In response to apoptotic stimuli, the pro-apoptotic protein Bax inserts in the outer mitochondrial membrane, resulting in the formation of pores and the release of several mitochondrial components, sealing the cell's fate. To study the binding of Bax to membranes, we recently used an in vitro system consisting of 50 nm diameter liposomes prepared with a lipid composition mimicking that of mitochondrial membranes in which recombinant purified full-length Bax was inserted via activation with purified tBid. We detected the association of the protein with the membrane using fluorescence fluctuation methods, and determined that at a high protein-to-liposome there were about 15 Bax proteins per 50 nm diameter liposome. We then obtained structural data using SANS, and observed that a significant amount of the protein mass protrudes above the membrane, in contrast to the conjecture that all of the membrane-associated Bax states are umbrella-like (Fig. 1). Upon protein binding, we also observed a thinning of the lipid bilayer accompanied by an increase in liposome radius, an effect reminiscent of the action of antimicrobial peptides on membranes. For details see: Satsoura et al., Biochim. Biophys. Acta 1818, 384 (2012)
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