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their specific chaperone (T3S encoded proteins that prevent premature polym-
erization of newly expressed needle components within the bacterial cytosol),
have been reported (
Deane et al., 2006
;
Zhang et al., 2006
;
Quinaud et al., 2007
;
Sun et al., 2008
;
Poyraz et al., 2010
; for review, see
Blocker et al., 2008
). In
all cases, a similar two-helix coiled-coil bundle, linked by a conserved turn
defined by a PxxP sequence motif is observed. Recently, elegant studies of the
Salmonella typhimurium
needle using solid-state NMR (
Poyraz et al., 2010
;
Loquet et al., 2012
) has allowed for an accurate model of its assembled struc-
ture, providing atomic-resolution insights into the orientation of individual PrgI
molecules within the needle, and their interaction with one another (
Loquet
et al., 2012
).
Importantly, the needle not only forms a hollow conduit to passage effectors
from the bacteria to the infected host, but has also been implicated in additional
roles including mammalian cell sensing, and, along with the inner rod, in sub-
strate switching during various stages of injectisome assembly and virulence
effector secretion (
Kenjale et al., 2005
;
Davis and Mecsas, 2007
).
At the distal, extracellular end of the needle, multiple copies of a terminat-
ing tip protein have been reported in various species (EspA in EPEC, IpaD
in
Shigella
); EM studies suggest the tip is pentameric, matching the cross-
sectional symmetry of the needle (
Deane et al., 2006
;
Broz et al., 2007
;
Epler
et al., 2012
). The proposed role of the tip is to sense the presence of mamma-
lian cells, through direct interaction with membrane lipids (
Olive et al., 2007
;
Veenendaal et al., 2007
), as well as to serve as an adapter between the needle
and the downstream translocon pore that inserts directly into the host cell mem-
brane (
Picking et al., 2005
). The structures of the tip protein from several sys-
tems (
Derewenda et al., 2004
;
Erskine et al., 2006
;
Johnson et al., 2007
) show
the presence of a core domain forming a two-helix coiled-coil similar to the
needle protein. In addition, a small N-terminal helical domain regulates binding
to the needle, requiring partial unfolding for the final assembly of the needle/tip
complex (
Wilharm et al., 2007
;
Lunelli et al., 2011
). A third domain, found at
the C-terminus, has been proposed to be involved in contacts with components
of the translocon (
Roehrich et al., 2010
).
Of note, in EPEC and related species, an unusual ortholog of the T3SS tip
protein, EspA, has been shown to be solely responsible for formation of the fila-
ment, a highly extended polymeric structure that protrudes from the bacteria, with
lengths in excess of 600 nm (
Knutton et al., 1998
;
Daniell et al., 2001
;
Sekiya
et al., 2001
). These extended translocation filaments in EPEC and EHEC are
thought to help span the significant mucosal layer of infected epithelial cells in
the host gut. EM helical reconstructions of sheared EspA filaments (
Wang et al.,
2006
) shows a structure similar to the T3SS needle, with approximately 5.5 pro-
teins per turn. Variations in helical parameters observed in these high-resolution
EM maps suggest a potential conformational lability or 'breathing', proposed to
help accommodate the significant cellular forces likely experienced at the bacte-
rial/host interface and potentially to help propel virulence proteins internalized
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