Earlier in vivo studies of the large protein family were stymied by the number of family members, and in vitro assays were inconclusive. The Newmeyer group has now successfully developed a minimal cell-free membrane model that behaves like mitochondrial outer membranes (OMMs). Adding recombinant Bid and Bax to the membranes allowed the exit of molecules over 100 times bigger than cytochrome c. With further minimalism, the authors showed that Bid and Bax also released large molecules from protein-free liposomes, as long as the liposomes contained cardiolipin, a signature mitochondrial lipid. Thus, Bax, Bid, and cardiolipin seem to be all a cell needs to induce mitochondria to spill their death-inducing guts. “That we can permeabilize membranes with Bid and Bax alone means we don't need to invoke any more complicated mechanism,” says Newmeyer.
The mechanism of hole formation is probably not simple, however. Tetramers of Bax were sufficient for the release of the macromolecules, but a Bax tetramer would not form a conventional protein channel of adequate size. Newmeyer hypothesizes that the insertion of Bax into the OMM may cause a rearrangement in or change the curvature of lipids, particularly cardiolipin, such that Bax and the lipids might both line the newly made opening. ▪