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cleation but proficient in protein translocation will be required to assess the contribution and significance of the actin-binding activity of SipC in bacterial entry.

Putting on the Brakes: A Lesson on Self-Restraint

In his pioneering electron microscopic description of the interaction of S. typhimurium with the intestinal epithelium of infected animals, Akio Takeuchi made the observation that the morphological changes of the brush border induced by the bacteria were reversible (44). He observed that shortly after infection, the brush border of intestinal epithelial cells infected by Salmonella regained their normal appearance despite the presence of numerous bacteria within an intracellular compartment. The molecular bases for the reversion of these marked morphological changes remained unknown until recently. The finding that Salmonella induces its own internalization by activating the small GTP binding proteins Cdc42 and Rac suggested the possibility that the endogenous cellular mechanisms controlling the down-regulation of these GTPases may mediate the reversion of the bacterial-induced responses. However, micro-injection into cultured cells of purified SopE in amounts equivalent to those injected by the bacterial type III secretion system led to actin cytoskeleton rearrangements and membrane ruffling that continued on for periods of time that far exceeded those resulting from bacterial infection (30). This result suggested a more active participation of Salmonella in mediating cellular recovery after bacterial infection. The finding that cells infected with a S. typhimurium strain carrying a null mutation in sptP failed to recover the integrity of their actin cytoskeleton after bacterial infection further supported this hypothesis (ref.21; Fig. 6). SptP is secreted by the invasion-associated type III secretion system and contains two modular domains: (i) an amino terminal domain with sequence similarity to the YopE and ExoS toxins of Yersinia spp. and Pseudomonas aeruginosa, respectively, and (ii) a carboxyl terminal domain homologous to Yersinia YopH and eukaryotic tyrosine phosphatases (10, 45).

How does SptP exert its function? A mechanism for its biochemical function was suggested by the observation that the actin cytoskeleton rearrangements stimulated by the exchange factor SopE could be effectively blocked by the comicroinjection of SptP, suggesting a Cdc42/Rac-1 antagonistic function for this protein (21). G proteins have an intrinsic GTPase activity that allows them to turn themselves off after activation by switching to the GDP-bound (inactive) conformation (46). However, such intrinsic activity is very low unless in the presence of GTPase activating proteins (GAPs), which can stimulate the intrinsic GTPase activity by several orders of magnitude (47). Consistent with its antagonistic function, SptP was found to be a potent GAP for Cdc42 and Rac-1 but not for other GTPases from the Rho or more distantly related families (21). The GAP activity is encoded in the amino terminal domain of SptP. Consistent with this finding, it now has been reported that the related bacterial toxins ExoS and YopE are also GAPs for Rho GTPases (33). Although different GAPs often do not share strong sequence similarity, they do share a short sequence motif that contains a conserved arginine that is essential for catalysis (47). SptP, ExoS, and YopE exhibit such a motif indicating a similarity between the catalytic mechanism of these bacterial proteins and those of eukaryotic GAP proteins (21). Consistent with this hypothesis, an SptP mutant in which the invariant arginine has been changed to an alanine is totally devoid of GAP activity. This mutant also is unable to reverse the actin cytoskeleton rearrangements that follow Salmonella infection, therefore confirming the role of the SptP GAP activity in this process. SptP not only reverses the actin cytoskeletal changes but also prevents the potential harm to the host cell derived from excessive signaling through Cdc42 and Rac that may lead to apoptosis (21). This function may allow the bacteria to preserve the integrity of its intracellular niche long enough to permit its replication and to allow reprogramming of gene expression, a necessary step to continue with the next phase of its pathogenic life cycle.

Lessons Learned from Salmonella

Salmonella entry into host cells requires the coordinated action of several bacterial effector proteins that, on delivery into the host cell, exert their activity in a temporally coordinated manner (Fig. 7). Thus, activation of Cdc42 and Rac by SopE and SopB is followed by actin cytoskeleton rearrangements that are further modulated by the activity of the actin-binding protein SipA. The bacterial-induced cellular responses subsequently are reversed by another bacterial effector protein, SptP, which opposes the activities of SopB and SopE. In addition, it is likely that the effector molecules themselves may have regulatory domains that control their activity. The existence of other putative effector proteins of unknown function that also are delivered into host cells by the centisome 63 type III secretion system indicates that we are just beginning to understand the complexities of this system. The interaction of Salmonella with host cells is therefore an eloquent example of the sophisticated nature of the mechanisms used by bacterial pathogens that have sustained long-standing associations with their hosts. Such sophistication is the result of evolutionary forces operating over extended periods of time, leading to a rather balanced interaction that allows bacterial replication while preventing excessive harm to the host. Because the study of microbial pathogens commonly focuses in the examination of the events that lead to overt harm to the host, it is frequently overlooked that the interaction of these “adapted pathogens” with their hosts most often does not lead to overt disease. In fact, the Salmonella example indicates that the bacteria can be actively engaged in preventing overt harm. This fact is particularly important when considering the design of novel therapeutic and prevention strategies because inadvertent interference with “down-modulators of virulence” could result in more harm to the host. As we increase our understanding of the molecular mechanisms that govern the interaction of adapted pathogens with their hosts, we will undoubtedly find more examples of “pathogen self-restraint. ”

We thank members of J.E.G.'s laboratory for careful review of this manuscript. Work in J.E.G. 's laboratory that was discussed in this article was supported by Grants AI30492 and GM52543 from the National Institutes of Health.

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