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Infection and Immunity, October 2000, p. 6066-6068, Vol. 68, No. 10
Medical College of Wisconsin, Microbiology
and Molecular Genetics, Milwaukee, Wisconsin
53226,1 and Institut für
Pharmakologie und Toxikologie, Albert-Ludwigs-Universität
Freiburg, D-79104 Freiburg, Germany2
Received 25 April 2000/Returned for modification 20 June
2000/Accepted 31 July 2000
Transient intracellular expression of ExoT in CHO cells stimulated
cell rounding and actin reorganization. Biochemical studies showed that
ExoT was a GTPase-activating protein for RhoA, Rac1, and Cdc42.
Together, these data show that ExoT interferes with Rho signal
transduction pathways, which regulate actin organization, exocytosis,
cell cycle progression, and phagocytosis.
Pseudomonas
aeruginosa is a gram-negative opportunistic pathogen
affecting patients with cystic fibrosis, AIDS, severe burns, neutropenia, and ocular infections. Recent studies have shown that
P. aeruginosa possesses a type-III system to deliver
cytotoxins directly into eukaryotic cells (3). To date, four
type-III cytotoxins have been identified: ExoS, ExoT, ExoY, and ExoU.
The enzymatic activity of exoenzyme S was purified from
the culture supernatant of P. aeruginosa 388 as an aggregate
composed of two proteins with apparent molecular masses of 49 kDa
(ExoS) and 53 kDa (ExoT). Subsequent studies showed ExoS (8)
and ExoT (14) were encoded by separate genes and had 76%
primary amino acid homology. ExoS is a bifunctional cytotoxin that
encodes two independent catalytic activities. The carboxyl terminus
encodes a cytotoxic FAS (factor activating exoenzyme
S)-dependent ADP-ribosyltransferase activity (6) while the
amino terminus stimulates the intrinsic GTPase activity of Rho
GTPases (4). ExoT also catalyzes a FAS-dependent ADP
ribosylation of eukaryotic proteins, but at a velocity that is only
0.2% of that of ExoS (9). Recently, Vallis et al. showed that delivery of ExoT by P. aeruginosa stimulates CHO
cells to round (13).
A transient transfection system was used to directly measure the effect
of ExoT on eukaryotic cell physiology (10). Hemagglutinin (HA) epitope-tagged ExoT (ExoT-HA) was engineered to encode the entire open reading frame of ExoT fused at its carboxyl terminus to the amino acid sequence of the hemagglutinin epitope
(11). ExoT-HA was subcloned into pEGFP-N1 and
expressed under the control of a cytomegalovirus promoter. Chinese
Hamster Ovary (CHO) K1 cells were seeded at 5 × 105 cells/dish in 100-mm dishes and 5 × 104 cells/dish in 12-well dishes the day before
transfection. Cotransfections were performed with
Lipofectamine-PLUS (Gibco-BRL) using 1 µg (100-mm dishes)
or 50 ng (12-well dishes) of reporter plasmid (pEGFP-N1) and the
indicated amount of effector DNA (pExoT-HA). Total DNA transfected was normalized with pCMV-10luc.
Twenty-four hours posttransfection,
pExoT-HA-transfected cells showed a rounded morphology that was
dose responsive (Fig.
1). To associate
the rounding phenotype with ExoT expression, Western blot
analyses of cell lysates from pExoT-HA-transfected CHO cells
were performed by enhanced chemiluminescence (ECL), using the
rabbit anti-HA immunoglobulin (IgG) or mouse anti-green fluorescent
protein (anti-GFP) IgG (1/1000 dilution; BABCO) as the primary antibody
and the appropriate horseradish peroxidase-conjugated IgG
(1:40,000; Pierce) (11). Lysates from
pExoT-HA-transfected CHO cells possessed two bands indicating
reactivity to HA with apparent molecular masses of 53 and 39 kDa (Fig.
1). The 53-kDa band migrated at the predicted molecular weight
as full-length ExoT-HA, while the presence of the 39-kDa band
suggested that ExoT-HA had been processed at its amino terminus,
since the HA epitope was fused to the carboxyl terminus of ExoT.
Coexpression of ExoT did not inhibit expression of the reporter
protein, EGFP, indicating that ExoT was not cytotoxic to CHO cells.
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Copyright © 2000, American Society for Microbiology. All rights reserved.
Pseudomonas aeruginosa ExoT Is a Rho
GTPase-Activating Protein
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FIG. 1.
Transfection of CHO cells with pExoT-HA. (A) CHO
cells were cotransfected with 50 ng of pEGFP alone or with 300 ng of
pExoT-HA. Twenty-four hours posttransfection, cells were fixed,
stained with phalloidin-rhodamine, and visualized by fluorescence
microscopy for GFP or rhodamine (phalloidin). (B) Lysates from CHO
cells transfected with pEGFP (lane 1) or pEGFP and pExoT-HA (lane
2) were subjected to ECL Western blot analysis, using rabbit anti-HA
IgG (upper section) or mouse anti-GFP IgG (lower section) as the
primary antibody and HRP-conjugated goat anti-rabbit or mouse IgG as
the secondary antibody. (C) CHO cells were transfected with the
indicated amount of pExoT-HA and 50 ng of reporter plasmid (pEGFP)
and, 24 h posttransfection, were scored for morphology. The
percentages of rounded cells shown are averages from analysis of three
fields of transfected cells (approximately 50 transfected cells per
field).
The ability of ExoT-HA to stimulate the reorganization of actin in CHO cells was also determined (11). CHO cells were transfected with pEGFP (control) or pEGFP and pExoT-HA. Twenty-four hours posttransfection CHO cells were fixed with 1% paraformaldehyde in phosphate-buffered saline (PBS), permeabilized with 4% paraformaldehyde containing 0.1% Triton X-100 in H2O, and incubated with 0.2 µg of phalloidin-rhodamine conjugate (Sigma) per ml in PBS to stain the actin cytoskeleton. Fluorescence microscopy showed that control-transfected cells had typical cellular morphology with a well-defined actin cytoskeleton, while pExoT-HA transfected cells were rounded and did not have a defined actin cytoskeleton or stress fibers (Fig. 1). Thus, ExoT stimulates the reorganization of actin.
Other studies in our laboratory localized the GTPase-activating
protein (GAP) domain of ExoS within residues 78 to 234 and showed that
a deletion peptide comprising this region could be expressed in
Escherichia coli as a soluble enzymatically active His(6)
fusion protein (K. J. Pederson and J. T. Barbieri,
unpublished data). Thus, a recombinant deletion peptide,
which comprised residues 78 to 237 of ExoT, was engineered, subcloned
into pET15b (pExoT78-237), and sequenced to confirm the absence
of secondary mutations. ExoT(78-237) was expressed in
E. coli BL21(DE3) as a His(6) fusion protein and purified by
Ni-affinity chromatography as previously described (6).
Purified ExoT(78-237) was dialyzed in 25 mM Tris, pH
7.6, plus 40% glycerol and stored at
20°C. ExoT(78-237) was
determined to have an apparent molecular mass of 22 kDa by sodium
dodecyl sulfate-polyacrylamide gel electrophoresis analysis and
purity of about 85%. ExoT(78-237) possessed limited
immunoreactivity to a polyclonal antibody that was prepared against
ExoS (7).
The GAP activity of ExoT(78-237) was determined with a
filter-binding assay (4). Rho GTPases were loaded with
10 µM [
-32P]GTP. Intrinsic GTPase activity was
initiated with the addition 3 mM MgCl2 and 1 mM GTP.
After 5 min, the reaction mixtures were spotted onto a
nitrocellulose filters, the filters were washed, and the remaining
radioactivity was measured by scintillation counting. Stimulation of
GTP hydrolysis by ExoT(78-237) or ExoS(78-234) was measured, using
a concentration of ExoT or ExoS that stimulated the hydrolysis of
less than 25% of the available Rho-GTP. Under these conditions the
rate of stimulation of GTP hydrolysis by ExoS was linear for 5 min
(4). The intrinsic rates of GTPase activities (in
picomoles of GTP per minute per microgram of protein) were greatest for
recombinant glutathione S-transferase-RhoA, followed by
Rac1 and Cdc42 (data not
shown).
ExoT(78-237) stimulated the intrinsic GTPase activity of RhoA
in a dose-dependent manner and at a rate similar to that stimulated by
ExoS(78-234) (Fig. 2). Figure 2 also shows that ExoT(78-237)
stimulated the GTPase activity of Rac1 and Cdc42.
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Rho GTPases regulate numerous eukaryotic cellular functions, including actin cytoskeleton reorganization (5), exocytosis (1), transcription (12), and phagocytosis (2). Like other members of the family of Ras monomeric G proteins, Rho GTPase modulate cell physiology through their ability to cycle between an inactive GDP bound form and an active GTP bound form that is able to interact with downstream effectors. The Rho GTPases possess low intrinsic capacity to perform guanine nucleotide exchange or to catalyze the hydrolysis of GTP to GDP. These two activities are modulated by two types of eukaryotic protein, guanine exchange factors and GAPs. The ability of ExoT to express Rho GAP activity identifies it as a biochemical activity, which may contribute to the pathogenic potential of P. aeruginosa.
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ACKNOWLEDGMENTS |
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J.T.B. was supported by AI30162 from the NIH-NIAID. K.A. was supported by the DFG.
We thank Kristin J. Pederson for technical assistance during the study.
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ADDENDUM IN PROOF |
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We acknowledge that Joanne Engel and coworkers have also observed that ExoT is a RhoGAP (B. I. Kazmierczak, T. S. Jou, K. Mostov, and J. N. Engel, Cell. Microbiol., in press).
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FOOTNOTES |
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* Corresponding author. Mailing address: Medical College of Wisconsin, Microbiology and Molecular Genetics, 8701 Watertown Plk. Rd., Milwaukee, WI 53226. Phone: (414) 456-8412. Fax: (414) 456-6535. E-mail: toxin{at}mcw.edu.
Editor: V. J. DiRita
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