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

Mol Microbiol. 2005 Dec;58(5):1406-20. doi: 10.1111/j.1365-2958.2004.04910.x.


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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Bacterial Outer Membrane Proteins / genetics
  • Bacterial Outer Membrane Proteins / metabolism*
  • Bacterial Proteins / genetics
  • Bacterial Proteins / metabolism*
  • Calcium / metabolism*
  • Cell Membrane / metabolism*
  • Ion Channels / metabolism
  • Lipid Bilayers / metabolism
  • Membrane Lipids / metabolism*
  • Membranes, Artificial
  • Oocytes / growth & development
  • Oocytes / metabolism
  • Plant Leaves / ultrastructure
  • Plants, Genetically Modified / genetics
  • Plants, Genetically Modified / growth & development
  • Plants, Genetically Modified / metabolism
  • Ralstonia solanacearum / metabolism
  • Ralstonia solanacearum / pathogenicity
  • Tobacco / genetics
  • Tobacco / growth & development
  • Tobacco / metabolism*
  • Xenopus laevis / growth & development
  • Xenopus laevis / metabolism


  • Bacterial Outer Membrane Proteins
  • Bacterial Proteins
  • Ion Channels
  • Lipid Bilayers
  • Membrane Lipids
  • Membranes, Artificial
  • PopA1 protein, Ralstonia solanacearum
  • harpin protein, Erwinia amylovora
  • Calcium