BCL-XL Dimerization by Three-dimensional Domain Swapping

Dimeric interactions among anti- and pro-apoptotic members of the BCL-2 protein family are dynamically regulated and intimately involved in survival and death functions. We report the structure of a BCL-XL homodimers a 3D-domain swapped dimer (3DDS). The X-ray crystal structure demonstrates the mutual exchange of carboxy-terminal regions including BH2 (Bcl-2 homology 2) between monomer subunits, with the hinge region occurring at the hairpin turn between the fifth and sixth alpha helices. Both BH3 peptide-binding hydrophobic grooves are unoccupied in the 3DDS dimer and available for BH3 peptide binding, as confirmed by sedimentation velocity analysis. BCL-XL 3DDS dimers have increased pore-forming activity compared to monomers, suggesting that 3DDS dimers may act as intermediates in membrane pore formation. Chemical crosslinking studies of Cys-substituted BCL-XL proteins demonstrate that 3DDS dimers form in synthetic lipid vesicles.