Proapoptotic protein

During the process of cell death, cytochrome c is released from mitochondria into the cytosol where it assists to activate the caspases, a family of killer caspases that trigger cell death, hi this chapter evidence that transmission of cell death signals into the release of cytochrome c involves phospholipids at several stages will be presented. Thus, phospholipids target proapoptotic proteins to mitochondria, enable these or other proteins to form channels or pores and to break the mitochondrial permeability barrier. Finally, peroxidation or increased levels of disrupt the ability of cardiolipin to interact with cytochrome c, initiating a sequence of events that ultimately lead to cell death. 

Proapototic signals direct these proteins to mitochondria where they compete with antiapoptotic members of the Bcl-2 family to regulate the cytochrome c release and to determine the fate of the cell life or death (Cosulich et al., 1999). Unlike proapoptotic proteins, the antiapoptotic Bcl-2 proteins reside in the outer mitochondrial membrane, anchored by a hydrophobic stretch of amino acids located at the COOH-termini,... 

Figure 3. Possible mechanisms of actions of Bcl-2 members. Two prevailing models through which Bcl-2 membas trigger cytochrome c release have been suggested. In both models phospholipids in the bilayer stnicture either individually and/or collectively induce a conformational change in Bcl-2 members, allowing them to insert into the outer mitochondrial membrane. In model 1 proapoptotic proteins destabilize the outer mitochondrial membrane, oligomerize and form channels through which cytochrome c and other proteins of the intermembrane space can escape.BcI-2 proteins such as Bax or tBid act in concert with other proteins of the BcI-2 family to form channels. In model 2 Bcl-2 members such as Bax interact with residoit proteins in the outer membrane (OM) such as the voltage-dependent anion...

It has been well established that Bcl-2 prevents most forms of apoptotic cell death as well as certain forms of necrotic cell death (T6). A large number of Bcl-2-related proteins have been isolated and divided into three categories (T6) (1) antiapoptotic members such as Bcl-2, BcI-Xl, Bcl-w, Mcl-l, A1 (Bfl-1), and Boo, all of which exert anti-cell death activity (2) proapoptotic members including Bax, Bak, Bad, Mtd, and Diva and (3) proapoptotic proteins, which include Bik, Bid, Bim, Hrk, Blk, Bnip3, and Bnip3L and share sequence homology only in Bcl-2 homology (BH)3. 

The action of Bcl-2-related proteins constitutes a major life-or-death decision of the mitochondria that depends on the ratio of the antiapoptotic and proapoptotic proteins (Fig. 1). It has been shown that mitochondria undergo a transition of permeability characterized by a breakdown of membrane potential followed by an outflow of AIF and cytochrome C into the cytoplasm. It seems that Bcl-2 and Bc1-Xl act against this permeability transition, whereas Bax and Bak suppress this antiapoptotic activity of Bcl-2 and Bc1-Xl (T6). 

The biochemical basis of the various activities of the Bcl-2 family members is only partially imderstood. The antiapoptotic Bcl-2 proteins may function by directly inhibiting the activation of the caspases (see 15.3.3). It is assumed that proapoptotic proteins interact with antiapoptotic proteins and halt their inhibition of apoptosis. 

The antiapoptotic Bcl-2 family members control apoptosis by various mechanisms without directly binding to the caspases. Bcl-2 proteins can interact with cofactors and inhibit their activity. They can also act antiapoptotically by binding to mitochondria and interfering with release of cytochrome c. Furthermore, they can interact with other proapoptotic proteins, e.g. with propapoptotic members of their own family. 

In this model, cellular stress mediates the release of cytochrome C from the mitochondrion. The proapoptotic proteins Bax and BH3 proteins support the release of cytochrome C, while the antiapoptotic Bcl2 protein has an inhibitory effect. Cytosolic cytochrome C binds to the cofactor Apaf 1, which then associates via its CARD motif with procaspase 9 to a complex termed apopto-some. In this complex, procaspase 9 is processed to the mature caspase which subsequently activates downstream effector caspases and commits the cell to death. 

The target of the caspase proteins are nuclear, regulatory, and cytoskeleton proteins, whose degradation triggers the apoptotic response. The proteins of the bcl gene family modulate caspase activity and are divided into proapoptotic and anti-apoptotic fractions (see Figure 11.3). The BCL-associated death (BAD) protein is one of the proapoptotic proteins, whereas BCL-2 is one of the antiapoptotic proteins. The BCL proteins that interact with the caspases to determine whether apoptosis occurs are themselves phosphorylated by PI-3K. 

Proapoptotic proteins can be further classified according to their BH domains (Figure 7.5). Some members, such as Bax and Bak, contain multiple domains and others, like Bid, Bad, Bim, and Bmf, contain only the BH3 domain (Gross et al., 1999 Kuwana and Newmeyer, 2003). These structural differences also reflect differences in their function. Multi-domain proteins directly induce outer mitochondrial membrane permeabi-... 

The exact manner of how proapoptotic proteins induce the release of apoptogenic factors from the mitochondrial intermembrane space into the cytoplasm and how antiapoptotic proteins prevent it remains obscure. 

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