Interaction with a membrane surface triggers a reversible conformational change in Bax normally associated with induction of apoptosis.
The Bcl-2 family member Bax is an apoptosis-promoting protein that normally resides in an inactive state within the cytoplasm of healthy cells. Upon induction of apoptosis by diverse stimuli, Bax undergoes a conformational change and translocates to mitochondria, where it oligomerizes and forms pores that allow the release of cytochrome c and other cytotoxic factors. Protein-protein interactions between Bax and other Bcl-2 family members are strongly implicated in Bax activation, but a compelling case has recently been made for the involvement of lipids in this process as well. Here we report that purified Bax undergoes a reversible conformational change upon incubation with lipid vesicles in the absence of other proteins. This Bax-liposome interaction does not depend on a specific lipid composition. Changes in Bax conformation were observed by immunoprecipitation with the conformation-specific antibody 6A7, circular dichroism spectroscopy, and differential scanning calorimetry. Although liposomes induced Bax to become 6A7-reactive (a feature normally associated with the onset of apoptosis), the protein did not insert into membranes, become oligomeric, or form pores, clearly indicating that other triggers are required for Bax to achieve its final pro-apoptotic state. Indeed, the lipid-induced Bax conformational change is shown to be required for tBid-induced Bax oligomerization and pore formation, putting it upstream of tBid activity in this molecular pathway to Bax activation. These data demonstrate that Bax is sensitized to activation by transient interaction with lipid membrane surfaces and provide evidence that Bax activation proceeds in a stepwise fashion, with multiple triggers and potential levels of regulation.
Yethon
Jeremy A
JA
Department of Biochemistry, McMaster University, Hamilton, Ontario L8N 3Z5, Canada.
Epand
Raquel F
RF
Leber
Brian
B
Epand
Richard M
RM
Andrews
David W
DW
D017209:Apoptosis / Q000502:physiology*; D002462:Cell Membrane / Q000378:metabolism; D002942:Circular Dichroism; D006801:Humans; D050356:Lipid Metabolism; D008081:Liposomes; D011487:Protein Conformation; D011518:Proto-Oncogene Proteins / Q000737:chemistry / Q000378:metabolism* / Q000502:physiology; D019253:Proto-Oncogene Proteins c-bcl-2; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D051028:bcl-2-Associated X Protein
C494826:BAX protein, human; D008081:Liposomes; D011518:Proto-Oncogene Proteins; D019253:Proto-Oncogene Proteins c-bcl-2; D011994:Recombinant Proteins; D051028:bcl-2-Associated X Protein