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

1 Department of Basic Sciences, College of Dentistry, New York University, New York, New York 10010; 2 Biotechnology Division, National Institute of Science and Technology, Gaithersburg, Maryland 20899-8313; 3 Departments of Pathology and Medicine, Harvard Medical School, Dana-Farber Cancer Institute, Howard Hughes Medical Institute, Boston, Massachusetts 02115; and 4 Serono Pharmaceutical Research Institute, Serono International, CH-1228 Plan-les Ouates, Geneva, Switzerland Submitted 6 May 2003 ; accepted in final form 10 December 2003 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. patch clamp; ion channels Address for reprint requests and other correspondence: K. W. Kinnally, Dept. of Basic Sciences, College of Dentistry, New York Univ., 345 East 24th St., New York, NY 10010 (E-mail: kathleen.kinnally{at}nyu.edu ).