Bid-induced Conformational Change of Bax Is Responsible for Mitochondrial Cytochrome c Release during Apoptosis

Abstract

Here we report that in staurosporine-induced apoptosis of HeLa cells, Bid, a BH3 domain containing protein, translocates from the cytosol to mitochondria. This event is associated with a change in conformation of Bax which leads to the unmasking of its NH2-terminal domain and is accompanied by the release of cytochrome c from mitochondria. A similar finding is reported for cerebellar granule cells undergoing apoptosis induced by serum and potassium deprivation. The Bax-conformational change is prevented by Bcl-2 and Bcl-xL but not by caspase inhibitors. Using isolated mitochondria and various BH3 mutants of Bid, we demonstrate that direct binding of Bid to Bax is a prerequisite for Bax structural change and cytochrome c release. Bcl-xL can inhibit the effect of Bid by interacting directly with Bax. Moreover, using mitochondria from Bax-deficient tumor cell lines, we show that Bid- induced release of cytochrome c is negligible when Bid is added alone, but dramatically increased when Bid and Bax are added together. Taken together, our results suggest that, during certain types of apoptosis, Bid translocates to mitochondria and binds to Bax, leading to a change in conformation of Bax and to cytochrome c release from mitochondria.

Role of cardiolipin on tBid and tBid/Bax synergistic effects on yeast mitochondria

“…Since the proapoptotic action of tBid is thought to involve the activation of Bax towards a conformation changeable to trigger the release of cytochrome c, 30 the role of CL on this activation was investigated. It clearly appeared that tBid was actually able to activate the cytochrome c-release activity of Bax, and that the absence of CL further amplified this stimulating effect of tBid.…”

“…Uncleaved Bid can also move to mitochondria via a still unclear mechanism, but it remains inactive. 30 A new hypothesis 31 established that tBid interacts with CL at contact sites and that this disturbs the electron transport chain enough to inhibit state-3 respiration in association with a mild uncoupling of the state-4 respiration. These damages may also be linked to superoxide anion production, 32 leading to modification of the mitochondrial membrane composition and favor Bax or Bak oligomerization and cytochrome c release.…”

BH3-only proteins and their roles in programmed cell death

“…Indications that certain BH3-only proteins might trigger the activation of Bax and Bak came from several observations that the activated form of Bid, truncated Bid, could induce conformational changes in either Bax or Bak (Desagher et al, 1999;Wei et al, 2000). Using a large array of synthetic BH3 peptides, two groups later showed that some of these peptides (Bid, Bim and Puma) could directly induce oligomerization of Bax and Bak to release cytochrome c, whereas the others could not (Letai et al, 2002;Kuwana et al, 2005;Kim et al, 2006b).…”

Mitochondria and cancer: is there a morphological connection?

“…Following a death signal, the proapoptotic proteins Bax and Bak undergo a conformational change, and their N terminus becomes exposed. Bax, which is usually in the cytosol or loosely associated with mitochondria, translocates to the mitochondria, oligomerizes and inserts in the OMM causing MMP (Hsu et al, 1997;Desagher et al, 1999;Antonsson et al, 2000). These events can be promoted by tBid, the activated form of Bid, and inhibited by Bcl2 and Bcl-x L Eskes et al, 2000;Cheng et al, 2001).…”