Adhesive pili of the chaperone-usher family - Escherichia coli: Pathotypes and Principles of Pathogenesis - page 355

( Sauer et al., 1999 ). In the case of PapD, residues L107, I105, and L103 are respec-
tively in register with the subunits' hydrophobic P1-P3 pockets. Mutating residue
105 is particularly detrimental to pilus biogenesis ( Hung et al., 1999 ). However,
mutagenesis of residue L107 does not alter pilus assembly, which may be due
to the plasticity of the P1 pocket ( Ford et al., 2012 ). These residues are termed
P1-P3 residues based on their corresponding acceptor sites, the subunit P1-P3
pockets ( Figure 12.2 D). Additionally, N101 forms hydrogen bonds above the shal-
low P4 subunit pocket. Thus, the interactions involved in DSC facilitate subunit
folding and prevent subunit aggregation. The presence of unfolded subunits in the
periplasmic space otherwise induces periplasmic stress responses targeting aggre-
gated subunits for degradation by the DegP protease ( Jones et al., 1997 , 2002 ).
Once the subunit is folded, it remains bound to the chaperone until its incor-
poration into the pilus. The chaperone is exchanged for the N-terminal exten-
sion (Nte) of the incoming subunit via a 'zip-in, zip-out' mechanism termed
donor strand exchange (DSE). DSE is initiated by the insertion of the Nte P5
residue into the open P5 pocket of the previously assembled subunit ( Figure
12.2 C-E). The chaperone G1 strand does not occupy the P5 pocket of subunits,
which remains easily accessible in a chaperone-subunit complex to the P5 resi-
due of the subunit Nte. Ultimately, insertion of Nte in an antiparallel fashion to
the P2-P5 subunit pockets facilitates the removal of the chaperone's parallel-
oriented G1 strand. The DSE interaction is more energetically favorable than
the DSC interaction, which allows DSE and the stable docking of subunits with
each other to occur ( Sauer et al., 2002 ; Remaut et al., 2006 ).
Usher-binding surface
PapD-like chaperones have a set of conserved, solvent-exposed, hydrophobic
residues (termed Set B: L32, Q34, T53, P54, P55, V56, R68, I93) situated at the
N-terminal domain ( Figure 12.2 A). X-ray crystallography studies demonstrated
that the FimC chaperone of the type 1 pilus system interacts with the N-terminal
domain of the usher via these Set B residues ( Nishiyama et al., 2003 ; Eidam
et al., 2008 ), demonstrating that the Set B patch is a surface that interacts with
the usher. Accordingly, point mutations in these residues negatively impact pilus
biogenesis ( Hung et al., 1999 ). In addition, small molecules called pilicides,
designed to block pilus biogenesis ( Pinkner et al., 2006 ; Chorell et al., 2010 ),
were shown to bind Set B residues and prevent the targeting of chaperone-
subunit complexes to the usher ( Pinkner et al., 2006 ). These results implicate
PapD's conserved hydrophobic patch Set B as the usher-targeting site. Other
chaperone residues that may be involved in interactions with other domains of
the usher are subjects of investigation.
Conserved cleft residues and the DRES motif
Another set of highly conserved residues (Set C: L78, P79, D81, R82, E83,
S84) is located at the elbow region of PapD, which is not known to interact