Biology Reference
In-Depth Information
Chapter 13
The type 2 secretion and type 4
pilus systems of
Escherichia coli
Leon G. De Masi, Courntey D. Sturey, Joshua A. Lieberman,
Michael S. Donnenberg
University of Maryland School of Medicine, Baltimore, MD, USA
INTRODUCTION
Definition
The type 2 secretion (T2S) and the type 4 pilus (T4P) systems are two broadly
conserved systems that play important roles in the virulence of pathogenic
Escherichia coli
and many other bacteria. These systems have been identified
in enteropathogenic
E. coli
(EPEC), enterotoxigenic
E. coli
(ETEC), enterohe-
morrhagic
E. coli
(EHEC), and non-pathogenic
E. coli
strains. The T2S system
exports exotoxins and important cell-surface anchored lipoprotein enzymes
that digest components of the host cell, including proteins, lipids, and sugars
(
Economou et al., 2006
). The T4P system produces pili that are involved in
adherence, motility, and aggregation. Both systems consist of multicompo-
nent machines that are broadly distributed but play distinct roles in bacterial
pathogenesis. Components of the T2S and T4P machines are homologous in
sequence, structure, and function. Only a few key differences account for the
ability of T4P systems to anchor polymerized substrates in the inner membrane
(IM) as opposed to the secretion of individual, folded substrates across the outer
membrane (OM) by T2S systems. Under artificial conditions, T4P systems can
assemble T2S pseudopilins into pili (
Sauvonnet et al., 2000
).
T2S represents one of at least six secretion systems in Gram-negative bac-
teria that function in the export of proteins to the extracellular environment
(
Economou et al., 2006
). T2S has been referred to as the main terminal branch
of the general secretory pathway (Gsp) and the second part of a two-step pro-
cess. The first step is transport of the exoprotein across the IM by the Sec or
Tat pathway (
Cianciotto, 2005
). The substrate precursor contains an N-terminal
signal peptide that targets the protein to be transported through the IM. The
signal peptide is cleaved off and the protein is released into the periplasm where
it folds into its functional confirmation. The protein is then transported by the
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