title: After embedding in membranes antiapoptotic Bcl-XL protein binds both Bcl-2 homology region 3 and helix 1 of proapoptotic Bax protein to inhibit apoptotic mitochondrial permeabilization.
abstract: Bcl-XL binds to Bax, inhibiting Bax oligomerization required for mitochondrial outer membrane permeabilization (MOMP) during apoptosis. How Bcl-XL binds to Bax in the membrane is not known. Here, we investigated the structural organization of Bcl-XL·Bax complexes formed in the MOM, including the binding interface and membrane topology, using site-specific cross-linking, compartment-specific labeling, and computational modeling. We found that one heterodimer interface is formed by a specific interaction between the Bcl-2 homology 1-3 (BH1-3) groove of Bcl-XL and the BH3 helix of Bax, as defined previously by the crystal structure of a truncated Bcl-XL protein and a Bax BH3 peptide (Protein Data Bank entry 3PL7). We also discovered a novel interface in the heterodimer formed by equivalent interactions between the helix 1 regions of Bcl-XL and Bax when their helical axes are oriented either in parallel or antiparallel. The two interfaces are located on the cytosolic side of the MOM, whereas helix 9 of Bcl-XL is embedded in the membrane together with helices 5, 6, and 9 of Bax. Formation of the helix 1·helix 1 interface partially depends on the formation of the groove·BH3 interface because point mutations in the latter interface and the addition of ABT-737, a groove-binding BH3 mimetic, blocked the formation of both interfaces. The mutations and ABT-737 also prevented Bcl-XL from inhibiting Bax oligomerization and subsequent MOMP, suggesting that the structural organization in which interactions at both interfaces contribute to the overall stability and functionality of the complex represents antiapoptotic Bcl-XL·Bax complexes in the MOM.
authors.affiliation: Department of Biochemistry and Molecular Biology, University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma 73126.
authors.forename: Jingzhen
authors.initials: J
authors.lastname: Ding
authors.affiliation: Department of Biochemistry and Molecular Biology, University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma 73126; Peggy and Charles Stephenson Cancer Center, University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma 73126.
authors.forename: Blaine H M
authors.initials: BHM
authors.lastname: Mooers
authors.affiliation: Department of Biochemistry and Molecular Biology, University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma 73126.
authors.forename: Zhi
authors.initials: Z
authors.lastname: Zhang
authors.affiliation: Department of Biochemistry and Biomedical Sciences, McMaster University, Hamilton, Ontario L8N 3Z5, Canada.
authors.forename: Justin
authors.initials: J
authors.lastname: Kale
authors.affiliation: Department of Biochemistry and Biomedical Sciences, McMaster University, Hamilton, Ontario L8N 3Z5, Canada.
authors.forename: Domina
authors.initials: D
authors.lastname: Falcone
authors.affiliation: Department of Biochemistry and Biomedical Sciences, McMaster University, Hamilton, Ontario L8N 3Z5, Canada.
authors.forename: Jamie
authors.initials: J
authors.lastname: McNichol
authors.affiliation: Institute of Biophysics, Chinese Academy of Sciences, Beijing 100101, China.
authors.forename: Bo
authors.initials: B
authors.lastname: Huang
authors.affiliation: Institute of Biophysics, Chinese Academy of Sciences, Beijing 100101, China.
authors.forename: Xuejun C
authors.initials: XC
authors.lastname: Zhang
authors.affiliation: Department of Medicinal Chemistry, University of Minnesota, Minneapolis, Minnesota 55455.
authors.forename: Chengguo
authors.initials: C
authors.lastname: Xing
authors.affiliation: Department of Biochemistry and Biomedical Sciences, McMaster University, Hamilton, Ontario L8N 3Z5, Canada; Biological Sciences, Sunnybrook Research Institute and Department of Biochemistry, University of Toronto, Toronto, Ontario M4N 3M5, Canada.
authors.forename: David W
authors.initials: DW
authors.lastname: Andrews
authors.affiliation: Department of Biochemistry and Molecular Biology, University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma 73126; Peggy and Charles Stephenson Cancer Center, University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma 73126. Electronic address: jialing-lin@ouhsc.edu.
authors.forename: Jialing
authors.initials: J
authors.lastname: Lin
mesh_terms: D000595:Amino Acid Sequence; D017209:Apoptosis; D001665:Binding Sites; D019281:Dimerization; D008928:Mitochondria / Q000378:metabolism*; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010539:Permeability; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000737:chemistry / Q000378:metabolism*; D051028:bcl-2-Associated X Protein / Q000737:chemistry / Q000378:metabolism*; D051020:bcl-X Protein / Q000737:chemistry / Q000378:metabolism*
chemical_list: D019253:Proto-Oncogene Proteins c-bcl-2; D051028:bcl-2-Associated X Protein; D051020:bcl-X Protein
keywords: Bax; Bcl-2 Proteins; Bcl-XL; Membrane Proteins; Mitochondrial Apoptosis; Protein Cross-linking