Allosteric regulation of BH3-in-groove interactions by tail anchors of BCL-xL complexes limits BH3 mimetic antagonism.

BCL-xL exerts an essential cell survival function which relies on its hydrophobic groove binding to BH3 domain of BH3-only initiators and downstream BAX and BAK executioners. Combining resonance energy transfer assays and molecular dynamics simulations, we unravel that the C-terminal tail mediated subcellular membrane anchoring of BCL-xL selectively advantages binding to membrane-anchored PUMA initiator over BH3 mimetic ligands of the groove. This is due to the combined allosteric effect on BH3-in-groove binding of BCL-xL and PUMA tail anchors. Moreover, doubly anchored PUMA / BCL-xL complexes recruit endogenous BAX, which favors their antagonism by BH3 mimetics. BAX C-terminal tail anchor alone is sufficient to enhance BH3 mimetics induced death in cells expressing PUMA / BCL-xL. Thus, the survival function of BCL-xL is regulated by a complex interplay between its tail anchor and those of its interacting partners. This enables both resistance to pharmacological inhibitors and modulation by BAX, which functions as a crucial feedback disruptor of the BCL-xL network.