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A more recent version of this article appeared on April 1, 2005
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Submitted on August 17, 2004
Revised on December 20, 2004
Accepted on January 12, 2005

*Department of Molecular Biology, Graduate School of Medical Science, Kyushu University, Fukuoka 812-8582, Japan;
CREST of Japan Science and Technology Agency, Ako Hyogo 678-1297, Japan;
Graduate School of Life Science, University of Hyogo, Ako Hyogo 678-1297, Japan
Monitoring Editor: Thomas Fox
Hydrophobic membrane proteins are cotranslationally targeted to the ER membrane, mediated by hydrophobic signal sequence. Mitochondrial membrane proteins escape this mechanism despite of the hydrophobic character. We examined sorting of membrane proteins into the mitochondria, using mitochondrial ABC transporter isoform (ABC-me). In the absence of 135-residue N-terminal hydrophilic segment (N135), the membrane domain was integrated into the ER membrane in COS7 cells. Other sequences that were sufficient to import soluble protein into mitochondria could not import the membrane domain. N135 imports other membrane proteins into mitochondria. N135 prevents cotranslational targeting of the membrane domain to ER and in turn achieves posttranslational import into mitochondria. In a cell-free system, N135 suppresses targeting to the ER membranes, although it does not affect recognition of hydrophobic segments by signal recognition particle. We conclude that the N135 segment blocks the ER targeting of membrane proteins even in the absence of mitochondria and switches the sorting mode from cotranslational ER integration to posttranslational mitochondrial import.
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