Adhesive pili of the chaperone-usher family - Escherichia coli: Pathotypes and Principles of Pathogenesis - page 351

efforts, groups led by Stanley Falkow and Gordon Dougan were the first to
clone pili gene clusters ( Hull et al., 1981 ; Morrissey and Dougan, 1986 ). Mean-
while, afimbrial adhesins of E. coli were also discovered and cloned ( Labigne-
Roussel et al., 1985 ; Walz et al., 1985 ). These pioneering studies led to work
focused on dissecting the biogenesis and structure of P pili of uropathogenic
E. coli (UPEC), which provided a model and blueprint for the next three
decades of work on these systems worldwide. The P pilus was discovered to be
a multicomponent structure consisting of a stalk and an adhesive tip ( Lindberg
et al., 1987 ; Kuehn et al., 1992 ). At around the same time, PapD became the
first pilus chaperone described in detail, a central factor in a molecular machine
assembled by a process we would come to name the chaperone-usher (CU)
pathway ( Hultgren et al., 1989 ; Lindberg et al., 1989 ). The crystal structure of
PapD described in 1989 was a landmark study that provided the first structural
insights into CU pilus biogenesis ( Holmgren et al., 1988 ; Holmgren and Bran-
den, 1989 ). The surprising immunoglobulin (Ig)-like fold of PapD catapulted
future structure-function analyses that led to the elucidation of fundamental
principles of chaperone-assisted subunit folding and pilus biogenesis ( Waksman
and Hultgren, 2009 ). Thus, from the structural resolution of PapD and the dis-
covery that it formed complexes with each of the pilus subunits, three decades
of investigations were nucleated worldwide, which only recently culminated in
the published crystal structure of an usher-chaperone-adhesin ternary complex
( Phan et al., 2011 ). These historical findings served as the beginnings of a scien-
tific era for understanding these virulence factors at the molecular level, which
has now led to the development of therapeutics interfering with pilus assembly,
pilus function, and hence, infection.
PILUS ARCHITECTURE
Pili of the CU system consist of multiple pilus subunits arranged into long,
linear protein polymers. The morphology of CU pili varies across the six major
clades - α, β, γ(1-4), κ, π, and σ - of the 189-membered CU pilus superfamily
(as of 2007), ranging from thin, fibrillar structures to thick, helical rods topped
by a fibrillar tip ( Nuccio and Baumler, 2007 ). Paradigms for CU pilus architec-
ture and assembly have been well established with the P pilus and type 1 pilus
of UPEC, members of the π and γ1 clades, respectively. These two archetypal
pili exhibit a bipartite organization, consisting of a long, helical rod connected
to a thin tip fibrillum. The P pilus subunits PapG, PapF, PapE, PapK, PapA,
and PapH arrange in order from fibrillar tip to rod base ( Figure 12.1 ). PapG,
the adhesin, lies at the distal end of the pilus. The tip adaptor PapF connects
PapG to the main tip component PapE, which appears in 5-10 copies and has
a width of ∼2 nm. The adaptor PapK anchors PapE to the main rod component
PapA, which appears in >1000 copies and gives rise to a right-handed, helical
structure that displays a 6.8 nm width, 2.5 nm pitch, and 3.3 subunits per turn
( Kuehn et al., 1992 ; Jacob-Dubuisson et al., 1993 ; Striker et al., 1994 ). Finally,