Structural biology of the Bcl-2 family of proteins

Abstract

The proteins of the Bcl-2 family are important regulators of programmed cell death. Structural studies of Bcl-2 family members have provided many important insights into their molecular mechanism of action and how members of this family interact with one another. To date, structural studies have been performed on six Bcl-2 family members encompassing both anti- (Bcl-x(L), Bcl-2, KSHV-Bcl-2, Bcl-w) and pro-apoptotic (Bax, Bid) members. They all show a remarkably similar fold despite an overall divergence in amino acid sequence and function (pro-apoptotic versus anti-apoptotic). The three-dimensional structures of Bcl-2 family members consist of two central, predominantly hydrophobic alpha-helices surrounded by six or seven amphipathic alpha-helices of varying lengths. A long, unstructured loop is present between the first two alpha-helices. The structures of the Bcl-2 proteins show a striking similarity to the overall fold of the pore-forming domains of bacterial toxins. This finding led to experiments which demonstrated that Bcl-x(L), Bcl-2, and Bax all form pores in artificial membranes. A prominent hydrophobic groove is present on the surface of the anti-apoptotic proteins. This groove is the binding site for peptides that mimic the BH3 region of various pro-apoptotic proteins such as Bak and Bad. Structures of Bcl-x(L) in complex with these BH3 peptides showed that they bind as an amphipathic alpha-helix and make extensive hydrophobic contacts with the protein. These data have not only helped to elucidate the interactions important for hetero-dimerization of Bcl-2 family members but have also been used to guide the discovery of small molecules that block Bcl-x(L) and Bcl-2 function. In the recently determined structure of the anti-apoptotic Bcl-w protein, the protein was also found to have a hydrophobic groove on its surface capable of binding BH3-containing proteins and peptides. However, in the native protein an additional carboxy-terminal alpha-helix interacts with the hydrophobic groove. This is reminiscent of how the carboxy-terminal alpha-helix of the pro-apoptotic protein Bax binds into its hydrophobic groove. This interaction may play a regulatory role and for Bax may explain why it is found predominately in the cytoplasm prior to activation. The hydrophobic groove of the pro-apoptotic protein, Bid protein, is neither as long nor as deep as that found in Bcl-x(L), Bcl-2, or Bax. In addition, Bid contains an extra alpha-helix, which is located between alpha1 and alpha2 with respect to Bcl-x(L), Bcl-2, and Bax. Although there are still many unanswered questions regarding the exact mechanism by which the Bcl-2 family of proteins modulates apoptosis, structural studies of these proteins have deepened our understanding of apoptosis on the molecular level.

A dynamic view of peptides and proteins in membranes

“…Such membrane structural arrangements of the protein helices agree with a model in which the lipids are part of the channel lining [60] analogous to the propositions made for amphipathic antimicrobial peptides [69]. The solution structures of several members of the Bcl-2 family, regardless of their pro-or anti-apoptotic activities, exhibit close relationships to those obtained from the channel-forming bacterial toxins discussed above [70]. Indeed channel activities have been measured for the anti-apoptotic Bcl-x L [71] and Bcl-2 proteins [72].…”

“…1B) as well as of the channel-forming bacterial toxins have been determined by X-ray diffraction and solution state NMR spectroscopy and represent the proteins in their soluble state, i.e., before insertion into the membrane. Their common globular fold is characterized by eight to ten a-helices arranged in a three-layered structure where the central layer is composed of a hydrophobic helical hairpin, which is surrounded by a layer of amphipathic helices on each side of the resulting sandwich [62]. During membrane insertion the colicin channel domains undergo pronounced conformational changes, adopt intermediate conformations, and finally insert into the membrane by refolding to better match the bilayer interfacial properties [63,64].…”

“…For example, the Cterminal domains of several colicins have been shown to insert into bilayers and to form voltage-gated channels in black lipid membranes, a process involving significant structural transitions [58 -60]. In solution the colicin pore-forming domains share structural similarities not only with each other but also with other bacterial toxins, such as proteins secreted in diphtheria, tetanus and botulinum, the Bacillus thuringensis d-toxin, or with the Bcl-2 family of proteins, the latter being key regulators of apoptosis [61,62]. The three-dimensional structures of a number of members of the Bcl-2 family (Fig.…”

Beyond Antiangiogenesis: Intratumorally Injected Bevacizumab Plays a Cisplatin-Sensitizing Role in Squamous Cell Carcinomas in Mice

“…The important proapoptosis protein Bcl-2 was chosen because the Bcl-2 family is the best-characterized group of apoptosis-mediating factors [35]. High expression of Bcl-2 is found in various human tumors.…”

Bad and Bid – potential background players in preneoplastic to neoplastic shift in human endometrium

“…Disruptions can occur at any stage of the apoptotic cascade. Disruption of the physiological apoptotic process is known to contribute to many pathological conditions such as developmental defects, neurodegenerative disorders, persistent infections, autoimmune responses and initiation of cancerogenesis [1,2].…”