Type 1 and 5 secretion systems and associated toxins - Escherichia coli: Pathotypes and Principles of Pathogenesis - page 487

( Dinh et al., 1994; Thanabalu et al., 1998 ). The OMP forms a trimeric, water-
filled channel through the outer membrane, open to the extracellular milieu but
constricted at the periplasmic end ( Koronakis et al., 2000 ). The interaction of the
secreted effector molecule with the ABC protein triggers the sequential assem-
bly of the secretion complex by generating interactions between the ABC, MFP,
and OMP ( Thanabalu et al., 1998 ). When the MFP protein binds the OMP,
it triggers the opening of the periplasmic end thus creating a complete chan-
nel from the cytoplasm to the extracellular medium ( Thanabalu et al., 1998 ).
Hydrolysis of ATP by the ABC protein provides the energy for the secretion of
the substrate protein through the channel ( Figure 16.1 ).
Proteins secreted via the T1SS can have diverse functions, however virtually
all T1SS proteins found in E. coli belong to the repeat-in-toxin (RTX) family;
the exception is dispersin of EAEC ( Linhartova et al., 2010 ). RTX proteins are
so named for the distinctive glycine-rich repeats (GGXGXDXXX) that specifi-
cally bind calcium ( Welch, 2001 ). All RTX toxins are made as an inactive pre-
cursor and must undergo a common post-translational maturation step before
export by the T1SS ( Linhartova, et al., 2010 ). RTX family members can have a
variety of functions, however the prominent and historically the first described
group consists of toxins exhibiting a cytotoxic pore-forming ability, often first
detected as a hemolytic halo surrounding bacterial colonies grown on blood
agar plates ( Goebel and Hedgpeth, 1982; Muller et al., 1983; Felmlee et al.,
1985; Welch, 1991 ). RTX proteins found in E. coli include TosA from UPEC
strains needed for adherence and colonization of the urinary tract ( Parham et al.,
2005; Iguchi et al., 2009; Lloyd et al., 2009; Vigil et al., 2011a,b , 2012 ), EhxA
from EHEC strains ( Bauer and Welch, 1996 ) and the prototypical member of
the T1SS, hemolysin (HlyA), from uropathogenic E. coli.
Hemolysin
Regulation and maturation
Hemolysin of UPEC was the first described T1SS protein. Hemolysin is an
important virulence factor in E. coli infections (see Chapter 9) owing to its
cytolytic and cytotoxic activity against a wide range of mammalian cell types
( Welch et al., 1981 ). The synthesis, activation and secretion of hemolysin is
determined by the hlyCABD operon ( Hess et al., 1986 ). In E. coli this operon
is located either on chromosome-bound pathogenicity islands or on transmis-
sible plasmids, suggesting that the T1SS has been acquired by horizontal gene
transfer. The operon encodes the toxin activation protein (HlyC), the hemolysin
itself (HlyA), the ABC transporter (HlyB), and the MFP protein (HlyD). The
outer-membrane component, the multifunctional protein TolC, is found in all
E. coli strains and is encoded elsewhere on the chromosome.
Full transcription of the hlyCABD operon is reliant on two factors, the
18-kDa protein RfaH ( Bailey et al., 1992 ) and a motif 2-kb upstream from the
operon termed the ops (operon polarity suppressor) ( Bailey et al., 1996; Nieto