The type 2 secretion and type 4 pilus systems of Escherichia coli - Escherichia coli: Pathotypes and Principles of Pathogenesis - page 383

canonical lipobox sequences (LxxC) in their N-termini and are likely lipose-
cretins ( Wu and Tokunaga, 1986 ; Viarre et al., 2009 ). Additionally, none of the
other E. coli T4P operons appears to include pilotin genes, further suggesting
that the secretins are all self-sorting lipoproteins.
Beyond the lack of a cognate pilotin, the T4P OM complex is noticeably dif-
ferent from the T2S system by virtue of two unique proteins. Both are soluble,
periplasmic proteins that associate with the OM in the presence of the secre-
tin ( Daniel et al., 2006 ). BfpG is the product of the second gene in the BFP
operon ( Schmidt et al., 2001 ; Daniel et al., 2006 ). The second, BfpU, is found in
both the cytoplasm and periplasm and has no apparent T2S or T4aP homologs
( Schreiber et al., 2002 ). Both of these proteins are essential for pilus biogenesis
and interact with BfpB to form the OM subassembly ( Daniel et al., 2006 ). Inter-
actions between the BfpU and BfpG have not been described.
Pseudopilus and pilus
According to generally accepted theory, both the T2S and T4P build filamen-
tous appendages that move through a central channel formed by the IM and OM
subassemblies. The T2S builds a pseudopilus that is of sufficient length to reach
the vestibule formed by the secretin ( Reichow et al., 2011 ). The pseudopilus is
primarily composed of GspG, the major pseudopilin ( Sauvonnet et al., 2000 ;
Durand et al., 2003 ) and a set of two to four minor pseudopilins depending upon
the system. In ETEC the four minor pseudopilins are GspH, I, J, and K ( Yanez
et al., 2008a,b ). All pseudopilins share N-terminal sequence homology and are
processed by a prepilin peptidase GspO ( Bally et al., 1992 ; Dupuy et al., 1992 ).
The T4P forms a structure that protrudes through the secretin and serves as
a fimbrial adhesin. The main pilus subunit precursor, pre-bundlin for BFP, is
cleaved by the prepilin peptidase, BfpP ( Zhang et al., 1994 ) to form the primary
component of the pilus. The solution structure of bundlin lacking the N-terminal
stretch of hydrophobic amino acids predicted to be buried in the pilus core has
been solved. Core features common to all T4P pilins and T2S pseudopilins,
including the N-terminal alpha helix, the variable alpha-beta region, and the
antiparallel beta sheet, are apparent ( Figure 13.2 ). However, the nature of the
alpha-beta region, the topological arrangement of the beta strands in the sheet,
and the enclosure of the sheet in alpha helices is unique. These features contrast,
not only with those of T4aP pilin proteins, but even with T4bP pilin proteins
such as CofA ( Fukakusa et al., 2012 ; Kolappan et al., 2012 ), with which it
shares only the most conserved pilin features ( Figure 13.2 ).
T4P systems also contain analogs of the minor pseudopilins called pilin-
like proteins: BfpI, BfpJ, and BfpK. These proteins, like the minor pilins in
other T4P systems, contain the same peptidase cleavage site as bundlin and are
processed by the prepilin peptidase ( Ramer et al., 2002 ). Like the pseudopilins,
the pilin-like proteins may form a complex with one another ( Koomey, 1995 ;
Ramer et al., 2002 ; Helaine et al., 2007 ). Immunoelectron microscopy has con-
firmed that BfpI is a true minor pilin.