title: Switching the sorting mode of membrane proteins from cotranslational endoplasmic reticulum targeting to posttranslational mitochondrial import.
abstract: 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.
authors.affiliation: Department of Molecular Biology, Graduate School of Medical Science, Kyushu University, Fukuoka, Japan.
authors.forename: Emi
authors.initials: E
authors.lastname: Miyazaki
authors.forename: Yuichiro
authors.initials: Y
authors.lastname: Kida
authors.forename: Katsuyoshi
authors.initials: K
authors.lastname: Mihara
authors.forename: Masao
authors.initials: M
authors.lastname: Sakaguchi
mesh_terms: D018528:ATP-Binding Cassette Transporters / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D000595:Amino Acid Sequence; D000818:Animals; D019556:COS Cells; D002462:Cell Membrane / Q000378:metabolism; D002522:Chlorocebus aethiops; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D008565:Membrane Proteins / Q000737:chemistry / Q000378:metabolism*; D051379:Mice; D008928:Mitochondria / Q000235:genetics / Q000378:metabolism*; D008969:Molecular Sequence Data; D014176:Protein Biosynthesis / Q000502:physiology*; D020033:Protein Isoforms / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D011499:Protein Processing, Post-Translational; D021381:Protein Transport; D012995:Solubility; D013379:Substrate Specificity
chemical_list: D018528:ATP-Binding Cassette Transporters; D008565:Membrane Proteins; D020033:Protein Isoforms