Proapoptotic BAX protein is largely cytosolic in healthy cells, but it oligomerizes and translocates to mitochondria upon receiving apoptotic stimuli. A long-standing challenge has been the inability to capture any structural information beyond the onset of activation. Here, we present solution structures of an activated BAX oligomer by means of spectroscopic and scattering methods, providing details about the monomer-monomer interfaces in the oligomer and how the oligomer is assembled from homodimers. We show that this soluble oligomer undergoes a direct conversion into membrane-inserted oligomer, which has the ability of inducing apoptosis and structurally resembles a membrane-embedded oligomer formed from BAX monomers in lipid environment. Structural differences between the soluble and the membrane-inserted oligomers are manifested in the C-terminal helices. Our data suggest an alternative pathway of apoptosis in which BAX oligomer formation occurs prior to membrane insertion.

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.

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Spin-labeled nanodiscs improve DEER distance measurement of membrane proteins.

The apoptotic function of Bid does not depend on its native structure.