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An Immunodominant Membrane Protein (Imp) of ‘Candidatus Phytoplasma mali’ Binds to Plant Actin

The phytopathogenic, cell-wall-less phytoplasmas exhibit a dual life cycle: they multiply in the phloem of their host plant and in the body of their insect vector. Their membrane proteins are in direct contact with both hosts and are supposed to play a crucial role in the phytoplasma spread within the plant as well as by the insect vector. Three types of nonhomologous but highly abundant and immunodominant membrane proteins (IDP) have been identified within the phytoplasmas: Amp, IdpA, and Imp. Although recent results indicate that Amp is involved in vector specificity interacting with insect proteins such as actin, little is known about the interaction of IDP with the plant. We could demonstrate that transiently expressed Imp of ‘Candidatus Phytoplasma mali’ as well as the Imp without transmembrane domain (Imp^▲Tm) bind with plant actins in vivo. Moreover, in vitro co-sediment and binding assays showed that Escherichia coli-expressed recombinant Imp^▲Tm-His binds to both G- and F-actins isolated from rabbit muscle. Transgenic plants expressing Imp- or Imp^▲Tm-green fluorescent protein did not exhibit any remarkable change of phenotype compared with the wild-type plant. These results indicate that Imp specifically binds to plant actin and a role of Imp-actin binding in phytoplasma motility is hypothesized.