BAX protein plays a key role in the mitochondria-mediated apoptosis. However, it 
remains unclear by what mechanism BAX is triggered to initiate apoptosis. Here, 
we reveal the mechanism using electron spin resonance (ESR) techniques. An 
inactive BAX monomer was found to exhibit conformational heterogeneity and exist 
at equilibrium in two conformations, one of which has never been reported. We 
show that upon apoptotic stimulus by BH3-only peptides, BAX can be induced to 
convert into either a ligand-bound monomer or an oligomer through a 
conformational selection mechanism. The kinetics of reaction is studied by means 
of time-resolved ESR, allowing a direct in situ observation for the 
transformation of BAX from the native to the bound states. In vitro 
mitochondrial assays provide further discrimination between the proposed BAX 
states, thereby revealing a population-shift allosteric mechanism in the 
process. BAX's apoptotic function is shown to critically depend on excursions 
between different structural conformations.