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ATPase, GspE (
Abendroth et al., 2005
;
Camberg and Sandkvist, 2005
;
Camberg
et al., 2007
). In addition, the T2S IM complex includes a membrane-anchored
periplasmic protein, GspC, which interacts with the secretin of the system
(
Korotkov et al., 2006
;
Lybarger et al., 2009
).
The IM complex of T4P biogenesis machines is very similar. Using the BFP
of EPEC as a prototype, there is also a polytopic IM protein, BfpE (
Blank and
Donnenberg, 2001
). The T4P system has a bitopic IM protein BfpC (
Milgotina
et al., 2011
;
Yamagata et al., 2012
). These two proteins interact with each other
as well as with BfpD, an ATPase that powers pilus extension (
Milgotina et al.,
2011
). Remarkably, despite the absence of sequence similarity, the cytoplasmic
domain of BfpC is a structural and functional homolog of GspL and of the
T4aP component PilM (
Yamagata et al., 2012
). The Longus and CFA/III pili
of ETEC have the same core IM proteins as BFP (
Table 13.1
). The same core
components are also found in R64, however this system has several IM proteins
with unknown function (
Kim and Komano, 1997
). BfpL is a bitopic IM protein
essential for BFP biogenesis that interacts with BfpC, but homologs of BfpL
have not yet been described in other systems (
De Masi et al., 2012
).
Although the core transmembrane proteins and ATPase are present in both
T2S and T4P systems there are several important differences. At present, no
homolog for GspC, which interacts with the secretin and is essential for stability
of the IM complex, has been found in an
E. coli
T4P system. Also, unlike T2S
systems, most T4P systems have the capacity to rapidly retract their pilus fibers,
and BFP is no exception. This is accomplished through a dedicated retraction
ATPase, BfpF (
Anantha et al., 1998
;
Bieber et al., 1998
). However, an equiva-
lent in the T4P systems of ETEC has not been found.
The OM subassembly
The T2S and T4P systems encounter a common problem in exporting sub-
strates: crossing the OM. Both machines solve this problem with pores formed
by proteins in the secretin superfamily. Secretins form multimeric pores in the
OM, most often consisting of 12 monomers, each of which is believed to be a
fully folded beta barrel (
Korotkov et al., 2011
). Many of these proteins require
a small lipoprotein, termed a pilotin, for stability and successful transport to the
OM. In the T2S system, the OM pore is formed by a dodecamer of the secre-
tin, GspD (
Genin and Boucher, 1994
;
Reichow et al., 2010
). This complex is
stabilized by the pilotin, GspS (
Hardie et al., 1996
). The T4P OM complex also
includes a secretin, BfpB (
Ramer et al., 1996
;
Schmidt et al., 2001
). However,
the BFP secretin is itself a lipoprotein (
Ramer et al., 1996
) and can assemble
and reach the OM when expressed alone (
Daniel et al., 2006
;
Lieberman et al.,
2012
). To date no pilotin has been identified in the BFP system. Four secretins
in both T2S and T4P systems have been experimentally validated as lipopro-
teins and none appear to have or require cognate pilotins (
Hu et al., 1995
;
Ramer
et al., 1996
;
Schmidt et al., 2001
;
Bose and Taylor, 2005
;
Viarre et al., 2009
).
Interestingly, the secretins of CFA/III, Longus, and R64 systems all contain
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