Properties of a cyclosporin-insensitive permeability transition pore in yeast mitochondria

Yeast mitochondria (Saccharomyces cerevisiae) contain a permeability transition pore which is regulated differently than the pore in mammalian mitochondria. In a mannitol medium containing 10 mM P-i and ethanol (oxidizable substrate), yeast mitochondria accumulate large amounts of Ca2+ (>400 nmol/mg of protein) upon the addition of an electrophoretic Ca2+ ionophore (ETH129). Pore opening does not occur following Ca2+ uptake, even though ruthenium red-inhibited rat liver mitochondria undergo rapid pore opening under analogous conditions. However, a pore does arise in yeast mitochondria when Ca2+ and P-i are not present, as monitored by swelling, ultrastructure, and matrix solute release, Pore opening is slow unless a respiratory substrate is provided (ethanol or NADH) but also occurs rapidly in response to ATP (2 mM) when oligomycin is present, P-i and ADP inhibit pore opening (EC50 similar to 1 and 4 mar, respectively), however, cyclosporin A (7 mu g/ml), oligomycin (20 mu g/ml), or carboxyatractyloside (25 mu M) have no effect. The pare arising during respiration is also inhibited by nigericin or uncoupler, indicating that an acidic matrix pH antagonizes the process, P-i also inhibits pore opening by lowering the matrix pH (P-i/OH- antiport), However, inhibition of the ATP-induced pore by P-i is seen in the presence of mersalyl, suggesting a second mechanism of action. Since pore induction by ATP is not sensitive to carboxyatractyloside, ATP appears to act at an external site and P-i may antagonize the interaction. Isoosmotic polyethylene glycol-induced contraction of yeast mitochondria swollen during respiration, or in the presence of ATP, is 50% effective at a solute size of 1.0-1.1 kDa. This suggests that the same pore is induced in bath cases and is comparable in size with the permeability transition pore of heart and Liver mitochondria.