Effects of cytochrome c on the mitochondrial apoptosis-induced channel MAC

Abstract

Recent studies indicate that cytochrome c is released early in apoptosis without loss of integrity of the mitochondrial outer membrane in some cell types. The high-conductance mitochondrial apoptosis-induced channel (MAC) forms in the outer membrane early in apoptosis of FL5.12 cells. Physiological (micromolar) levels of cytochrome c alter MAC activity, and these effects are referred to as types 1 and 2. Type 1 effects are consistent with a partitioning of cytochrome c into the pore of MAC and include a modest decrease in conductance that is dose and voltage dependent, reversible, and has an increase in noise. Type 2 effects may correspond to "plugging" of the pore or destabilization of the open state. Type 2 effects are a dose-dependent, voltage-independent, and irreversible decrease in conductance. MAC is a heterogeneous channel with variable conductance. Cytochrome c affects MAC in a pore size-dependent manner, with maximal effects of cytochrome c on MAC with conductance of 1.9-5.4 nS. The effects of cytochrome c, RNase A, and high salt on MAC indicate that size, rather than charge, is crucial. The effects of dextran molecules of various sizes indicate that the pore diameter of MAC is slightly larger than that of 17-kDa dextran, which should be sufficient to allow the passage of 12-kDa cytochrome c. These findings are consistent with the notion that MAC is the pore through which cytochrome c is released from mitochondria during apoptosis.

The Bax pore in liposomes, Biophysics

“…25 This was a membrane lifetime that could not be reproduced in the liposome experiments. This activity is like that observed by others in mitochondria [65][66][67][68] and may represent another level of organization, which we have not been able to study.…”

Is MAC the knife that cuts cytochrome c from mitochondria during apoptosis?

“…[26][27][28] The conductance of MAC is both variable and high. 23,24 The mean conductance of MAC of apoptotic HeLa and FL5.12 cells is 3.3 and 4.5 nS, respectively. 20,23,24 MAC is typically a voltage-independent channel, 20,23,24,29 although it has also been reported to occupy less than peak conductances at higher potentials.…”

“…Although flickering between conductance states is sometimes observed, current traces usually show MAC occupying a stable open conductance state with relatively infrequent transitions. 20,23,24 The activity of MAC is significantly different from the constitutive channels of the mitochondrial outer membrane, TOM and VDAC. The single-channel parameters of peak conductance, transition size, selectivity, and voltage dependence for these channels are in Table 1 and illustrated by the current traces of Figure 1.…”

“…These observations indicate that, instead of rupturing, a more selective mechanism of permeabilization is operating, like the formation of a pore in the outer membrane. 11,[20][21][22][23][24] The mitochondrial apoptosis-induced channel (MAC), was detected by directly patch clamping mitochondria isolated from cells in early apoptosis. MAC activity is exquisitely regulated by Bcl-2 family proteins and can initiate release of apoptotic mediators from mitochondria to commit the cell to die.…”

“…MAC activity is exquisitely regulated by Bcl-2 family proteins and can initiate release of apoptotic mediators from mitochondria to commit the cell to die. 20,23,24 Nevertheless, MAC and PTP transient opening may act alone or in combination, depending on cell type and death stimulus, to relocalize Bax to the mitochondria, remodel the cristae, and maximize cytochrome c release to amplify the death signal. 22,25 The focus of this review is a biophysical characterization of MAC, cytochrome c permeability, and how Bcl-2 family proteins regulate the permeability of the mitochondrial outer membrane through the formation of MAC.…”