Solution structure of BID, an intracellular amplifier of apoptotic signaling.

We report the solution structure of BID, an intracellular cross-talk agent that can amplify FAS/TNF apoptotic signal through the mitochondria death pathway after Caspase 8 cleavage. BID contains eight alpha helices where two central hydrophobic helices are surrounded by six amphipathic ones. The fold resembles poreforming bacterial toxins and shows similarity to BCL-XL although sequence homology to BCL-XL is limited to the 16-residue BH3 domain. Furthermore, we modeled a complex of BCL-XL and BID by aligning the BID and BAK BH3 motifs in the known BCL-XL-BAK BH3 complex. Additionally, we show that the overall structure of BID is preserved after cleavage by Caspase 8. We propose that BID has both BH3 domain-dependent and -independent modes of action in inducing mitochondrial damage.