and, in 2006, reported that some of the stapled peptides bound BAX. But it was not clear where the peptides were binding.
Gavathiotis, research fellow in pediatrics at DFCI, Walensky, and colleagues, working with researchers at the National Institutes of Health, conducted nuclear magnetic resonance (NMR) studies and were able to identify the binding site. They went on to solve the structure of the bound complex, using a specialized method, paramagnetic relaxation enhancement (PRE) NMR. To see if the binding was linked to BAX activation, they mutated the binding site at various spots, both in vitro and in intact cells. Under both conditions, they found BAX’s ability to carry out its death-inducing activities was greatly impaired.
“We’ve now determined how BAX is directly triggered,” said Walensky, adding that the trigger might be manipulated to combat a variety of diseases. “In the case of premature cell death that occurs in stroke, heart attack, or neurodegeneration, where you want to turn the death process off in a cell, you would want to prevent access to the trigger site. To kill unwanted cells as in cancer, you would want to mobilize BAX by deploying a compound that directly binds and activates the trigger site.”
For Students: Contact Loren Walensky at Loren_Walensky@dfci.harvard.edu for further information about this and other lab projects.
Conflicts of interest: L.D.W. is a scientific advisory board member and consultant for Aileron Therapeutics, Inc.
Funding Sources: The National Cancer Institute, the Burroughs Wellcome Fund, the Goldman Philanthropic Partnerships, the American Society of Hematology the William Lawrence Children’s Foundation, the National Heart, Lung, and Blood Institute, and the Searle Scholars Program
