Regulation of IcsP, the Outer Membrane Protease of the Shigella Actin Tail Assembly Protein IcsA, by Virulence Plasmid Regulators VirF and VirB

Abstract

The Shigella outer membrane protease IcsP removes the actin assembly protein IcsA from the bacterial surface, and consequently modulates Shigella actin-based motility and cell-to-cell spread. Here, we demonstrate that IcsP expression is undetectable in mutants lacking either of two transcriptional activators, VirF and VirB. In wild-type Shigella spp., virB expression is entirely dependent on VirF; therefore, to circumvent this regulatory cascade, we independently expressed VirF or VirB in Shigella strains lacking both activators and measured both IcsP levels and transcription from the icsP promoter. Our results show that VirB significantly enhanced icsP transcription, even in the absence of VirF. In contrast, when VirF was induced in the absence of VirB, VirF had variable effects. The regulation of icsP is distinctly different from the regulation of the gene encoding its major substrate, icsA, which is activated by VirF and not VirB. We propose that the different pathways regulating icsA and icsP may be critical to the modulation of IcsA-mediated actin-based motility by IcsP.Shigella spp., gram-negative bacterial pathogens cause severe and bloody diarrhea in their human hosts by invading and spreading through the colonic epithelium. Shigella movement within the host cell cytoplasm is dependent on the ability of the bacterium to recruit host cell actin to its surface to form an "actin tail," which propels the bacterium from one cell to another (5,16,29). Actin tail assembly is mediated by a single bacterial protein, IcsA, which is found on the outer surface at one pole of the bacterium (17). This asymmetric localization of IcsA ensures that actin assembly occurs in a directional manner. In its mature form, IcsA is comprised of two domains: the ␣ domain (residues 53 to 758) contains the determinant for actin assembly (14) and extends from the bacterial surface into the extracellular environment, whereas the ␤ domain (residues 759 to 1102) is embedded in the outer membrane (33). The amount of IcsA ␣ domain exposed on the bacterial surface correlates with the efficiency of actin tail formation in the cytoplasm of infected cells (21).IcsP, an outer membrane protease of Shigella, cleaves IcsA between Arg 758 and Arg 759 , removing the entire IcsA ␣ domain from the bacterial surface (8, 13, 15a, 31). Overexpression of IcsP leads to complete removal of the IcsA ␣ domain from the bacterial cell surface (32), whereas genetic disruption of icsP increases the total amount of cell associated IcsA ␣ domain, leading to an increase in the rate of actin-based movement of Shigella (31). Although IcsP is not required for polar localization of IcsA (6,28), it contributes to the maintenance of a tight polar cap of IcsA on the bacterial surface (31). Furthermore, as Shigella enter stationary phase, the amount of cell-associated IcsA ␣ domain decreases dramatically, an effect due at least in part to IcsP (18,32).These data demonstrate that IcsP plays an important role in modulating the amount of the IcsA ␣ domain present on the bacter...

Characterization of the ospZ Promoter in Shigella flexneri and Its Regulation by VirB and H-NS

“…Our previous studies at the icsP and ospZ locus have revealed that H-NS is able to repress the icsP promoter at 37°C (14). Therefore, we hypothesized that H-NS would also repress transcription of the divergent gene ospZ at 37°C.…”

“…Upon a switch to 37°C, a complex regulatory cascade is triggered, which results in the production of VirF and subsequently VirB (3,24,25). Although some virulence genes are derepressed solely by the increase in temperature (23), others are derepressed at 37°C by either VirF or VirB, which function to relieve the H-NS-mediated repression of these virulence gene promoters on the large virulence plasmid (3,14,21,26).…”

“…␤-Galactosidase activity was determined using the Miller protocol (16). Overnight cell cultures were back-diluted 1:100 and grown for 5 h at 37°C before being assayed, because prior experiments have shown that both icsP and ospZ promoter activities are maximal after 5 h (14). In experiments in which virB was induced from the pBAD18 derivative, Shigella and E. coli strains were grown for 3 h and 5 h, respectively, at either 30 or 37°C prior to induction with 0.2% (wt/vol) L-arabinose.…”

“…IcsP is an outer membrane protease that cleaves IcsA, the actin tail assembly protein, from the surface of Shigella and functions to regulate the actin-based motility of Shigella within epithelial cells (12,13). Like many virulence genes in Shigella, icsP is transcriptionally repressed by H-NS and derepressed by VirB (14). This mechanism of gene regulation has been termed "silencing and anti-silencing" (4).…”

Shigella

“…icsP is also regulated by the H-NS antagonist VirB. 114 icsA is expressed upon invasion of the host through its regulators H-NS, VirF and a 450 nt antisense RNA (RnaG) acting as transcriptional attenuator. H-NS was shown to repress icsA transcription at 30°C but not at 37°C.…”