Ca2+-dependent lipid binding and membrane integration of PopA, a harpin-like elicitor of the hypersensitive response in tobacco

Authors

  • Judith Racapé,

    1. Unité Mixte de Recherches Interactions Plantes-Microorganismes et Santé Végétale, INRA-CNRS-UNSA, 400 Route des Chappes, 06903 Sophia Antipolis, France.
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  • Lassaad Belbahri,

    1. Unité Mixte de Recherches Interactions Plantes-Microorganismes et Santé Végétale, INRA-CNRS-UNSA, 400 Route des Chappes, 06903 Sophia Antipolis, France.
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  • Stefan Engelhardt,

    1. Eberhard-Karls-Universität Tübingen, Zentrum für Molekularbiologie der Pflanzen, Auf der Morgenstelle 5, 72076 Tübingen, Germany.
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  • Benoit Lacombe,

    1. Unité Mixte de Recherches Biochimie et Physiologie Moleculaire des Plantes, AgroM-CNRS-INRA-UMII, 2 Place Viala, 34060 Montpellier, France.
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  • Justin Lee,

    1. Department of Stress and Developmental Biology, Leibniz Institute of Plant Biochemistry, Weinberg 3, 06120 Halle (Saale), Germany.
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  • Jan Lochman,

    1. Department of Biochemistry, Faculty of Science, Masaryk University, Kotlarska 2, 61137 Brno, Czech Republic.
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  • Antoine Marais,

    1. Unité Mixte de Recherches Interactions Plantes-Microorganismes et Santé Végétale, INRA-CNRS-UNSA, 400 Route des Chappes, 06903 Sophia Antipolis, France.
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  • Michel Nicole,

    1. Unité Mixte de Recherches Diversite et Genome des Plantes Cultivees, CIRAD-INRA-IRD, BP 64501, 34394 Montpellier, France.
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  • Thorsten Nürnberger,

    1. Eberhard-Karls-Universität Tübingen, Zentrum für Molekularbiologie der Pflanzen, Auf der Morgenstelle 5, 72076 Tübingen, Germany.
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  • Francis Parlange,

    1. Unité Mixte de Recherches Interactions Plantes-Microorganismes et Santé Végétale, INRA-CNRS-UNSA, 400 Route des Chappes, 06903 Sophia Antipolis, France.
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  • Sandrine Puverel,

    1. Laboratoire Reponse des Organismes aux Stress Environnementaux, Universite de Nice-Sophia Antipolis, Faculte des Sciences, 06108 Nice, France.
    2. Centre Scientifique de Monaco, Avenue Saint Martin, MC-98000, Monaco.
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  • Harald Keller

    Corresponding author
    1. Unité Mixte de Recherches Interactions Plantes-Microorganismes et Santé Végétale, INRA-CNRS-UNSA, 400 Route des Chappes, 06903 Sophia Antipolis, France.
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E-mail keller@antibes.inra.fr; Tel. (+33) 492386594; Fax (+33) 492386587.

Summary

PopA is released by type III secretion from the bacterial plant pathogen Ralstonia solanacearum and triggers the hypersensitive response (HR) in tobacco. The function of PopA remains obscure, mainly because mutants lacking this protein are not altered in their ability to interact with plants. In an attempt to identify the site of PopA activity in plant cells, we generated transgenic tobacco plants expressing the popA gene under the control of an inducible promoter. Immunocytologic analysis revealed that the HR phenotype of these plants correlated with the presence of PopA at the plant plasma membrane. Membrane localization was observed irrespective of whether the protein was designed to accumulate in the cytoplasm or to be secreted by the plant cell, suggesting a general lipid-binding ability. We found that the protein had a high affinity for sterols and sphingolipids in vitro and that it required Ca2+ for both lipid binding and oligomerization. In addition, the protein was integrated into liposomes and membranes from Xenopus laevis oocytes where it formed ion-conducting pores. These characteristics suggest that PopA is part of a system that aims to attach the host cell plasma membrane and to allow molecules cross this barrier.

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