Trypanosomes have divergent kinesin-2 proteins that function differentially in flagellum biosynthesis and cell viability.

Trypanosoma brucei, the causative agent of African sleeping sickness, has a flagellum that is crucial for motility, pathogenicity, and viability. In most eukaryotes, the intraflagellar transport (IFT) machinery drives flagellum biogenesis, and anterograde IFT requires kinesin-2 motor proteins. In this study, we investigated the function of the two T. brucei kinesin-2 proteins, TbKin2a and TbKin2b, in bloodstream form trypanosomes. We found that, compared to kinesin-2 proteins across other phyla, TbKin2a and TbKin2b show greater variation in neck, stalk and tail domain sequences. Both kinesins contributed additively to flagellar lengthening. Silencing TbKin2a inhibited cell proliferation, cytokinesis and motility, whereas silencing TbKin2b did not. TbKin2a was localized on the flagellum and colocalized with IFT components near the basal body, consistent with it performing a role in IFT. TbKin2a was also detected on the flagellar attachment zone, a specialized structure that connects the flagellum to the cell body. Our results indicate that kinesin-2 proteins in trypanosomes play conserved roles in flagellar biosynthesis and exhibit a specialized localization, emphasizing the evolutionary flexibility of motor protein function in an organism with a large complement of kinesins.

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

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.

Identification of a bacterial factor required for actin-based motility of Burkholderia pseudomallei.

Burkholderia pseudomallei is a Gram-negative facultative intracellular pathogen that enters and escapes from eukaryotic cells using the power of actin polymerization. We have identified a bacterial protein (BimA) that is required for the ability of B. pseudomallei to induce the formation of actin tails. BimA contains proline-rich motifs and WH2-like domains and shares limited homology at the C-terminus with the Yersinia autosecreted adhesin YadA. BimA is located at the pole of the bacterial cell at which actin polymerization occurs and mutation of bimA abolished actin-based motility of the pathogen in J774.2 cells. Transient expression of BimA in HeLa cells resulted in F-actin clustering reminiscent of that seen on WASP overexpression. Antibody-mediated clustering of a CD32 chimera in which the cytoplasmic domain was replaced with BimA resulted in localization of the chimera to the tips of F-actin enriched membrane protrusions. We report that purified truncated BimA protein binds monomeric actin in a concentration-dependent manner in cosedimentation assays and that BimA stimulates actin polymerization in vitro in a manner independent of the cellular Arp2/3 complex.

A Rickettsia WASP-like protein activates the Arp2/3 complex and mediates actin-based motility.

Spotted fever group Rickettsia are obligate intracellular pathogens that exploit the host cell actin cytoskeleton to promote motility and cell-to-cell spread. Although other pathogens such as Listeria monocytogenes use an Arp2/3 complex-dependent nucleation mechanism to generate comet tails consisting of Y-branched filament arrays, Rickettsia polymerize tails consisting of unbranched filaments by a previously unknown mechanism. We identified genes in several Rickettsia species encoding proteins (termed RickA) with similarity to the WASP family of Arp2/3-complex activators. Rickettsia rickettsii RickA activated both the nucleation and Y-branching activities of the Arp2/3 complex like other WASP-family proteins, and was sufficient to direct the motility of microscopic beads in cell extracts. Actin tails generated by RickA-coated beads consisted of Y-branched filament networks. These data suggest that Rickettsia use an Arp2/3 complex-dependent actin-nucleation mechanism similar to that of other pathogens. We propose that additional Rickettsia or host factors reorganize the Y-branched networks into parallel arrays in a manner similar to a recently proposed model of filopodia formation.

Mycobacterium marinum escapes from phagosomes and is propelled by actin-based motility.

Mycobacteria are responsible for a number of human and animal diseases and are classical intracellular pathogens, living inside macrophages rather than as free-living organisms during infection. Numerous intracellular pathogens, including Listeria monocytogenes, Shigella flexneri, and Rickettsia rickettsii, exploit the host cytoskeleton by using actin-based motility for cell to cell spread during infection. Here we show that Mycobacterium marinum, a natural pathogen of fish and frogs and an occasional pathogen of humans, is capable of actively inducing actin polymerization within macrophages. M. marinum that polymerized actin were free in the cytoplasm and propelled by actin-based motility into adjacent cells. Immunofluorescence demonstrated the presence of host cytoskeletal proteins, including the Arp2/3 complex and vasodilator-stimulated phosphoprotein, throughout the actin tails. In contrast, Wiskott-Aldrich syndrome protein localized exclusively at the actin-polymerizing pole of M. marinum. These findings show that M. marinum can escape into the cytoplasm of infected macrophages, where it can recruit host cell cytoskeletal factors to induce actin polymerization leading to direct cell to cell spread.

Salmonella effectors translocated across the vacuolar membrane interact with the actin cytoskeleton.

A family of nine Salmonella typhimurium type III secretion effectors with a conserved amino-terminus have been defined. Three family members (SifA, SifB and SseJ) have previously been demonstrated to localize to the Salmonella-containing vacuole and to Salmonella-induced filaments. In contrast, we demonstrate that two other family members, SspH2 and SseI, co-localized with the polymerizing actin cytoskeleton. These proteins also interacted with the mammalian actin cross-linking protein filamin in the yeast two-hybrid assay through their highly conserved amino-terminal domains. This amino-terminus was sufficient to direct localization to the polymerizing actin cytoskeleton, suggesting that the interaction with filamin is important for this subcellular localization. In addition, SspH2 co-localized with vacuole-associated actin polymerizations (VAP) induced by intracellular bacteria through the Salmonella pathogenicity island (SPI)-2 type III secretion system (TTSS). SspH2 interacted with the actin-binding protein profilin in the yeast two-hybrid assay and by affinity chromatography. This interaction was highly specific to SspH2 and was mediated by its carboxy-terminus. Furthermore, SspH2 inhibited the rate of actin polymerization in vitro, suggesting that it functions to reduce or remodel VAP. Strains with mutations in sspH2 and sseI retained the ability to form VAP. However, a third intracellular virulence factor, spvB, which ADP-ribosylates actin, strongly inhibited VAP formation in HeLa cells, suggesting a more subtle effect for SspH2 and SseI on the actin cytoskeleton.

Motility determinants in WASP family proteins.

In response to upstream signals, proteins in the Wiskott-Aldrich Syndrome protein (WASP) family regulate actin nucleation via the Arp2/3 complex. Despite intensive study of the function of WASP family proteins in nucleation, it is not yet understood how their distinct structural organization contributes to actin-based motility. Herein, we analyzed the activities of WASP and Scar1 truncation derivatives by using a bead-based motility assay. The minimal region of WASP sufficient to direct movement was the C-terminal WCA fragment, whereas the corresponding region of Scar1 was insufficient. In addition, the proline-rich regions of WASP and Scar1 and the Ena/VASP homology 1 (EVH1) domain of WASP independently enhanced motility rates. The contributions of these regions to motility could not be accounted for by their direct effects on actin nucleation with the Arp2/3 complex, suggesting that they stimulate motility by recruiting additional factors. We have identified profilin as one such factor. WASP- and Scar1-coated bead motility rates were significantly reduced by depletion of profilin and VASP and could be more efficiently rescued by a combination of VASP and wild-type profilin than by VASP and a mutant profilin that cannot bind proline-rich sequences. Moreover, motility of WASP WCA beads was not affected by the depletion or addback of VASP and profilin. Our results suggest that recruitment of factors, including profilin, by the proline-rich regions of WASP and Scar1 and the EVH1 domain of WASP stimulates cellular actin-based motility.