A novel chloroplastic outer membrane-targeting signal that functions at both termini of passenger polypeptides.
Several components in the machinery mediating the import of nuclear-encoded chloroplastic precursor proteins have been identified. One of the components, OEP34, is an outer membrane protein and is synthesized at its mature size in the cytosol without a distinguishable chloroplast-targeting signal. To address the question of how components in the transport machinery are imported to chloroplasts themselves, we first identified the chloroplastic outer membrane-targeting signal of OEP34. Using an Arabidopsis homologue of the originally isolated pea OEP34, we show that the outer membrane-targeting signal of OEP34 is located within a 10-amino acid hydrophobic core of the C-terminal membrane anchor. Interestingly, this signal can target a passenger protein to the chloroplastic outer membrane no matter whether it is placed at the N or C terminus of a passenger protein. Proper insertion of fusion proteins into the outer membrane requires in addition the C-terminal hydrophilic region following the hydrophobic core. Furthermore, passenger proteins fused to the C terminus of the targeting/insertion signal were most likely imported into the intermembrane space of the envelope.
Li
H m
Hm
Institute of Molecular Biology, Academia Sinica, Nankang, Taipei 11529, Taiwan, Republic of China.
Chen
L J
LJ
D000595:Amino Acid Sequence; D017360:Arabidopsis; D002467:Cell Nucleus / Q000378:metabolism; D002736:Chloroplasts / Q000378:metabolism*; D003001:Cloning, Molecular; D019204:GTP-Binding Proteins / Q000737:chemistry* / Q000302:isolation & purification / Q000378:metabolism*; D007425:Intracellular Membranes / Q000378:metabolism*; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D018532:Pisum sativum; D010940:Plant Proteins / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism; D011993:Recombinant Fusion Proteins / Q000737:chemistry / Q000378:metabolism; D017384:Sequence Deletion; D017386:Sequence Homology, Amino Acid
D010940:Plant Proteins; D011993:Recombinant Fusion Proteins; C093715:outer envelope membrane protein 34-kDa; D019204:GTP-Binding Proteins