title: Targeting of a polytopic membrane protein to the inner envelope membrane of chloroplasts in vivo involves multiple transmembrane segments.
abstract: The inner envelope membrane (IEM) of the chloroplast plays crucial roles in forming an osmotic barrier and controlling metabolite exchange between the organelle and the cytosol. The IEM therefore harbours a number of membrane proteins and requires the import and integration of these nuclear-encoded proteins for its biogenesis. Recent studies have demonstrated that the transmembrane segment of single-spanning IEM proteins plays key roles in determining their IEM localization. However, few studies have focused on the molecular mechanisms by which polytopic membrane proteins are targeted to the IEM. In this study, we investigated the targeting mechanism of polytopic IEM proteins using the protein Cor413im1 as a model substrate. Cor413im1 does not utilize a soluble intermediate for its targeting to the IEM. Furthermore, we show that the putative fifth transmembrane segment of Cor413im1 is necessary for its targeting to the IEM. The C-terminal portion containing this transmembrane segment is also able to deliver Cor413im1 protein to the IEM. However, the fifth transmembrane segment of Cor413im1 itself is insufficient to target a fusion protein to the IEM. These data suggest that the targeting of polytopic membrane proteins to the chloroplast IEM in vivo involves multiple transmembrane segments and that chloroplasts have evolved a unique mechanism for the integration of polytopic proteins to the IEM.
authors.affiliation: Department of Agricultural and Environmental Sciences, Faculty of Agriculture, University of Miyazaki, Miyazaki 889-2192, Japan.
authors.forename: Kumiko
authors.initials: K
authors.lastname: Okawa
authors.affiliation: Department of Biochemistry and Molecular Biology, University of Massachusetts, Amherst, 01003 MA, USA.
authors.forename: Hitoshi
authors.initials: H
authors.lastname: Inoue
authors.affiliation: Department of Agricultural and Environmental Sciences, Faculty of Agriculture, University of Miyazaki, Miyazaki 889-2192, Japan.
authors.forename: Fumi
authors.initials: F
authors.lastname: Adachi
authors.affiliation: Department of Agricultural and Environmental Sciences, Faculty of Agriculture, University of Miyazaki, Miyazaki 889-2192, Japan.
authors.forename: Katsuhiro
authors.initials: K
authors.lastname: Nakayama
authors.affiliation: Organization for Promotion of Tenure Track, University of Miyazaki, Miyazaki 889-2192, Japan.
authors.forename: Yasuko
authors.initials: Y
authors.lastname: Ito-Inaba
authors.affiliation: Department of Biochemistry and Molecular Biology, University of Massachusetts, Amherst, 01003 MA, USA.
authors.forename: Danny J
authors.initials: DJ
authors.lastname: Schnell
authors.affiliation: Department of Agricultural and Environmental Sciences, Faculty of Agriculture, University of Miyazaki, Miyazaki 889-2192, Japan.
authors.forename: Susumu
authors.initials: S
authors.lastname: Uehara
authors.affiliation: Department of Agricultural and Environmental Sciences, Faculty of Agriculture, University of Miyazaki, Miyazaki 889-2192, Japan tinaba@cc.miyazaki-u.ac.jp.
authors.forename: Takehito
authors.initials: T
authors.lastname: Inaba
mesh_terms: D017360:Arabidopsis / Q000378:metabolism*; D029681:Arabidopsis Proteins / Q000378:metabolism*; D002736:Chloroplasts / Q000378:metabolism*; D007425:Intracellular Membranes / Q000378:metabolism*; D008565:Membrane Proteins / Q000378:metabolism*; D021381:Protein Transport / Q000502:physiology
chemical_list: D029681:Arabidopsis Proteins; D008565:Membrane Proteins
keywords: Arabidopsis; Cor413im1; chloroplast; inner envelope membrane; polytopic proteins; protein targeting.