Full anti-apoptotic function of Bcl-XL complexed with Beclin-1 verified by live-cell FRET assays

Binding of Bcl-XL to Beclin-1 reduces Beclin-1's capacity to induce autophagy. This report aims to explore whether this interaction affects Bcl-XL's anti-apoptotic function. Using fluorescence resonance energy transfer (FRET) two-hybrid assay to quantify the stoichiometry of Bcl-XL-Beclin-1 complex in living cells coexpressing Bcl-XL-CFP and Beclin-1-YFP, we showed that Bcl-XL bond to Beclin-1 to form hetero-oligomers whose stoichiometry increases from 1:1 to 2:1 or higher with the increasing relative expression level of Bcl-XL, indicating the multiple binding sites of Beclin-1 with Bcl-XL. Co-expression of Bcl-XL and Beclin-1 exhibited consistent anti-apoptotic ability against staurosporine (STS)-induced apoptosis with expression of Bcl-XL alone irrespective of the relative expression level between Beclin-1 and Bcl-XL. Collectively, Bcl-XL complexed with Beclin-1 maintains full anti-apoptotic ability independent of the stoichiometry of Bcl-XL-Beclin-1 complex.