A novel BH3-like domain in BID is required for intramolecular interaction and autoinhibition of pro-apoptotic activity

Upon activation of the Fas apoptotic signaling pathway, Bid, a "BH3 domain- only" pro-apoptotic molecule, is cleaved by caspase-8 into a 6.5-kDa N-term inal and a 15-kDa BH3 domain-containing C-terminal fragment, referred to as t(n)-Bid and t(c)-Bid, respectively. t(c)-Bid is a more potent inducer of apoptosis than full-length Bid, suggesting that the N-terminal region of Bi d has an inhibitory effect on its pro-apoptotic activity. Here, we report t he identification of a novel BH3-like motif (amino acid residues 35-43) in t(n)-Bid. Although Bid does not homodimerize, t(n)-Bid is able to associate avidly with t(n)-Bid. Site directed mutagenesis revealed that both the nov el BH3-like and BH3 domains are necessary for direct binding between t(n)-B id and t(c)-Bid. While full-length Bid does not associate with t(n)-Bid, su bstitution of Leu(35) a critical residue in mediating t(n)-Bid/t(c)-Bid int eraction, with Ala in full-length Bid is sufficient to establish Bid/t(n)-B id interaction. Interestingly, the L35A Bid mutant is as effective as t(c)- Bid in inducing apoptosis and binding Bcl-X-L. We propose that the intramol ecular interaction involving the BH3-like and BH3 domains serves to regulat e the pro-apoptotic potential of Bid.