Interaction between the membrane protein of a pathogen and insect microfilament complex determines insect-vector specificity

Many insect-transmissible pathogens are transmitted by specific insect species and not by others, even if they are closely related. The molecular mechanisms underlying such strict pathogen–insect specificity are poorly understood. Candidatus Phytoplasma asteris, OY strain, line W (OY), is a phytopathogenic bacterium transmitted from plant to plant by sap-feeding insect vectors (leafhoppers). Our study focused on an abundant cell-surface membrane protein of the phytoplasma named antigenic membrane protein (Amp), which is not homologous with any reported functional protein. Immunofluorescence microscopy of the phytoplasma-infected insect showed that OY phytoplasma was localized to the microfilaments of the visceral smooth muscle surrounding the insect’s intestinal tract. The affinity column assay showed that Amp forms a complex with three insect proteins: actin, myosin heavy chain, and myosin light chain. Amp–microfilament complexes were detected in all OY-transmitting leafhopper species, but not in the non-OY-transmitting leafhoppers, suggesting that the formation of the Amp–microfilament complex is correlated with the phytoplasma-transmitting capability of leafhoppers. Although several studies have reported interactions between pathogens and mammalian microfilaments, this is an example of host-specific interactions between a bacterial surface protein and a host microfilament in insect cells. Our data also suggest that the utilization of a host microfilament may be a universal system for pathogenic bacteria infecting mammals or insects