Chloroplast SecY is complexed to SecE and involved in the translocation of the 33-kDa but not the 23-kDa subunit of the oxygen-evolving complex.
SecY is a component of the protein-conducting channel for protein transport across the cytoplasmic membrane of prokaryotes. It is intimately associated with a second integral membrane protein, SecE, and together with SecA forms the minimal core of the preprotein translocase. A chloroplast homologue of SecY (cpSecY) has previously been identified and determined to be localized to the thylakoid membrane. In the present work, we demonstrate that a SecE homologue is localized to the thylakoid membrane, where it forms a complex with cpSecY. Digitonin solubilization of thylakoid membranes releases the SecY/E complex in a 180-kDa form, indicating that other components are present and/or the complex is a higher order oligomer of the cpSecY/E dimer. To test whether cpSecY forms the protein-conducting channel of the thylakoid membrane, translocation assays were conducted with the SecA-dependent substrate OE33 and the SecA-independent substrate OE23, in the presence and absence of antibodies raised against cpSecY. The antibodies inhibited translocation of OE33 but not OE23, indicating that cpSecY comprises the protein-conducting channel used in the SecA-dependent pathway, whereas a distinct protein conducting channel is used to translocate OE23.
Schuenemann
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Department of Plant Biology, Carnegie Institution of Washington, Stanford, California 94305, USA.
Amin
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Hartmann
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Hoffman
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NE
D000595:Amino Acid Sequence; D017360:Arabidopsis / Q000378:metabolism; D001426:Bacterial Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D001692:Biological Transport; D002736:Chloroplasts / Q000378:metabolism*; D003001:Cloning, Molecular; D029968:Escherichia coli Proteins; D007425:Intracellular Membranes / Q000378:metabolism; D008969:Molecular Sequence Data; D010100:Oxygen / Q000378:metabolism*; D011485:Protein Binding; D000069816:SEC Translocation Channels; D017386:Sequence Homology, Amino Acid
D001426:Bacterial Proteins; D029968:Escherichia coli Proteins; D000069816:SEC Translocation Channels; C065493:SecE protein, E coli; C057841:SecY protein, E coli; D010100:Oxygen