Distinct lipid effects on tBid and Bim activation of membrane permeabilization by pro-apoptotic Bax

After exposure to stressful stimuli, apoptotic signals can be relayed to mitochondria by pro-apoptotic activator proteins, tBid and Bim, which activate Bax and or Bak to induce mitochondrial outer membrane permeabilization (MOMP). These protein-protein and protein-membrane interactions are critical for apoptosis regulation since MOMP irreversibly leads to cell death. While the distinct roles of tBid and Bim as sensors of different types of stress are well recognized, it is not known whether the molecular mechanisms whereby they initiate MOMP are the same. Here we compare membrane permeabilization by Bax activated by either cBid or Bim and we examine the role of membrane lipids in the recruitment and activation of these three Bcl-2 pro-apoptotic proteins. We employ fluorescently labeled proteins and liposomes, to quantify the effects of specific lipids on each of the well-characterized steps in Bax-mediated membrane permeabilization. We extend previous observations that high levels of cholesterol in the membrane inhibit permeabilization by categorically identifying the recruitment of Bax by the activators and Bax insertion in the membrane as the steps being hindered by cholesterol. Furthermore, we show that binding of both cBid and Bim to membranes is facilitated by electrostatic interactions with anionic phospholipids. However, while Bim does not require any particular anionic lipids, the conformational change in tBid depends on cardiolipin. This suggests that cardiolipin can...
Source: BJ Cell - Category: Biochemistry Authors: Tags: BJ Cell Source Type: research