Dear Fadwa,

I have read an article entitled " Cytosolic Bax DOES IT REQUIRE BINDING PROTEINS TO KEEP ITS PRO-APOPTOTIC ACTIVITY IN CHECK?" by Vogel et al. published in J Biol Chem. 2012 Mar 16; 287(12): 9112–9127 and found that the paper's introduction has some important information which may cover the answer to your question.

Herein some of the important text depicted in the article's introduction. I have highlighted the important sentenced with bold,

It has become widely accepted that the pro-apoptotic members of the Bcl-2 family, Bax and Bak, are crucial for the permeabilization of the outer mitochondrial membrane (MOMP),4 a prerequisite of the release of apoptogenic factors such as cytochrome c from the intermembrane space and a point-of-no-return for caspase-dependent and -independent apoptotic processes (1). Whereas Bak is an integral MOM protein inhibited by pro-survival Bcl-2 proteins (2, 3) and/or voltage-dependent anion channel-2 (4, 5), Bax is soluble and resides inactive in the cytosol or loosely attached to mitochondria (6, 7). In response to various apoptotic stimuli, both Bax and Bak undergo conformational changes (8–10), oligomerize (11, 12), and form proteinaceous (13, 14) or lipidic pores (15, 16) through yet unknown mechanisms (1). These conformational changes occur in the MOM and require a tight cooperation between Bax or Bak, BH3-only proteins, and the lipid bilayer. The current model postulates that some BH3-only proteins such as Bid, Bim, and Puma rapidly translocate to the MOM after their synthesis and/or posttranslational modification (17–19) and then serve as activators of Bax and Bak (20–25). As Bak is already an inserted protein, the role of Bid, Bim, and Puma is probably to disengage Bak from its inhibitors (2, 20), allowing its conformational change and oligomerization. However, Bax needs to first translocate from the cytosol to mitochondria and then to insert into the MOM before activation can take place (26, 27). It is thought that Bid, Bim, and Puma may act as MOM receptors of Bax for this process (21–25). Moreover, components of the TOM complex and endophilin B1/Bif-1 appear to fine-tune Bax insertion and oligomerization (28–30). Recently, Montessuit et al. (16) reported that the fission protein Drp1 further assists Bax oligomerization on the MOM through a GTPase-independent mechanism, probably involving a hemifusion-like membrane remodeling process.

To view the full paper, please use the following link:

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3308772/

Hoping this will be helpful,

Rafik

Take a look at this article. It may help you

http://www.sciencedirect.com/science/article/pii/S0014482702955862