Poster Presentation Melbourne Protein Group Student Symposium 2013

Structural Investigations into the Control of Pro-Apoptotic Bcl-2 Family Proteins (#21)

Angus D Cowan 1
  1. The Walter and Eliza Hall Institute, Parkville, VIC, Australia

The Bcl-2 protein family regulates the intrinsic pathway of apoptosis. Proteins in this family are defined by the presence of at least one of four Bcl-2 homology (BH) domains and function either in a pro-apoptotic or pro-survival manner. Pro-survival family members sequester pro-apoptotic relatives by binding the amphipathic BH3 helix of the pro-apoptotic protein within a hydrophobic surface groove on the pro-survival protein. BH3 helices also play important roles in activation of critical pathway effectors Bax and Bak, both of which oligomerise on the mitochondrial outer membrane (MOM) to permeabilise it, releasing cytochrome c which initiates apoptosis1 . Another subfamily, called BH3-only proteins, are directly or indirectly responsible for Bax and Bak activation2 . Current understanding of the molecular interactions occurring during sequestration and activation of Bax and Bak is based on bound BH3 peptides rather than whole protein-protein or protein domain interactions. Other molecular interactions that may occur within these complexes are yet to be characterized and could lead to the discovery of drug targets for cancer, autoimmune and degenerative disease therapy. Bax and Bak oligomerisation on the MOM is another poorly understood process. Recently, a potential membrane interface on Bax has been identified3 , investigation of which may further our understanding of how Bax binds to and permeabilises this compartment.

Bok is a third potential effector protein that was originally thought to function by permeabilising the MOM like Bax and Bak. However, recent data suggest it cannot permeabilise the MOM in the absence of Bax and Bak, but may instead exert its function at the endoplasmic reticulum and Golgi apparatus4 . The exact function of Bok remains undefined despite the fact it was discovered 16 years ago5 . Molecular studies of Bok have previously been intractable due to a lack of suitable recombinant expression systems. Successful expression and purification of recombinant Bok would allow for functional and structural studies and further our understanding of this poorly understood Bcl-2 protein family member.

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  3. Czabotar, P.E., Westphal, D., Dewson, G., Ma, S., Hockings, C., Fairlie, W.D., Lee, E.F., Yao, S., Robin, A.Y., Smith, B.J., Huang, D.C.S., Kluck, R.M., Adams, J.M., Colman, P.M. Bax crystal structures reveal how BH3 domains activate Bax and nucleate its oligomerization to induce apoptosis. Cell 152, 519–531 (2013).
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  5. Hsu, S.Y., Kaipia, A., McGee, E., Lomeli, M., and Hsueh, A.J. Bok is a pro-apoptotic Bcl-2 protein with restricted expression in reproductive tissues and heterodimerizes with selective anti-apoptotic Bcl-2 family members. Proc Natl Acad Sci USA 94, 12401–12406 (1997)