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predicted for the majority of TPS-secreted TpsA proteins. The large size, the
abundance of repetitive sequences and the predominance of β-structure thus
seem to be common characteristics of both AT and TpsA proteins (reviewed
extensively in Kajava and Steven, 2006 ). However TpsA proteins vary greatly
in size, sequence, and function and thus other structures may exist.
PROCESSING OF T5SS PASSENGER DOMAINS
The fate of the passenger domain of T5SS proteins is dependent on the extent
of processing that occurs at the bacterial cell surface. For AT proteins, some
passenger domains remain intact ( Wells et al., 2008, 2009 ), whereas others are
cleaved from the β-domains and secreted ( Eslava et al., 1998; Yen et al., 2008 ).
Intriguingly, some AT passenger domains are cleaved but remain strongly asso-
ciated with the translocation domain ( Benz and Schmidt, 1989; Sherlock et al.,
2005a ). The mechanisms of passenger domain cleavage have been elucidated
for a few proteins, however they already seem to be diverse, including pro-
cessing by exogenous host proteases ( Shere et al., 1997 ), processing by other
AT proteins ( van Ulsen et al., 2003 ) and autocatalytic reactions ( Charbonneau
et al., 2009 ).
The passenger domains of TpsA proteins can remain stably, but non-
covalently, attached to the cell surface after translocation, whereas others are
released into the extracellular milieu. Many TpsA proteins are synthesized
with large pro-domains of unknown function, either at the N-terminus or the
C-terminus, and these are removed at some point during the translocation
process ( Mazar and Cotter, 2007 ). In contrast to AT and TAA proteins, the pas-
senger domains of all TAA proteins remain attached to the translocation domain.
Finally, Intimin remains covalently attached to the cell surface after secretion.
DISTRIBUTION, FUNCTION, AND REGULATION
The T5SS is widely used by E. coli, with strains usually carrying up to 12 differ-
ent T5SS encoding genes ( Wells et al., 2010 ). These proteins are found across
all E. coli pathotypes and evolutionary lineages ( Chaudhuri et al., 2010 ) and are
often associated with virulence. Although E. coli strains usually have a large
complement of T5SS proteins only a few of these are conserved across the
majority of strains. The T5SS proteins with specific virulence properties tend to
be restricted to a single pathotype. Due to the variable nature of the passenger
domain, T5SS proteins have a wide variety of functions, however some pheno-
types such as autoaggregation, biofilm formation, and adhesion are common
to many T5SS proteins. A summary of the functions of all known E. coli T5SS
proteins is found in Table 16.1 . As T5SS proteins are widespread in E. coli
many different methods of regulation of the genes have been reported. A more
detailed examination of function and regulation of four E. coli T5SS proteins
from representative groups is presented below.
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