Dynamic regulation of the Bcl-xL-BAD interaction

Abstract Background The interaction between the anti-apoptotic protein Bcl-xL and the BH3-only sensitizer BAD represents a critical regulatory checkpoint in the intrinsic apoptotic pathway. Although this interaction is known to influence mitochondrial fate, its dynamic regulation and structural determinants in living cells remain poorly understood. Here, we developed a fluorescence lifetime imaging microscopy-based Förster resonance energy transfer (FLIM-FRET) platform to visualize and quantify Bcl-xL-BAD interactions in real-time. Methods We developed a quantitative fluorescence lifetime-based FRET (FLIM-FRET) approach to visualize and measure Bcl-xL-BAD interactions in single living glioblastoma cells. Stable GFP/Venus-Bcl-xL and mCherry-BAD FRET pairs were created, followed by acceptor photobleaching FRET, FLIM-FRET, Annexin V-BFP-based apoptosis assays, pharmacological perturbation using BH3 mimetics, and molecular dynamics simulations with MM/GBSA analysis. Statistical significance was assessed using appropriate parametric tests across multiple independent experiments. Results Using this platform, we observed that apoptotic stress markedly enhances the engagement of Bcl-xL and BAD. Increased FRET efficiency coincided with Annexin V positivity and nuclear condensation, indicating that maximal BAD binding reflects a higher level of apoptotic commitment. Structure-function analysis using targeted Bcl-xL mutants revealed distinct binding requirements: disruption of the core hydrophobic groove (Y101K) abolished BAD binding and impaired BH3 mimetic sensitivity, whereas mutation within the BH1 domain (G138A) preserved BAD interaction and sensitivity to BH3 mimetics. Molecular dynamics simulations corroborated these observations by revealing preserved BAD-binding energetics in the G138A mutant, but destabilization in the Y101K mutant. Conclusions Together, these findings demonstrate the utility of a live-cell FLIM-FRET platform for resolving protein-protein interactions involving apoptotic proteins at the single-cell level. By linking interaction dynamics, structural determinants, and functional outcomes, this approach provides a broadly applicable framework for studying apoptotic priming, structural tolerance at BCL-2 family interfaces, and cellular responses to BH3-mimetic therapies.