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ER quality control can lead to retrograde transport from the ER lumen to the cytosol and the nucleoplasm in plants

Authors

  • Federica Brandizzi,

    Corresponding author
    1. Research School of Biological and Molecular Sciences, Oxford Brookes University, Gipsy Lane, Oxford OX3 0BP, UK,
      For correspondence (fax +44 1865 483955; e-mail fbrandizzi@brookes.ac.uk).
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  • Sally Hanton,

    1. Centre for Plant Sciences, Leeds Institute for Plant Biotechnology and Agriculture, School of Biology, University of Leeds, Leeds LS2 9JT, UK, and
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  • Luis L. Pinto DaSilva,

    1. Centre for Plant Sciences, Leeds Institute for Plant Biotechnology and Agriculture, School of Biology, University of Leeds, Leeds LS2 9JT, UK, and
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  • Petra Boevink,

    1. Scottish Crop Research Institute, Dundee DD2 5DA, UK
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  • David Evans,

    1. Research School of Biological and Molecular Sciences, Oxford Brookes University, Gipsy Lane, Oxford OX3 0BP, UK,
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  • Karl Oparka,

    1. Scottish Crop Research Institute, Dundee DD2 5DA, UK
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  • Jürgen Denecke,

    1. Centre for Plant Sciences, Leeds Institute for Plant Biotechnology and Agriculture, School of Biology, University of Leeds, Leeds LS2 9JT, UK, and
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    • Both laboratories contributed equally.

  • Chris Hawes

    1. Research School of Biological and Molecular Sciences, Oxford Brookes University, Gipsy Lane, Oxford OX3 0BP, UK,
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    • Both laboratories contributed equally.


For correspondence (fax +44 1865 483955; e-mail fbrandizzi@brookes.ac.uk).

Summary

Quality control in the secretory pathway is a fundamental step in preventing deleterious effects that may arise by the release of malfolded proteins into the cell or apoplast. Our aims were to visualise and analyse the disposal route followed by aberrant proteins within a plant cell in vivo using fluorescent protein technology. A green fluorescent protein (GFP) fusion was detected in the cytosol and the nucleoplasm in spite of the presence of an N-terminal secretory signal peptide. In contrast to secreted GFP, the fusion protein was retained in the cells where it was degraded slowly, albeit at a rate much higher than that of the endoplasmic reticulum (ER)-retained derivative GFP-HDEL. The fusion protein could not be stabilised by inhibitors of transport or the cytosolic proteasome. However, the protein is a strong lumenal binding protein (BiP) ligand. Complete signal peptide processing even after long-term expression in virus-infected leaves rules out the possibility that the documented accumulation in the cytosol and nucleoplasm is because of the bypassing of the translocation pores. The data are consistent with the hypothesis that the fusion protein is disposed off from the ER via a retrograde translocation back to the cytosol. Moreover, accumulation in the nucleoplasm was shown to be microtubule dependent unlike the well-documented diffusion of cytosolically expressed GFP into the nucleoplasm. The apparent active transport of the GFP fusion into the nucleoplasm may indicate an as yet undiscovered feature of the ER-associated degradation (ERAD) pathway and explain the insensitivity to degradation by proteasome inhibitors.

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