In our recent paper,10
we found that the L. pneumophila
effector VipA binds and nucleates actin polymerization in vitro.
During infection of macrophages, translocated VipA co-localizes with actin filaments and early endosomes, an association also observed when VipA-GFP was ectopically expressed in S. cerevisiae
and in mammalian CHO cells. In previous work,11
it had been shown that this effector impaired vacuolar protein trafficking in S. cerevisiae
, interfering with the Multivesicular Body (MVB) pathway, which is supported by its colocalization with some MVB components. In addition, we isolated a VipA mutant carrying a mutation in the N-terminal region of the protein (VipA-1), which is affected in its binding and polymerization of actin, in its ability to interfere with the MVB pathway and subcellular localization. Taken together, these results suggest a role for VipA as a link between the host actin cytoskeleton and endosomal trafficking, a novel function among prokaryotic virulence factors.
To date, only a small number of bacterial actin nucleators have been identified and characterized (). Chlamydia Tarp and Salmonella SipC are involved in actin-mediated uptake of the pathogen, while Rickettsia Sca2 is a formin-like nucleator that mediates intracellular movement of the bacterium via polymerization of actin comet tails.12-14
In addition, the extracellular bacterium Vibrio parahaemolyticus
injects into host epithelial cells VopL and VopF, which lead to a rearrangement of the actin network and cause disruption of cell-cell contacts, facilitating enteric infection.15,16
None of these bacterial actin nucleators have however been implicated in vesicle trafficking, and to our knowledge only two effectors have given clues as to how the pathogen may link actin dynamics to endosomal trafficking for their own benefit. The Salmonella enterica
phosphoinositide phosphatase SopB plays multiple roles during infection. In addition to mediating actin-dependent internalization of the bacteria, it also prevents the transition of PI(3)P to PI(3,5)P in the Salmonella containing vacuole (SCV) membrane and therefore may contribute to arresting SCV maturation along the endosomal pathway and consequently lysosomal fusion.17-19
Enteropathogenic Escherichia coli
(EPEC) EspF nucleates a multiprotein complex containing the Arp2/3 activator N-WASP and the endocytic regulator sorting nexin 9 (SNX9), which mediates clathrin-dependent endocytosis. Although the mechanism or pathogenic significance of this has not been yet elucidated, EspF has been proposed to alter SNX9 regulation of endocytosis during EPEC infection, leading to the formation of aberrant tubular vesicles.20-22
Figure 1. Bacterial actin nucleators and their role in infection. Schematic diagram of cell rearrangements led by bacterial effector-driven actin polymerization. In Chlamydia, a cooperative mechanism leading to the formation of cell protrusions (more ...)