A SecY Homolog in Arabidopsis thaliana

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.Thylakoid membranes consist of proteins synthesized by both nuclear and chloroplast genomes. Nuclear encoded thylakoid proteins are first targeted to the chloroplast by means of the transit peptide, which initiates the translocation of the protein across the envelope membranes into the stroma (1). Translation initiation of chloroplast encoded thylakoid proteins appears to occur in the stroma (2, 3), and then synthesis appears to continue on thylakoid bound ribosomes through a co-translational targeting mechanism (4). Considerable progress has been made in defining mechanisms by which nuclear encoded thylakoid proteins insert or translocate posttranslationally across the membrane. One class of proteins insert into the membrane in the absence of an energy supply, soluble factors, or membrane components (5-8). A second class of proteins does not require any soluble factors but requires a transthylakoid pH gradient and the membrane protein encoded by

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“…In addition to SecA, chloroplasts contain a thylakoid protein related to SecY (39). However, the cpSec translocase is largely uncharacterized.…”

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confidence: 99%

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

Schünemann

Amin

Hartmann³

et al. 1999

Journal of Biological Chemistry

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“…Biochem. 249) 1995; Laidler et al, 1995;Nohara et al, 1995;Berghofer and Klosgen, 1996), and is assumed to be simlar in mechanistic terms also to the Sec pathway of bacteria, the main secretion route for proteins in prokaryotes (Pugsley, 1993). This pathway is specific for plastocyanin, the 33-kDa subunit of the oxygenevolving system and the photosystem I subunit F (Theg et al, 1989;Mould et al, 1991 ;Hulford et al, 1994;Kamauchov et al, 1994;Mant et al, 1994;Robinson et al, 1994;.…”

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confidence: 99%

Transport of CtpA Protein from the Cyanobacterium Synechocystis 6803 Across the Thylakoid Membrane in Chloroplasts

Karnauchov

Herrmann

Pakrasi

et al. 1997

European Journal of Biochemistry

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The CtpA protein in the cyanobacterium Synechocystis 6803 is a C-terminal processing protease that is essential for the assembly of the manganese cluster of the photosystem I1 complex. When fused to different chloroplast-targeting transit peptides, CtpA can be imported into isolated spinach chloroplasts and is subsequently translocated into the thylakoid lumen. Thylakoid transport is mediated by the cyanobacterial signal peptide which demonstrates that the protein transport machinery in thylakoid membranes is functionally conserved between chloroplasts and cyanobacteria. Transport of CtpA across spinach thylakoid membranes is affected by both nigericin and sodium azide indicating that the SecA protein and a transthylakoidal proton gradient are involved in this process. Saturation of the Sec-dependent thylakoid transport route by high concentrations of the precursor of the 33-kDa subunit of the oxygen-evolving system leads to a strongly reduced rate of thylakoid translocation of CtpA which demonstrates transport by the Sec pathway. However, thylakoid transport of CtpA is affected also by excess amounts of the 23-kDa subunit of the oxygen-evolving system, though to a lesser extent. This suggests that the cyanobactenal protein is capable of also interacing with components of the ApH-dependent route and that transport of a protein across the thylakoid membrane may not always be restricted to a single pathway.

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“…One possible difference, when compared with the E. coli pathway, is that the Sec apparatus may not be used for the insertion of membrane proteins. Thylakoids do contain a Sec system (13)(14)(15), but the available evidence suggests that it is used solely for the transport of lumenal proteins (16). Although it has not been fully established that this Sec system is not involved in membrane protein insertion, current models suggest that Alb3 acts independently, as appears to be the case with its mitochondrial counterpart.…”

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A Homolog of Albino3/OxaI Is Essential for Thylakoid Biogenesis in the Cyanobacterium Synechocystis sp. PCC6803

Spence

Bailey

Nenninger

et al. 2004

Journal of Biological Chemistry

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YidC/OxaI play essential roles in the insertion of a wide range of membrane proteins in Eschericha coli and mitochondria, respectively. In contrast, the chloroplast thylakoid homolog Albino3 (Alb3) facilitates the insertion of only a specialized subset of proteins, and the vast majority insert into thylakoids by a pathway that is so far unique to chloroplasts. In this study, we have analyzed the role of Alb3 in the cyanobacterium Synechocystis sp. PCC6803, which contains internal thylakoids that are similar in some respects to those of chloroplasts. The single alb3 gene (slr1471) was disrupted by the introduction of an antibiotic cassette, and photoautotrophic growth resulted in the generation of a merodiploid species (but not full segregation), indicating an essential role for Alb3 in maintaining the photosynthetic apparatus. Thylakoid organization is lost under these conditions, and the levels of photosynthetic pigments fall to ϳ40% of wild-type levels. Photosynthetic electron transport and oxygen evolution are reduced by a similar extent. Growth on glucose relieves the selective pressure to maintain photosynthetic competence, and under these conditions, the cells become completely bleached, again indicating that Alb3 is essential for thylakoid biogenesis. Full segregation could not be achieved under any growth regime, strongly suggesting that the slr1471 open reading frame is essential for cell viability.

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A SecY Homolog in Arabidopsis thaliana

Cited by 100 publications

References 23 publications

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

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

Transport of CtpA Protein from the Cyanobacterium Synechocystis 6803 Across the Thylakoid Membrane in Chloroplasts

A Homolog of Albino3/OxaI Is Essential for Thylakoid Biogenesis in the Cyanobacterium Synechocystis sp. PCC6803

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