Type 1 and 5 secretion systems and associated toxins - Escherichia coli: Pathotypes and Principles of Pathogenesis - page 496

STRUCTURE OF T5SS DOMAINS
The structure of many AT proteins has been solved, including EspP and Hbp
from E. coli strains. These structures have revealed a domain architecture com-
prising a signal sequence, the passenger domain, an autochaperone domain, an
α-helix linker followed by the translocation domain. The translocation domain
of AT proteins is highly conserved and consists of β-pleated sheets in the form
of a β-barrel akin to most other integral outer-membrane proteins ( Loveless
and Saier, 1997; Barnard et al., 2007; Tajima et al., 2010 ). Although diverse in
sequence, all solved AT β-barrels contain 12 antiparallel strands connected by
extracellular loops and periplasmic turns of varying length, with a narrow (~1 ×
1.25 nm) hydrophilic pore. Translocation domains also share a consensus amino
acid motif at the C-terminus ( Struyve et al., 1991; Jose et al., 1995; Loveless and
Saier, 1997 ). The C-terminal 9 aa are generally alternating aromatic/hydropho-
bic and charged/hydrophilic with the last residue a tryptophan or phenylalanine.
This sequence (in particular the C-terminal three residues) is predicted to play a
role in outer-membrane localization and/or stability of outer-membrane proteins.
Encoded just before the translocation domain, AT proteins have a single
α-helix linker, found to reside within the barrel lumen. The length of the linker
is determined by whether the passenger domain remains attached (long), is
cleaved extracellularly (long) or cleaved within the barrel (short) ( Barnard
et al., 2007; Tajima et al., 2010 ). The α-helix also ensures the pore is blocked
post-translocation preserving the integrity of the outer membrane. In addition
to this α-helix, in the two solved E. coli ATs, a long extracellular loop of the
translocation domain also folds into the barrel, closing the pore from the outside
( Barnard et al., 2007 ).
Passenger domains of AT proteins are diverse in sequence and function
and thus structures are different between proteins, however there are some
commonalities. Above 97% of AT passenger domains are predicted to form
a right-handed β-helical structure despite large diversity in sequence, length,
and function. The right-handed β-helical structure was first solved for the
T5SS protein, Pertactin ( Emsley et al., 1994, 1996 ). β-Helical structures typi-
cally contain three β sheets separated by three turns giving the protein a 'V'
shape in cross-section ( Jenkins and Pickersgill, 2001 ). A complete turn of the
β-helix is known as a coil, with different proteins having varying numbers
of coils. The structure displays extensive 'stacking' across its coils, whereby
similar aliphatic residues occupy equivalent positions in neighboring β sheets
leading to ridges of aliphatic residues across the coils ( Jenkins and Pickersgill,
2001 ). This right-handed β-helical structure is thought to be conserved in
AT proteins because the passenger domain adopts a predominantly unfolded
conformation during its passage through the outer membrane, a process that
occurs independent of ATP and proton gradients ( Junker et al., 2006 ). The
conserved β-helical structure may contribute to protein folding after transport
through the translocation domain and may also play a role in presenting an