A CHARACTERIZATION OF THE DYNAMIC INTERACTION BETWEEN THE PRO-APOPTOTIC PROTEIN BID AND THE MITOCHONDRIAL OUTER MEMBRANE

Abstract

<p>Bcl-2 family of proteins regulate apoptosis at the level of the mitochondrial outer membrane (MOM) through both protein-protein and protein-membrane interactions. While the role of the membrane as the “locus of action” has been recognized, the detailed molecular mechanisms and the consequences of the interactions of Bcl-2 family members with the membrane are yet to be fully understood. The findings presented here focus on the dynamic interactions of Bcl-2 proteins, most notably tBid with the MOM, and their functional significance on mitochondrial outer membrane permeabilization. We show that the activation of tBid is a multi-step process that is regulated by MOM lipids and proteins. The rate-limiting step in the activation of tBid is an elaborate conformational change that is facilitated by Mtch2, and is required for the activation and recruitment of Bax to the MOM. Furthermore, we demonstrate that binding of both tBid and Bax to the membrane is reversible and is governed by dynamic equilibria that potentially contribute to the propagation of the permeabilization signal within the cell for the regulation of apoptosis. We report that the transfer of tBid between membranes is accelerated by Bax and restricted by Bcl-XL, whereas the transfer of Bax between membranes is slower than and not influenced by tBid. Finally, by studying the effect of varying lipid composition on Bax-mediated permeabilization, we establish that electrostatic interactions mediate the binding of both tBid and Bim to the membrane. We demonstrate that while Bim does not exhibit any preference for a specific anionic lipid, tBid requires cardiolipin in order to undergo its conformational change at the membrane in the absence of Mtch2. Taken together, our work contributes to the growing understanding of the dynamic interactions and changes in conformation of Bcl-2 proteins at the MOM.</p>