POSTERS - BLAST X - University of Utah

BLAST X Poster #63
STRUCTURAL EVIDENCE SUGGESTS THAT ANTIACTIVATOR ExsD FROM
PSEUDOMONAS AERUGINOSA IS A DNA BINDING PROTEIN
Robert C. Bernhards, Xing Jing, Nancy J. Vogelaar, Howard Robinson#, Florian D.
Schubot*
Department of Biological Sciences, Virginia Polytechnic Institute & State University,
Washington Street, Blacksburg, VA 24060; #Biology Department, Brookhaven National
Laboratory, Upton, NY 11973-5000.
The opportunistic pathogen P.aeruginosa utilizes a type III secretion system (T3SS) to
support acute infections in predisposed individuals. In this bacterium expression of all T3SSrelated
genes is dependent on the AraC-type transcriptional activator ExsA. Prior to host
contact, the T3SS is inactive and ExsA is repressed by the antiactivator protein ExsD. The
repression, thought to occur through direct interactions between the two proteins, is relieved
upon opening of the type III secretion (T3S) channel when secretion chaperone ExsC
sequesters ExsD. We have solved the crystal structure of Δ20ExsD, a protease-resistant
fragment of ExsD that lacks only the twenty amino terminal residues of the wild type protein at
2.6 Å. Surprisingly the structure revealed similarities between ExsD and the DNA binding
domain of transcriptional repressor KorB. A model of an ExsD-DNA complex constructed on the
basis of this homology produced a realistic complex that is supported by the prevalence of
conserved residues in the putative DNA binding site and the results of differential scanning
fluorimetry studies. Our findings challenge the currently held model that ExsD solely acts
through interactions with ExsA and raise new questions with respect to the underlying
mechanism of ExsA regulation.
114