Vaccinia virus, the prototypic member of the orthopoxvirus genus, encodes the 
mitochondrial-localized protein F1L that functions to protect cells from 
apoptotic death and inhibits cytochrome c release. We previously showed that F1L 
interacts with the pro-apoptotic Bcl-2 family member Bak and inhibits activation 
of Bak following an apoptotic stimulus (Wasilenko, S. T., Banadyga, L., Bond, 
D., and Barry, M. (2005) J. Virol. 79, 14031-14043). In addition to Bak, the 
pro-apoptotic protein Bax is also capable of initiating cytochrome c release 
suggesting that vaccinia virus infection could also inhibit Bax activity. Here 
we show that F1L inhibits the activity of the pro-apoptotic protein Bax by 
inhibiting oligomerization and N-terminal activation of Bax. F1L expression also 
inhibited the subcellular redistribution of Bax to the mitochondria and the 
insertion of Bax into the outer mitochondrial membrane. The ability of F1L to 
inhibit Bax activation does not require Bak, because F1L expression inhibited 
cytochrome c release and Bax activation in Bak-deficient cells. No interaction 
between Bax and F1L was detected during infection, suggesting that F1L functions 
upstream of Bax activation. Notably, F1L was capable of interacting with the 
BH3-only protein BimL as shown by co-immunoprecipitation, and F1L expression 
inhibited apoptosis induced by BimL. These studies suggest that, in addition to 
interacting with the pro-apoptotic protein Bak, F1L also functions to indirectly 
inhibit the activation of Bax, likely by interfering with the pro-apoptotic 
activity of BH3-only proteins such as BimL.