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© The Rockefeller University Press, 0021-9525/2001//1151 $5.00
The Journal of Cell Biology, Volume 153, Number 6, , 2001 1151-1160

Tom40, the Pore-Forming Component of the Protein-Conducting Tom Channel in the Outer Membrane of Mitochondria

^a Institut für Physiologische Chemie, Universität München, D-81377 München, Germany
^b Laboratoire de Neurobiologie, Cellulaire et Moléculaire, Centre National de Recherche Scientifique, F-91198 Gif-sur-Yvette, France
^c Abteilung für Molekulare Strukturbiologie, Max-Planck Institut für Biochemie, D-82152 Martinsried, Germany
Institut für Physiologische Chemie, Universität München, Butenandtstrasse 5, D-81377 München, Germany.49-89-2180-709349-89-2180-7086

neupert{at}bio.med.uni-muenchen.de

Tom40 is the main component of the preprotein translocase of the outer membrane of mitochondria (TOM complex). We have isolated Tom40 of Neurospora crassa by removing the receptor Tom22 and the small Tom components Tom6 and Tom7 from the purified TOM core complex. Tom40 is organized in a high molecular mass complex of 350 kD. It forms a high conductance channel. Mitochondrial presequence peptides interact specifically with Tom40 reconstituted into planar lipid membranes and decrease the ion flow through the pores in a voltage-dependent manner. The secondary structure of Tom40 comprises 31% β-sheet, 22% -helix, and 47% remaining structure as determined by circular dichroism measurements and Fourier transform infrared spectroscopy. Electron microscopy of purified Tom40 revealed particles primarily with one center of stain accumulation. They presumably represent an open pore with a diameter of 2.5 nm, similar to the pores found in the TOM complex. Thus, Tom40 is the core element of the TOM translocase; it forms the protein-conducting channel in an oligomeric assembly.

Key Words: TOM complex • Tom40 • mitochondria • protein translocation channel • protein targeting

© 2001 The Rockefeller University Press

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