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Vol. 16, Issue 4, 1788-1799, April 2005

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Switching the Sorting Mode of Membrane Proteins from Cotranslational Endoplasmic Reticulum Targeting to Posttranslational Mitochondrial Import

Emi Miyazaki ^*, Yuichiro Kida ^* , Katsuyoshi Mihara ^*, and Masao Sakaguchi ^*  

^* Department of Molecular Biology, Graduate School of Medical Science, Kyushu University, Fukuoka 812-8582, Japan; Core Research for Evolutional Science and Technology of Japan Science and Technology Agency, Ako Hyogo 678-1297, Japan; and Graduate School of Life Science, University of Hyogo, Ako Hyogo 678-1297, Japan

Submitted August 17, 2004; Revised December 20, 2004; Accepted January 12, 2005
Monitoring Editor: Thomas Fox

Hydrophobic membrane proteins are cotranslationally targeted to the endoplasmic reticulum (ER) membrane, mediated by hydrophobic signal sequence. Mitochondrial membrane proteins escape this mechanism despite their hydrophobic character. We examined sorting of membrane proteins into the mitochondria, by using mitochondrial ATP-binding cassette (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.

Abbreviations used: DHFR, dihydrofolate reductase; Endo H, endoglycosidase H; MTX, methotrexate; N135, N-terminal 135 residue hydrophilic segment of ABC-me; PNGaseF, peptide N-glycosidase F; ProK, proteinase K; OTC, ornithine transcarbamylase; SMPB, succinimidyl 4-[p-maleimidophenyl]butyrate; SRP, signal recognition particle; SytII, synaptotagmin II; TM, transmembrane segment.

Address correspondence to: Masao Sakaguchi (sakag{at}sci.u-hyogo.ac.jp).

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