¹Department of Cell Biology, University of Geneva, 30 quai Ernest-Ansermet, Geneva, Switzerland

Correspondence: J-C Martinou, Department of Cell Biology, University of Geneva, Sciences III, 30 quai Ernest Ansermet, 1211 Geneva 4, Switzerland. Tel: +41 22 379 64 43; Fax: +41 22 379 64 42; E-mail: jean-claude.martinou@cellbio.unige.ch

Received 2 October 2007; Revised 19 December 2007; Accepted 8 January 2008; Published online 8 February 2008.

Abstract

Under many apoptotic conditions, Bax undergoes conformational rearrangements, leading to its insertion in the mitochondrial outer membrane as a transmembrane oligomer. At the same time, mitochondria undergo fragmentation and activated Bax was reported to localize to fission sites. We studied how lipid composition and membrane curvature regulate Bax activation. When isolated mitochondria were incubated with phospholipase A₂, which led to phosphatidylethanolamine and cardiolipin hydrolysis, tBid and Bax insertion were hindered. We thus studied in liposomes how phosphatidylethanolamine, cardiolipin, and its hydrolysis products affect Bax activation. Whereas phosphatidylethanolamine, a lipid with negative curvature, did not affect Bax insertion, it inhibited Bax oligomerization. Conversely, Bax insertion required cardiolipin, and was not blocked by cardiolipin hydrolysis products. These experiments support a direct role for cardiolipin in the recruitment and activation of Bax. To examine if the increase in membrane curvature that accompanies mitochondrial fission participates in Bax activation, we studied how liposome size affects the process, and observed that it was inhibited in small liposomes (200 nm diameter). Therefore, the localization of Bax to mitochondrial scission sites does not result from a preference for curved bilayers. Our experiments show that membrane properties can control the process of Bax activation, providing an additional level to the mechanisms of regulation of mitochondrial permeability.