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Cytoplasmic Actin Is an Extracellular Insect Immune Factor which Is Secreted upon Immune Challenge and Mediates Phagocytosis and Direct Killing of Bacteria, and Is a Plasmodium Antagonist

Fig 6

Globular and filamentous actin display different immune properties.

(A) Actin content in 0.3–0.5M NaCl eluates (E3–E5 fractions) or pellets (P) of E. coli and S. aureus incubated with recombinant globular (G Ac) or filamentous (F Ac) actin. (B) FITC-labeled E. coli (green) incubated with LPS-stimulated Sua5B cell supernatants and stained for AgMDL1 (blue) and actin (red). Co-localization is indicated in white. (C) The percentage of phagocytosing S2 cells containing at least one E. coli bacterium incubated with recombinant globular (G Ac) or filamentous (F Ac) actin with or without AgMDL1 or the control protein GUS as compared to untreated bacteria (PBS). For each assay, at least 16 fields were counted, and the data are representative of three independent experiments. Each bar represents the mean ± the standard deviation. Statistical significance was determined using Student’s t-test. (D) Viability of E. coli was determined after 1- or 24-hr incubations with recombinant globular (G Ac) or filamentous actin (F Ac), with or without AgMDL1, or the control protein GUS as compared to untreated bacteria (PBS). Error bars represent the mean ± the standard deviation. Statistical significance was determined using Student’s t-test. (E) Scanning electron microscopy image of E. coli cells after incubation with G actin (G Ac), F actin (F Ac), AgMDL1 (MDL1) alone or together (G Ac+MDL1, F Ac+MDL1) or with the control protein GUS. Untreated cells were incubated with PBS. Scale bar 200 nm.

Fig 6

doi: https://doi.org/10.1371/journal.ppat.1004631.g006