Abstract
An NMR-based fragment screen identified a BAX-interacting compound, BIF-44, that enhances BAX activity.
Major finding: An NMR-based fragment screen identified a BAX-interacting compound, BIF-44, that enhances BAX activity.
Mechanism: BIF-44 binds to a deep hydrophobic pocket to induce conformation changes that sensitize BAX activation.
Impact: Allosteric BAX sensitization may represent a therapeutic strategy to promote apoptosis of cancer cells.
The proapoptotic BAX protein is comprised of nine α-helices (α1–α9) and is a critical regulator of the mitochondrial apoptosis pathway. In the conformationally inactive state, BAX is primarily cytosolic and can be activated by BH3-only activator proteins, which bind to ab α6/α6 “trigger site” to induce a conformational change that activates BAX and promotes its oligomerization. Conversely, antiapoptotic BCL2 proteins or the cytomegalovirus vMIA protein can bind to and inhibit BAX. Efforts to therapeutically enhance apoptosis have largely focused on inhibiting antiapoptotic proteins. Pritz, Wachter, and colleagues took the opposite approach and sought to identify compounds that directly activate the proapoptotic BAX protein to potentially promote cancer cell death. An NMR spectroscopy–based fragment screen to discover molecules that interact with full-length human BAX identified the BAX-interacting compound BIF-44. BIF-44 bound specifically to BAX and enhanced BAX activation triggered by the α-helical BH3 motif of the BIM protein. BIF-44 bound competitively to the same region as vMIA, in a deep hydrophobic pocket formed by the junction of the α3–α4 and α5–α6 hairpins that normally maintain BAX in an inactive state. Binding of BIF-44 induced a structural change that resulted in allosteric mobilization of the α1–α2 loop, which is involved in BH3-mediated activation, and the BAX BH3 helix, which is involved in propagating BAX oligomerization, resulting in sensitization of BAX activation. In addition to identifying a BAX allosteric sensitization site and mechanism of conformational regulation of BAX, these findings suggest the potential for therapeutic enhancement of BAX activity to reduce the apoptotic threshold and promote apoptosis of cancer cells.
Note: Research Watch is written by Cancer Discovery editorial staff. Readers are encouraged to consult the original articles for full details. For more Research Watch, visit Cancer Discovery online at http://cancerdiscovery.aacrjournals.org/content/early/by/section.
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