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FIGURE 13.2
Structures of bundlin and CofA highlight similarities and differences among T4bP
pilin proteins. Structures of soluble, N-terminally truncated versions of (A) bundlin (PDB 1zwt)
and (B) CofA (PDB 3vor). The hydrophobic amino termini of both proteins, predicted to form
contiguous alpha helices buried in the core of the pili, were omitted to facilitate purification and
structure determination (
Ramboarina et al., 2005
;
Fukakusa et al., 2012
;
Kolappan et al., 2012
).
Conserved features include the N-terminal alpha helix (color) and the central anti-parallel beta
sheet (sheet). When overlaid (C), marked differences between bundlin (blue) and CofA (green) are
apparent. Figures were drawn using PyMOL (The PyMOL Molecular Graphics System, Version
1.3, Schrödinger, LLC) by Kurt Piepenbrink.
There are several important differences between the T4P systems in
E. coli
.
Unlike BFP, the CFA/III, Longus and R64 systems contain only one pilin-like
protein (
Gómez-Duarte et al., 1999
;
Yoshida et al., 1999
;
Taniguchi et al., 2001
).
Each of these is much larger in size than the pilin-like proteins in BFP. Second,
the first amino acid after the prepilin peptidase cleavage site of BfpA is leucine
and, as is the case with most T4P systems, is
N-
methylated (
Donnenberg, 2012
).
However, in the R64 major pilin tryptophan is the first amino acid after the pep-
tidase cleavage site and undergoes a distinct, but uncharacterized modification
(
Yoshida et al., 1998
).
MECHANISM OF ACTION
Structure and function of type 2 secretion systems in
E. coli
The T2S pseudopilus is believed to interact with substrate exoproteins in the
periplasmic vestibule of the secretin and force the exoprotein into the extra-
cellular space through an as yet undocumented piston mechanism (
Reichow
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