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Infection and Immunity, April 2003, p. 2230-2233, Vol. 71, No. 4
0019-9567/03/$08.00+0 DOI: 10.1128/IAI.71.4.2230-2233.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.
Protein Binding between PcrG-PcrV and PcrH-PopB/PopD Encoded by the pcrGVH-popBD Operon of the Pseudomonas aeruginosa Type III Secretion System
Leonard R. Allmond,1 Timur J. Karaca,1 Vinh N. Nguyen,2 Thong Nguyen,2 Jeanine P. Wiener-Kronish,1 and Teiji Sawa1*
Department of Anesthesia and Perioperative Care, School of Medicine, University of California, San Francisco ,1
College of Osteopathic Medicine of the Pacific, Western University of Health Sciences, Pomona, California2
Received 5 July 2002/
Returned for modification 26 September 2002/
Accepted 23 December 2002

ABSTRACT
Of the proteins encoded by the
pcrGVH-popBD operon of the
Pseudomonas aeruginosa type III secretion system, PcrG bound to PcrV and
PcrH bound to PopB/PopD. In addition,
Yersinia LcrG bound to
PcrV, and
Yersinia LcrH bound to PopD. The results imply a highly
functional conservation of type III secretion between
P.
aeruginosa and
Yersinia species.

TEXT
Pseudomonas aeruginosa possesses a type III secretion system
that is highly homologous to that of
Yersinia species (
39,
40).
In type III secretion, bacteria inject their effector proteins
directly into adjacent host cells (
15,
20). In
P.
aeruginosa infections, exoenzyme S (ExoS) and its coregulated type III
secreted toxins (ExoT, ExoU, and ExoY) are responsible for causing
acute lung injury and sepsis (
8,
9,
12,
21,
33,
38). In type
III secretion of
Yersinia species, translocation, a process
of toxin transfer directly into the eukaryotic cytosol across
the eukaryotic plasma membrane, involves LcrG, LcrV, LcrH, YopB,
and YopD. These proteins are encoded by the
lcrGVH-yopBD operon
in the Yop regulon of
Yersinia pathogenic plasmids (
5,
6). In
P.
aeruginosa, a chromosomal operon,
pcrGVH-popBD, encodes five
proteins, PcrG, PcrV, PcrH, PopB, and PopD, that are homologous
to
Yersinia LcrG, LcrV, LcrH, YopB, and YopD, respectively.
For
Yersinia pestis, protective antigenic characteristics of
LcrV were reported previously as a V antigen (
1,
3,
4,
18,
22,
23,
25,
35). LcrV likely forms the translocation pore in eukaryotic
cell membranes in conjunction with YopB and YopD (
16,
17,
19,
27,
29,
30,
31,
34). LcrG, which forms a stable complex with
LcrV, acts as a negative regulator that blocks secretion of
Yops (
7,
24,
28,
37). LcrH was reported previously as a cognate
chaperone of YopD (
26,
32) and was found to be necessary for
YopD stabilization before secretion (
10). Recently, an active
role of LcrH in Yop regulation was also reported (
2,
10,
11).
The importance of V antigen in cytotoxicity has been well established. Isogenic mutants of P. aeruginosa lacking the genes for pcrV or popD were unable to intoxicate eukaryotic cells (36). Active and passive immunization against PcrV in animal models of P. aeruginosa-induced lung injury greatly increased survival (36). Functional conservation from PopB and PopD of P. aeruginosa to YopB and YopD of Yersinia pseudotuberculosis was previously reported (14). However, there have been fewer studies analyzing the proteins encoded by the pcrGVH-popBD operon of the P. aeruginosa type III secretion system. In this study, we examined the interactions among the proteins encoded by the pcrGVH-popBD operon to investigate the functional homology between the type III secretion systems of P. aeruginosa and Yersinia.
In Escherichia coli, we induced the expression of glutathione S-transferase (GST) fusion PcrG, PcrV, PcrH, PopB, and PopD proteins whose genes were subcloned in pGEX plasmids under the lac promoter. We also induced the expression of the thioredoxin (Thio) fusion PcrG, PcrV, PcrH, PopB, and PopD proteins from genes subcloned into the pThio plasmid under the lac promoter. Induction of PopB fusion proteins appeared to decrease E. coli density after isopropyl-ß-D-thiogalactopyranoside (IPTG) induction, suggesting bactericidal activity. We performed affinity immunoblotting to examine the interaction between PcrV and other proteins encoded by the pcrGVH-popBD operon. We applied E. coli lysate containing Thio-PcrV to a membrane blotted with the lysates of E. coli expressing a series of GST tag-fused proteins. From this experiment, only the GST-PcrG band was visualized (Fig. 1A). Next, we applied GST-PcrG to a membrane blotted with the lysates of E. coli expressing Thio tag-fused proteins. From this experiment, only the Thio-PcrV band was intensely visualized (Fig. 1B). Next, we performed affinity immunoblotting with purified recombinant nontagged PcrV and applied it to membrane-bound Thio fusion proteins to determine whether PcrV-blocking antibodies could detect the PcrV-PcrG complex. Both rabbit polyclonal anti-PcrV antibody (data not shown) and murine anti-PcrV monoclonal antibody (MAb) 166 detected PcrV bound to Thio-PcrG (13) (Fig. 1C). All affinity immunoblotting resulted in the detection of a PcrV-PcrG interaction.
Because LcrH, a
Yersinia homolog of PcrH, was reported as a
chaperone protein for
Yersinia YopD, we purified recombinant
GST-PcrH from
E.
coli transformed with pGEX-
pcrH and examined
the interaction between PcrH and other proteins in the same
format as that previously used to find the PcrV-PcrG interaction.
Affinity immunoblotting was performed with recombinant purified
GST-PcrH to a membrane blotted with the lysates of
E.
coli expressing
Thio tag-fused proteins. GST-PcrH bound to both Thio-PopB and
Thio-PopD in this affinity immunoblot assay (Fig.
2). In order
to verify protein interactions, a GST pull-down assay was performed
on PA103 lysates with recombinant GST-PcrG and GST-PcrH. As
a result, GST-PcrG coprecipitated with native PcrV, and GST-PcrH
coprecipitated with PopD (data not shown).
We performed affinity immunoblotting to examine the cross-species
interaction between
Yersinia and
P.
aeruginosa type III proteins.
From this experiment, we found that GST-LcrG binds to Thio-PcrV
(Fig.
3A) and GST-LcrH binds to Thio-PopD (Fig.
3B). Therefore,
the protein binding between LcrG and PcrV and between LcrH and
PopD occurred in a cross-species manner between
Yersinia and
P.
aeruginosa.
These findings imply high functional and structural homology
among these proteins despite the fact that their amino acid
sequence similarities range from 56 to 57%. Our results suggest
that PcrG serves the role of a potential negative regulator
of PcrV. The neutralizing epitope on PcrV appears to be different
from the PcrG binding site, given that the blocking anti-PcrV
MAb 166 clearly detected the PcrV-PcrG complex in our study.
Since
P.
aeruginosa PcrH and PopD are homolog equivalents of
Yersinia LcrH and YopD, respectively, our findings suggest that
PcrH is a chaperone for PopD secretion. Although PcrH binds
to PopB in the immunoblot that we made, a similar interaction
between LcrH and PopB was not found. The experimental conditions
that we tested may have been affected by the fact that the expression
of recombinant PopB in
E.
coli was bactericidal. This phenomenon
has been reported elsewhere for
Yersinia YopB expression in
E.
coli without coexpression of LcrH (
26). LcrV, YopB, and YopD
are thought to form a pore complex involved in the translocation
of type III secreted proteins across the eukaryotic plasma membrane,
but interactions with LcrV among these proteins have not been
experimentally elucidated. We tested PcrV binding to PopB and
PopD, but no interaction was found.
In conclusion, PcrG binds to PcrV and PcrH binds to PopB and PopD. From interactions between Pcr and Lcr proteins, a highly functional conservation of type III system translocators was also confirmed between P. aeruginosa and Yersinia. Thus, investigations of the roles and mechanisms of PcrV secretion and anti-PcrV blockade and of LcrV secretion and anti-LcrV blockade may complement each other.

ACKNOWLEDGMENTS
This research was supported by National Institutes of Health
grants RO1 HL59239 and AI44101 to J.P.W.-K. and HL067600 to
T.S.; a supplement to AI44101 to L.R.A.; a Faculty Development
Award, University of California, San Francisco, and the American
Lung Association, RG004N, to T.S.; and grants from National
Medical Fellowships and the A

A Medical Society to L.R.A.

FOOTNOTES
* Corresponding author. Mailing address: 513 Parnassus, S-261, Anesthesia, University of California, San Francisco, CA 94143-0542. Phone: (415) 476-6784. Fax: (415) 476-8841. E-mail:
teiji{at}itsa.ucsf.edu.

Editor: V. J. DiRita

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Infection and Immunity, April 2003, p. 2230-2233, Vol. 71, No. 4
0019-9567/03/$08.00+0 DOI: 10.1128/IAI.71.4.2230-2233.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.
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