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

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.

Comparison of the TIM and TOM channel activities of the mitochondrial protein import complexes.

Water-filled channels are central to the process of translocating proteins since they provide aqueous pathways through the hydrophobic environment of membranes. The Tom and Tim complexes translocate precursors across the mitochondrial outer and inner membranes, respectively, and contain channels referred to as TOM and TIM (previously called PSC and MCC). In this study, little differences were revealed from a direct comparison of the single channel properties of the TOM and TIM channels of yeast mitochondria. As they perform similar functions in translocating proteins across membranes, it is not surprising that both channels are high conductance, voltage-dependent channels that are slightly cation selective. Reconstituted TIM and TOM channel activities are not modified by deletion of the outer membrane channel VDAC, but are similarly affected by signal sequence peptides.