The C. elegans B-cell Lymphoma 2 (bcl-2) Homolog Cell Death Abnormal 9 (ced-9) Associates with and Remodels Lipid Membranes

Bcl-2 proteins associate with and remodel mitochondria to regulate apoptosis. While the C. elegans Bcl-2 homolog CED-9 constitutively associates with mitochondria, it is unclear whether or not this association reflects an innate ability of CED-9 to directly remodel mitochondrial membranes. To address this question, we have characterized the effects of recombinantly expressed and purified CED-9 on synthetic lipid vesicles. We found that CED-9 associates with anionic lipid vesicles at neutral pH, and that association can occur independently of the C-terminal transmembrane domain. Membrane association changes the environment of CED-9 tryptophans and results in an apparent increase in alpha-helical structure. Upon association, CED-9 alters the permeability of membranes resulting in leakage of encapsulated dyes. Furthermore, this membrane remodeling promotes membrane fusion upon protonation of CED-9. Bypass of this protonation trigger can be achieved by mutating two conserved glutamates (E187K/E190K) or removing the N-terminal 67 residues. Together, these in vitro results suggest that CED-9 retains the amphitropic ability of mammalian Bcl-2 proteins to associate with cellular membranes. We therefore discuss the possibility that CED-9 and other Bcl-2 homologs localize at mitochondria to regulate mitochondrial homeostasis by either modulating mitochondrial membrane permeability or fusion.