The type 2 secretion and type 4 pilus systems of Escherichia coli - Escherichia coli: Pathotypes and Principles of Pathogenesis - page 385

et al., 2011 ). The pseudopilus consists of the major pseudopilin, GspG, and
minor pseudopilins GspH, I, J, and K, all of which are translocated across the
IM via the Sec-dependent pathway ( Francetic et al., 2007 ). A short, positively
charged segment at the N-terminal region of each pseudopilin is cleaved off
by the prepilin peptidase GspO ( Bally et al., 1992 ; Dupuy et al., 1992 ). The
peptidase is an aspartic acid protease with eight putative transmembrane heli-
ces and its catalytic residues in cytoplasmic loops ( LaPointe and Taylor, 2000 ).
The enzyme is bifunctional and also N -methylates the cleavage product ( Strom
et al., 1993 ). The pseudopilins GspI, J, and K appear to be incorporated as a
trimer at the tip of the pseudopilus ( Korotkov and Hol, 2008 ; Douzi et al., 2009 )
with GspK at the top and GspI/J forming the base. How monomers of GspG
and the pseudopilin GspH are added to the pseudopilus remains unknown. All
five pseudopilins have markedly different globular structures but share common
features: an N-terminal alpha helix (the first half of which is hydrophobic) fol-
lowed by a variable region and a conserved beta-sheet that varies in length with
the various proteins.
T2S biogenesis requires the insertion of the IM proteins, GspC, F, L, and
M, into the IM. These proteins create a complex together and appear to pro-
tect each other from proteolysis ( Sandkvist et al., 1999 ). GspC contains a short
cytoplasmic segment, a transmembrane helix, and two periplasmic domains: the
homology region (HR) and PDZ domains ( Bleves et al., 1999 ) although the PDZ
domain is not universal across all T2S systems ( Korotkov et al., 2006 ). The HR
domain of GspC interacts with the periplasmic N-terminal regions of the secretin,
GspD ( Korotkov et al., 2011 ). The HR domain may also influence formation of
the IM complex ( Lybarger et al., 2009 ). GspF is the only polytopic protein in the
complex, consisting of two cytoplasmic domains sharing some sequence homol-
ogy and three transmembrane helices ( Abendroth et al., 2009a,b ). As discussed
above, this polytopic transmembrane protein is one of the most highly conserved
members of the T2S system and related machines ( Peabody et al., 2003 ).
The GspL component recruits pseudopilins processed by GspO, the prepilin
peptidase, to the IM complex ( Sandkvist et al., 1995 ) and also recruits the ATPase,
GspE, to the cytoplasmic face of the IM ( Sandkvist et al., 2000 ; Abendroth et al.,
2005 ). GspE is a member of a large ATPase family ( Robien et al., 2003 ) provid-
ing the necessary energy for the secretion apparatus to function through ATP
hydrolysis and, unlike most other proteins of its type contains zinc ( Camberg
and Sandkvist, 2005 ). The protein contains typical Walker boxes A and B ( Planet
et al., 2001 ) and is active as a hexamer ( Satyshur et al., 2007 ). GspM contains a
short cytoplasmic sequence, a transmembrane helix, and a ferrodoxin fold in its
periplasmic domain that could serve as a binding site for another protein ( Aben-
droth et al., 2004b ). GspM interacts directly with GspL through contacts in the
transmembrane domain and periplasm ( Sandkvist et al., 1999 ; Py et al., 2001 ).
The secretin GspD is part of a large protein superfamily of multimeric, pore-
forming OM proteins ( Linderoth et al., 1996 ; Collins et al., 2004 ; Reichow
et al., 2010 ). Although variations exist, some common themes of secretin