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.

Electrical properties and regulation of the mitochondrial ATP-dependent potassium channel were studied. The channel protein was solubilized from the mitochondrial membrane using an ethanol/water mixture. Reconstituted into a bilayer lipid membrane BLM), the protein formed a slightly voltage-dependent channel with a conductance of 10 pS in 100 mM KCl. Often, several channels worked simultaneously (clusters) when many channels were incorporated into the BLM. The elementary channel and the clusters were both highly potassium selective. At concentrations of 1 to 10 microM, ATP favors channel opening, while channels become closed at 1-3 mM ATP. GDP (0.5 mM) reactivated the ATP-closed channels without affecting the untreated channels. The sulfhydryl-reducing agent ditiothreitol increased the open probability at concentrations of 1 to 3 mM, but damaged the selectivity of the channel.