Bid-induced conformational change of Bax is responsible for mitochondrial cytochrome c release during apoptosis

Here we report that in staurosporine-induced apoptosis of HeLa cells, Bid, a BH3 domain containing protein, translocates from the cytosol to mitochond ria. This event is associated with a change in conformation of Bar which le ads to the unmasking of its NH2-terminal domain and is accompanied by the r elease of cytochrome c from mitochondria. A similar finding is reported for cerebellar granule cells undergoing apoptosis induced by serum and potassi um deprivation. The Bar-conformational change is prevented by Bcl-2 and Bcl -x(L) but not by caspase inhibitors. Using isolated mitochondria and variou s BH3 mutants of Bid, we demonstrate that direct binding of Bid to Bar is a prerequisite for Bar structural change and cytochrome c release. Bcl-x(L) can inhibit the effect of Bid by interacting directly with Bar. Moreover, u sing mitochondria from Bax-deficient tumor cell lines, we show that Bid-ind uced release of cytochrome c is negligible when Bid is added alone, but dra matically increased when Bid and Bar are added together. Taken together, ou r results suggest that, during certain types of apoptosis, Bid translocates to mitochondria and binds to Bar, leading to a change in conformation of B ar and to cytochrome c release from mitochondria.