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Infection and Immunity, July 2000, p. 3971-3982, Vol. 68, No. 7
Section of Microbial Pathogenesis, Yale
University School of Medicine, Boyer Center for Molecular Medicine,
New Haven, Connecticut 06536-0812
Received 14 February 2000/Returned for modification 30 March
2000/Accepted 24 April 2000
The gram-negative respiratory pathogen Legionella
pneumophila infects and grows within mammalian macrophages and
protozoan host cells. Upon uptake into macrophages, L. pneumophila establishes a replicative organelle that avoids
fusion with endocytic vesicles. There are 24 dot/icm genes
on the L. pneumophila chromosome required for biogenesis of
this vacuole. Many of the Dot/Icm proteins are predicted to be
components of a membrane-bound secretion apparatus similar to type IV
conjugal transfer systems. We have been investigating the function of
L. pneumophila dot/icm gene products that do not have
obvious orthologs in other type IV transfer systems, since these
determinants could govern processes unique to phagosome biogenesis. The
icmX gene product falls into this category. To understand
the role of the IcmX protein in pathogenesis, we have detailed
interactions between an L. pneumophila icmX deletion mutant
and murine bone marrow-derived macrophages. These data demonstrate that
icmX is required for biogenesis of the L. pneumophila replicative organelle. Immunoblot analysis indicates
that the icmX gene product is a polypeptide with an
estimated molecular mass of 50 kDa. The IcmX protein was localized to
the bacterial periplasm, and periplasmic translocation was mediated by
an N-terminal sec-dependent leader peptide. A truncated
IcmX product was secreted into culture supernatants by wild-type
L. pneumophila growing extracellularly in liquid media;
however, transport of the IcmX protein into eukaryotic host cells was
not detected. Proteins similar in molecular weight to IcmX were
identified in other Legionella species by immunoblot
analysis using a monoclonal antibody specific for L. pneumophila IcmX protein. From these data, we conclude that the
IcmX protein is an essential component of the dot/icm secretion apparatus, and that a conserved mechanism of host cell parasitism exists for members of the Legionellaceae family.
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Copyright © 2000, American Society for Microbiology. All rights reserved.
Identification and Subcellular Localization of the
Legionella pneumophila IcmX Protein: a Factor Essential
for Establishment of a Replicative Organelle in Eukaryotic
Host Cells
*
Corresponding author. Mailing address: Section of
Microbial Pathogenesis, Yale University School of Medicine, Boyer
Center for Molecular Medicine, Room 354, 295 Congress Ave., New Haven, CT 06536-0812. Phone: (203) 737-2408. Fax: (203) 737-2630. E-mail: craig.roy{at}yale.edu.
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