THE PREPROTEIN TRANSLOCATION CHANNEL OF THE OUTER-MEMBRANE OF MITOCHONDRIA

The preprotein translocase of the outer membrane of mitochondria (TOM complex) facilitates the recognition, insertion, and translocation of nuclear-encoded mitochondrial preproteins. We have purified the TOM co mplex from Neurospora crassa and analyzed its composition and function al properties. The TOM complex contains a cation-selective high-conduc tance channel. Upon reconstitution into liposomes, it mediates integra tion of proteins into and translocation across the lipid bilayer. TOM complex particles have a diameter of about 138 Angstrom, as revealed b y electron microscopy and image analysis; they contain two or three ce nters of stain-filled openings, which we interpret as pores with an ap parent diameter of about 20 Angstrom. We conclude that the structure r eported here represents the protein-conducting channel of the mitochon drial outer membrane.