(34) DEATH SIGNAL LEADS TO THE FORMATION OF A NOVEL HIGH-CONDUCTANCE CHANNEL IN THE MITOCHONDRIAL OUTER MEMBRANE

E. Pavlov¹, E. Cheng², M. Magut³, S.J. Korsmeyer² and K.W. Kinnally^1
1College of Dentistry, New York University, New York, NY, 10010
²Dana Farber Cancer Institute, Harvard University, Boston, MA, 02115
³Division of Molecular Medicine, Wadsworth Center, Albany, NY, 12201

The permeability of the mitochondrial outer membrane increases during apoptosis.  This permeability increase allows for the release of cytochrome c and other pro-apoptotic factors from the intermembrane space, which ultimately triggers activation of caspases and cell death. While the mechanism is not known, the formation of novel channels with pore diameters of >3 nm in the outer membrane could account for cytochrome c release from mitochondria. To test this possibility, patch clamp techniques were used to compare the ion-transporting properties of mitochondrial outer membranes isolated from normal and apoptotic FL5.12 cells. The overall permeability of the outer membrane, estimated by mean conductance, increased two-fold during apoptosis. Furthermore, a novel, high conductance channel was detected in the mitochondrial outer membranes of apoptotic, but not normal cells.  The peak conductance was 2 nS and transition size was ~1 nS. The novel channel was slightly cationic-selective and voltage-independent. This new channel activity was detected more frequently than VDAC or TOM channel activity. These observations support the idea that the increase in the permeability of the mitochondrial outer membrane is associated with opening of a novel high-conductance channel. As the estimated pore diameter is ~3.4 nm (assuming pore length of 50 nm), this channel is a good candidate for participation in cytochrome c release from mitochondria during apoptosis.
It has been shown that the death signal induces the insertion of BAX, a pro-apoptotic protein, into mitochondrial membranes of FL5.12 cells.  It is also known that BAX is able to form ion channels and modulate VDAC in artificial lipid membranes.  Although the molecular nature of this novel channel is presently unknown, these findings suggest BAX insertion into mitochondrial membranes plays a crucial role in channel formation. 

This work was supported by NIH grant GM57249 to KWK.

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