Although Bcl-XL and Bax are structurally similar, activated Bax forms large 
oligomers that permeabilize the outer mitochondrial membrane, thereby committing 
cells to apoptosis, whereas Bcl-XL inhibits this process. Two different models 
of Bcl-XL function have been proposed. In one, Bcl-XL binds to an activator, 
thereby preventing Bax activation. In the other, Bcl-XL binds directly to 
activated Bax. It has been difficult to sort out which interaction is important 
in cells, as all three proteins are present simultaneously. We examined the 
mechanism of Bax activation by tBid and its inhibition by Bcl-XL using 
full-length recombinant proteins and measuring permeabilization of liposomes and 
mitochondria in vitro. Our results demonstrate that Bcl-XL and Bax are 
functionally similar. Neither protein bound to membranes alone. However, the 
addition of tBid recruited molar excesses of either protein to membranes, 
indicating that tBid activates both pro- and antiapoptotic members of the Bcl-2 
family. Bcl-XL competes with Bax for the activation of soluble, monomeric Bax 
through interaction with membranes, tBid, or t-Bid-activated Bax, thereby 
inhibiting Bax binding to membranes, oligomerization, and membrane 
permeabilization. Experiments in which individual interactions were abolished by 
mutagenesis indicate that both Bcl-XL-tBid and Bcl-XL-Bax binding contribute to 
the antiapoptotic function of Bcl-XL. By out-competing Bax for the interactions 
leading to membrane permeabilization, Bcl-XL ties up both tBid and Bax in 
nonproductive interactions and inhibits Bax binding to membranes. We propose 
that because Bcl-XL does not oligomerize it functions like a dominant-negative 
Bax in the membrane permeabilization process.