Caspase Cofactor

Activation of the caspases requires the help of a number of cofactors that are also known as activators or adaptors. Different cofactors are involved depending on the trigger mechanism of caspase activation. A central function of the cofactors is to bring about aggregation and thus activation of the procaspases. This occurs by specific protein-protein interactions with the help of common structural motives. Examples of such motives are the death domains (DD), death effector domains (DED) and the caspase recruitment domains (CARD), which all have a similar structure of six a-heh-ces. 

The Apafl protein has been identified as a central cofactor in cytotoxically-triggered apoptosis. The Apafl protein is homologous to Ced4 protein of C. elegans. It binds to the initiator caspase with the help of a CARD motif A CARD motif is found in the Apafl protein and in various caspases (caspases 1, 2, 4, 5 and 9). 

A further cofactor in activation in this system is cytochrome c. The ATP-dependent activation occins in a complex of caspase, Apafl protein and Cyt c, which is also known as the apoptosome. 

The abihty of the cofactors to activate caspases is regulated by a number of other proteins that appear to interact directly with the cofactors. 

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. 

Binding of the ligand of the Fas receptor triggers clustering of the receptor and association of the cofactor FADD (fas-assodated protein with death domain) which interacts with the receptor via its death domain (DD). Procaspase 8 binds to FADD via a common DED (death effector domain) motif and is thereby also recruited into the Fas-receptor associated complex. Due to the clustering of the proteins, proximity-induced cleavage of procaspase 8 to the mature initiator caspase 8 takes place. This activates the effector caspases and triggers cell death. 

Following the outer mitochondrial membrane permeabilization, the apoptogenic factors are released into the cytoplasm. Among them, cytochrome c has an important role in caspase activation, because it is the cofactor for assembling a large caspase 9 activating complex in the cytoplasm, called apoptosome. Along with cytochrome c, the Apaf-1 protein and dATP or ATP are required to form this complex in the cytoplasm (Hill et al., 2003). 

Two structural abnormahties in the mitochondria are considered important pathogenetic factors during ischemia. One is characterized by pore formation in the itmer mitochondrial membrane and high amplitude swelling (mitochondrial permeability transition or MPT) [30, 31]. The second involves leakage of cytochrome C from the inter-membrane space into the cytosol [32]. Because of its role as an electron shuttle, dislocation of cytochrome c compromises respiration [33, 34], and as a cytosolic cofactor cytochrome C activates caspase 9, and triggers apoptosis [33-35] (see below). 

Recruitment of the initiator procaspases into a multiprotein complex results from a regulated series of protein-protein interactions mediated by interaction modules . Four types of interaction modules are involved in the activation of initiator caspases and thus play important roles in the initiation of apoptosis (review Weber and Vin-cenz, 2001). These domains have been named the death domain (DD),, the death effector domain (DED), the caspase activation and recruitment domain (CARD), and the less characterized pyrin domain. The domains are found on several components of the apoptotic signaling pathways and mediate homotypic protein-protein interactions, i. e., a given module will interact only with a member of the same family and not with members of the other families. Since members of the same module are found on different proteins, these modules mediate the assembly of hetero-oligomeric protein complexes. As examples, DDs are found on death receptors and their cofactors, D EDs on cofactors and the initiator caspase-8, and CARDS on cofactors, caspase-2, and caspase-9. 

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