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      A twin arginine signal peptide and the pH gradient trigger reversible assembly of the thylakoid ΔpH/Tat translocase

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          Abstract

          The thylakoid ΔpH-dependent/Tat pathway is a novel system with the remarkable ability to transport tightly folded precursor proteins using a transmembrane ΔpH as the sole energy source. Three known components of the transport machinery exist in two distinct subcomplexes. A cpTatC–Hcf106 complex serves as precursor receptor and a Tha4 complex is required after precursor recognition. Here we report that Tha4 assembles with cpTatC–Hcf106 during the translocation step. Interactions among components were examined by chemical cross-linking of intact thylakoids followed by immunoprecipitation and immunoblotting. cpTatC and Hcf106 were consistently associated under all conditions tested. In contrast, Tha4 was only associated with cpTatC and Hcf106 in the presence of a functional precursor and the ΔpH. Interestingly, a synthetic signal peptide could replace intact precursor in triggering assembly. The association of all three components was transient and dissipated upon the completion of protein translocation. Such an assembly–disassembly cycle could explain how the ΔpH/Tat system can assemble translocases to accommodate folded proteins of varied size. It also explains in part how the system can exist in the membrane without compromising its ion and proton permeability barrier.

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          The Tat protein export pathway.

          The Tat (twin-arginine translocation) system is a bacterial protein export pathway with the remarkable ability to transport folded proteins across the cytoplasmic membrane. Preproteins are directed to the Tat pathway by signal peptides that bear a characteristic sequence motif, which includes consecutive arginine residues. Here, we review recent progress on the characterization of the Tat system and critically discuss the structure and operation of this major new bacterial protein export pathway.
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            Thylakoid ΔpH-dependent precursor proteins bind to a cpTatC–Hcf106 complex before Tha4-dependent transport

            The thylakoid ΔpH-dependent pathway transports folded proteins with twin arginine–containing signal peptides. Identified components of the machinery include cpTatC, Hcf106, and Tha4. The reaction occurs in two steps: precursor binding to the machinery, and transport across the membrane. Here, we show that a cpTatC–Hcf106 complex serves as receptor for specific binding of twin arginine–containing precursors. Antibodies to either Hcf106 or cpTatC, but not Tha4, inhibited precursor binding. Blue native gel electrophoresis and coimmunoprecipitation of digitonin-solubilized thylakoids showed that Hcf106 and cpTatC are members of an ∼700-kD complex that lacks Tha4. Thylakoid-bound precursor proteins were also associated with an ∼700-kD complex and were coimmunoprecipitated with antibodies to cpTatC or Hcf106. Chemical cross-linking revealed that precursors make direct contact with cpTatC and Hcf106 and confirmed that Tha4 is not associated with precursor, cpTatC, or Hcf106 in the membrane. Precursor binding to the cpTatC–Hcf106 complex required both the twin arginine and the hydrophobic core of the signal peptide. Precursors remained bound to the complex when Tha4 was sequestered by antibody, even in the presence of ΔpH. These results indicate that precursor binding to the cpTatC–Hcf106 complex constitutes the recognition event for this pathway and that subsequent participation by Tha4 leads to translocation.
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              Structural changes linked to proton translocation by subunit c of the ATP synthase.

              F1F0 ATP synthases use a transmembrane proton gradient to drive the synthesis of cellular ATP. The structure of the cytosolic F1 portion of the enzyme and the basic mechanism of ATP hydrolysis by F1 are now well established, but how proton translocation through the transmembrane F0 portion drives these catalytic changes is less clear. Here we describe the structural changes in the proton-translocating F0 subunit c that are induced by deprotonating the specific aspartic acid involved in proton transport. Conformational changes between the protonated and deprotonated forms of subunit c provide the structural basis for an explicit mechanism to explain coupling of proton translocation by F0 to the rotation of subunits within the core of F1. Rotation of these subunits within F1 causes the catalytic conformational changes in the active sites of F1 that result in ATP synthesis.
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                Author and article information

                Journal
                J Cell Biol
                The Journal of Cell Biology
                The Rockefeller University Press
                0021-9525
                1540-8140
                15 April 2002
                : 157
                : 2
                : 205-210
                Affiliations
                Horticultural Sciences and Plant Molecular and Cellular Biology, University of Florida, Gainesville, FL 32611
                Author notes

                Address correspondence to Kenneth Cline, Horticultural Sciences Department, Fifield Hall, University of Florida, Gainesville, FL 32611. Tel.: (352) 392-4711 ext. 219. Fax: (352) 392-5653. E-mail: kcline@ 123456ufl.edu

                Article
                0202048
                10.1083/jcb.200202048
                2199252
                11956224
                Copyright © 2002, The Rockefeller University Press
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