Biology Reference
In-Depth Information
T2S across the OM into the extracellular environment through a pore formed by
the secretin protein (
Genin and Boucher, 1994
).
The T2S system in
E. coli
consists of 12 core proteins found in all T2S sys-
tems and two accessory proteins (
Sandkvist, 2001
). Mutations in genes encoding
most of these proteins result in defects in the secretion process and accumula-
tion of exoproteins in the periplasm (
Filloux, 2004
). There are chromosomal
genes in the non-pathogenic
E. coli
K-12 and most other strains that encode
a T2S system; however, under standard conditions secretion does not occur.
The operon encoding this system is silenced by the nucleoid-structuring protein
H-NS, and mutants deficient for H-NS can express the T2S system (
Francetic
et al., 2000
). In
E. coli
pathogens, proteins secreted by accessory T2S systems
play multiple roles, including adherence to eukaryotic cells and damage to host
tissues. Examples include the EHEC StcE metalloprotease, which promotes
adherence to host eukaryotic cells (
Grys et al., 2005
) and the ETEC heat-labile
toxin (LT), a multimeric AB
5
enterotoxin functionally and structurally similar to
cholera toxin of
Vibrio cholerae
(
Sixma et al., 1993
) that induces fluid secretion
and promotes colonization of intestinal epithelia (
Tauschek et al., 2002
).
T4P are filamentous surface appendages required for adherence, motility,
aggregation, and transformation in a wide array of bacterial and archaeal species.
T4P were first identified in
Pseudomonas aeruginosa
and
Myxococcus xanthus
(
Ottow, 1975
). Further studies revealed that all T4P have a conserved set of core
proteins (
Pelicic, 2008
). T4P are long, thin, flexible, polymeric, three-start helical
filaments approximately 85 Å in diameter (
Craig et al., 2004
;
Ramboarina et al.,
2005
). The pili are composed primarily of the major pilin protein (
Craig et al.,
2004
), and are assembled by a complex biogenesis machine consisting of 10-18
proteins that spans both bacterial membranes (
Sohel et al., 1996
;
Stone et al.,
1996
;
Hwang et al., 2003
;
Peabody et al., 2003
;
Pelicic, 2008
). Major pilins are
first synthesized as precursor prepilins and then processed by a prepilin peptidase
to create mature pilin proteins incorporated into the pilus (
Strom et al., 1993
).
T4P can be subdivided into two distinct classes, type 4 class A pili (T4aPs)
and type 4 class B pili (T4bPs), based on differences in component structure,
genetic organization, distribution, and function. T4bPs are characterized by
larger major pilins compared to those of T4aPs that begin with an amino acid
other than phenylalanine and are encoded in a contiguous operon often found
on plasmids (
Craig et al., 2004
). As with the common T2S system, most strains
of
E. coli
have the genes for a T4aP that has been called, somewhat confusingly,
the hemorrhagic coli pilus (HCP) (
Xicohtencatl-Cortes et al., 2007
). In addi-
tion, some
E. coli
pathotypes express accessory T4bPs that play important roles
in infection.
History
The T2S system was initially discovered in
Klebsiella oxytoca
and
Pseudomo-
nas aeruginosa
and is widely conserved among Gram-negative bacteria (
Filloux
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