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J. Cell Biol.,
Volume 145, Number 5, May 31, 1999 973-978
Department of Biochemistry, McGill University, Montreal, Quebec, Canada H3G 1Y6
Insertion of newly synthesized proteins into
or across the mitochondrial outer membrane is initiated
by import receptors at the surface of the organelle. Typically, this interaction directs the precursor protein into
a preprotein translocation pore, comprised of Tom40.
Here, we show that a prominent 
-barrel channel protein spanning the outer membrane, human voltage-
dependent anion-selective channel (VDAC), bypasses
the requirement for the Tom40 translocation pore during biogenesis. Insertion of VDAC into the outer membrane is unaffected by plugging the translocation pore
with a partially translocated matrix preprotein, and mitochondria containing a temperature-sensitive mutant
of Tom40 insert VDAC at the nonpermissive temperature. Synthetic liposomes harboring the cytosolic domain of the human import receptor Tom20 efficiently
insert newly synthesized VDAC, resulting in transbilayer transport of ATP. Therefore, Tom20 transforms
newly synthesized cytosolic VDAC into a transmembrane channel that is fully integrated into the lipid bilayer.
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