THE ISOLATED COMPLEX OF THE TRANSLOCASE OF THE OUTER-MEMBRANE OF MITOCHONDRIA - CHARACTERIZATION OF THE CATION-SELECTIVE AND VOLTAGE-GATED PREPROTEIN-CONDUCTING PORE

The complex of the translocase mitochondrial outer membrane (TOM), med iates recognition, unfolding, and translocation of preproteins, We hav e used a combination of biochemical and electrophysiological methods t o study the properties of the preprotein-conducting pore of the purifi ed TOM complex. The pore is cation-selective and voltage-gated. It sho ws three main conductance levels with characteristic slow and fast kin etics transitions to states of lower conductance following application of transmembrane voltages. These electrical properties distinguish it from the mitochondrial voltage-dependent anion channel (porin) and ar e identical to those of the previously described peptide-sensitive cha nnel. Binding of antibodies to the C terminus of Tom40 on the intermem brane space side of the outer membrane modifies the channel properties and allows determination of the orientation of the channel within the lipid bilayer. Mitochondrial presequence peptides specifically intera ct with the pore and decrease the ion flow through the channel in a vo ltage-dependent manner,We propose that the presequence-induced closure s of the pore are related to structural alterations of the TOM complex observed during the various stages of preprotein movement across the mitochondrial outer membrane.