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structure have emerged. Like many of the secretin family members, GspD forms
a dodecameric ring structure in the OM approximately 200 Å in height and 150-
160 Å in width with a central pore through which exoprotein substrates pass
(
Korotkov et al., 2009
;
Reichow et al., 2010
). The periplasmic side of the pore
consists of a chamber that appears to serve as a docking site for secreted exopro-
teins (
Reichow et al., 2011
) with an opening of ∼70 Å, which narrows to ∼50 Å
at a periplasmic gate (
Korotkov et al., 2009
;
Reichow et al., 2010, 2011
). The
periplasmic chamber is formed by concentric rings composed of the N-termini
of the monomers. How the secretin accommodates the passage of substrate pro-
teins through the gates and narrow center of the channel, and how it prevents
secretion of other molecules that are not substrates, remains unknown. The pore
opens into a smaller chamber on the extracellular face that is capped. Since
GspD is not a liposecretin, the pilotin GspS is required for secretin stability and
transport to the OM via an interaction with the distal C-terminus of the secretin
(
Daefler et al., 1997
;
Nickerson et al., 2011
). GspS is itself a lipoprotein that is
acylated and transported to the OM via the Lol pathway (
Collin et al., 2011
).
Despite accumulating knowledge regarding the structures of the various
T2S components, the actual mechanism of secreting a protein like LT of ETEC
through the T2S apparatus has not been well established. However, a theoreti-
cal process has been proposed. Initially, the target proteins are synthesized in
the cytoplasm and translocated via the Sec or Tat export pathways depending
on whether the protein is folded in the periplasm or cytoplasm, respectively
(
Pugsley, 1993
;
Berks et al., 2005
). Molecular modeling strongly suggests an
interaction between the correctly folded substrate protein, the secretin, the pseu-
dopilus tip complex of Gsp I/J/K, and GspC (
Reichow et al., 2011
), although
interactions with additional machine components are possible. The binding of
the exoprotein then stimulates the ATPase activity of GspE, leading to the addi-
tion of GspG monomers to the pseudopilus (
Hobbs and Mattick, 1993
;
Shev-
chik et al., 1997
). It is not known how the system chooses substrates from the
plethora of periplasmic proteins available, or how the ATPase is specifically
activated by substrate binding. The exoprotein is pushed upward through the
secretin pore with the pseudopilus acting like a piston. However, it is not known
how the pseudopilus is retracted. The trimer tip of the pseudopilus may inter-
act with the secretin pore, inducing a conformational change destabilizing the
GspG monomers below it, causing the pseudopilus to depolymerize (
Korotkov
et al., 2012
). It notable that the degree of expression of the equivalent of GspK
in the
Pseudomonas aeruginosa
T2S is inversely proportional to the length of
the pseudopilus and an interaction between the GspK and GspG equivalent sub-
units destabilizes GspG (
Durand et al., 2005
).
Structure and function of type 4 pilus systems in
E. coli
BFP biogenesis is believed to start with the Sec-dependent translocation of
pre-bundlin and the pre-pilin-like proteins across the IM. The subunits are
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