Defining a core set of actin cytoskeletal proteins critical for actin-based motility of Rickettsia

Abstract

Many Rickettsia species are intracellular bacterial pathogens that use actin-based motility for spread during infection. However, while other bacteria assemble actin tails consisting of branched networks, Rickettsia assemble long parallel actin bundles, suggesting the use of a distinct mechanism for exploiting actin. To identify the underlying mechanisms and host factors involved in Rickettsia parkeri actin-based motility, we performed an RNAi screen targeting 115 actin cytoskeletal genes in Drosophila cells. The screen delineated a set of four core proteins-profilin, fimbrin/T-plastin, capping protein, and cofilin--as crucial for determining actin tail length, organizing filament architecture, and enabling motility. In mammalian cells, these proteins were localized throughout R. parkeri tails, consistent with a role in motility. Profilin and fimbrin/T-plastin were critical for the motility of R. parkeri but not Listeria monocytogenes. Our results highlight key distinctions between the evolutionary strategies and molecular mechanisms employed by bacterial pathogens to assemble and organize actin.

Figure 1. RNAi identified numerous targets that affect actin tail length and morphology

(A, B) Graphs of actin tail lengths (A) and widths (B) following RNAi of the indicated targets. Data are mean ± SEM. Black bar indicates the untreated control, grey bars indicate no difference from the control based on a pairwise Student’s t-test (p>0.05), and white bars indicate a statistically significant difference from the control (p<0.05). In cases where 2 distinct dsRNAs were used, targets are designated with −1 and −2. (C) Z-sections from deconvolved images of untreated cells, or cells treated with dsRNAs targeting profilin, fimbrin and capping protein. R. parkeri (red) was visualized by immunofluorescence, and actin tails (green) were visualized with Alexa Fluor 488 phalloidin. Scale bar, 10 μm.

Figure 2. RNAi of profilin, fimbrin, capping protein and cofilin resulted in reduced motility rates

(A) Graph of R. parkeri motility rates (mean ± SD) in untreated cells, and cells treated with dsRNAs targeting profilin, fimbrin, capping protein and cofilin. Rates for all dsRNA-treated cells are different from the untreated control (Kruskal-Wallis and Dunn’s Multiple Comparison tests; p<0.05), and rates for profilin RNAi are different from all other RNAi targets (p<0.01). (B) Scatter plot showing the mean tail length and the corresponding mean motility rate for individual bacteria in untreated control cells (black) or cells treated with dsRNAs targeting profilin (red), fimbrin (blue), capping protein (yellow) and cofilin (green). The grey line is the best linear fit for the combined data set. See also Movies S1, S2 and S3.

Figure 3. Localization of GFP-tagged cytoskeletal proteins to R. parkeri and L. monocytogenes tails

Images taken from movies of COS7 cells expressing the indicated GFP-tagged proteins or GFP alone. Cells were either infected with R. parkeri or L. monocytogenes. Arrowheads indicate motile bacteria. Scale bar, 5 μm. See also Movies S4, S5, S6, S7 and S8.

Figure 4. RNAi of T-plastin in mammalian cells reduces the frequency and rate of motility

COS7 cells were treated with siRNA targeting T-plastin (PLS3-1, -2), or control GAPDH (GAPD) or non-specific (NS) siRNAs, and infected with either R. parkeri (A-C, G) or L. monocytogenes (D-F). (A, D) Graphs of the percentage of bacteria (mean ± SD) associated with actin tails after transfection with the indicated siRNA. Triple asterisks indicate a statistically significant difference after pairwise comparison to the nonspecific siRNA control (p<0.001, Student’s t-test). (B, E) Scatter plots of tail length after transfection with the indicated siRNA (red bar indicates mean). (C, F) Scatter plots of the mean tail length and the corresponding mean motility rate for individual bacteria in cells transfected with control non-specific (black) or T-plastin siRNA (blue). (G) R. parkeri (red) visualized by immunofluorescence and actin (green) visualized with Alexa Fluor 488 phalloidin in cells transfected with non-specific (NS) or T-plastin (PLS3-1) siRNAs. Scale bar, 5 μm. See also Movies S9 and S10.

Figure 5. RNAi of profilin 1 in mammalian cells reduces the length of R. parkeri actin tails and the motility rate

COS7 cells were treated with siRNA targeting profilin 1 (PFN1-1, -2), or control GAPDH (GAPD) or non-specific (NS) siRNAs, and infected with either R. parkeri (A-C, G) or L. monocytogenes (D-F). (A, D) Graphs of the percentage of bacteria associated with actin tails (mean ± SD) after transfection with the indicated siRNA. (B, E) Scatter plots of the length of actin tails after transfection with the indicated siRNA (red bar indicates mean). Triple asterisks indicate a statistically significant difference after pairwise comparison to the nonspecific siRNA control (p<0.001); Student’s t-test). (C, F) Scatter plots showing the mean tail length and the corresponding mean motility rate for individual bacteria in cells transfected with control non-specific siRNA (black) or with profilin 1 siRNA (red). (G) R. parkeri (red) visualized by immunofluorescence and actin (green) visualized with Alexa Fluor 488 phalloidin in cells transfected with non-specific (NS) or profilin 1 (PFN1-1) siRNAs. Scale bar, 5 μm. See also Movies S9 and S11.

Figure 6. Model of the molecular mechanism of actin-based Rickettsia motility

Actin filament assembly at the bacterial surface is induced by a nucleator that is likely of bacterial origin (red drop). Actin monomers (green circles) associated with profilin (blue circles) assemble onto filament barbed ends at the bacterial surface, generating motile force. Fimbrin/T-plastin (yellow cross) bundles the growing filaments into long helical strands, while displacing cofilin (orange triangles). Cofilin binds to older ADP-actin filaments and facilitates severing and disassembly at the pointed ends, replenishing the monomer pool. Capping protein (purple arches) blocks addition of ATP-actin monomers to the barbed ends of older filaments, enhancing the flux of profilin-bound actin monomers onto newer barbed ends.