Structural and dynamic basis of indirect apoptosis inhibition by Bcl-xL: A case study with Bid

Abstract

Intrinsic apoptosis is a form of cell death which is activated, executed, and inhibited by the Bcl-2 protein family. The structural basis of the inhibition mechanisms remains elusive. Here, we characterize the ensemble structural model of the inhibitory Bcl-xL/tBid complex at the mitochondrial membrane by probing interresidue distances and dynamic solvent accessibilities complemented by integrative modeling and molecular dynamics simulations. We show that Bcl-xL and tBid form a heterodimer anchored to the membrane by the C-terminal helix of Bcl-xL. The BH3 domain of tBid is wedged between the exposed hydrophobic groove of Bcl-xL and the membrane headgroups, while tBid’s C-terminal helices remain dynamically engaged with the bilayer. This dynamic architecture sheds light on the mechanism of indirect inhibition of apoptosis.