How does the Bax-α1 targeting sequence interact with mitochondrial membranes? The role of cardiolipin

A key event in programmed cell death is the translocation of the apoptotic Bax protein from the cytosol towards mitochondria. The first helix localized at the N-terminus of Bax (Bax-α1) can act here as an addressing sequence, which directs activated Bax towards the mitochondrial surface. Solid state NMR (nuclear magnetic resonance), CD (circular dichroism) and ATR (attenuated total reflection) spectroscopy were used to elucidate this recognition process of a mitochondrial membrane system by Bax-α1. Two potential target membranes were studied, with the outer mitochondrial membrane (OM) mimicked by neutral phospholipids, while mitochondrial contact sites (CS) contained additional anionic cardiolipin. 1H and 31P magic angle spinning (MAS) NMR revealed Bax-α1 induced pronounced perturbations in the lipid headgroup region only in presence of cardiolipin. Bax-α1 could not insert into CS membranes but at elevated concentrations it inserted into the hydrophobic core of cardiolipin-free OM vesicles, thereby adopting β-sheet-like features, as confirmed by ATR. CD studies revealed, that the cardiolipin mediated electrostatic locking of Bax-α1 at the CS membrane surface promotes conformational changes into an α-helical state; a process which seems to be necessary to induce further conformational transition events in activated Bax which finally causes irreversible membrane permeabilization during the mitochondrial apoptosis.