Adhesive pili of the chaperone-usher family - Escherichia coli: Pathotypes and Principles of Pathogenesis - page 354

Caf1M forms tetramers as a self-capping mechanism to prevent unfavorable
interactions and proteolysis ( Hung et al., 1999 ; Knight et al., 2002 ; Zavialov
and Knight, 2007 ).
Interactive surfaces of the chaperone
Active site
Upon translocation of subunits across the SecYEG translocase to the periplas-
mic space, they are taken up by their cognate periplasmic chaperones, which
use several interactive surfaces to provide stability to the subunits. The active
site of the chaperone is comprised of residues R8 and K112, a conserved basic
patch in the cleft formed between the two domains. Mutagenesis of these
basic residues abolishes the ability of the chaperone to mediate pilus assembly
( Slonim et al., 1992 ; Kuehn et al., 1993 ). Crystallography studies carried out
on all of the P pili chaperone-subunit complexes ( Sauer et al., 1999 , 2002 ;
Verger et al., 2006 , 2007 , 2008 ; Ford et al., 2012 ) further confirmed these
interactions and the extended conformation by which the C-termini of sub-
units are anchored at the invariant R8 and K112 residues of the chaperone
( Kuehn et al., 1993 ).
The G1 β-strand
The absence of the seventh β-strand in the subunit results in a deep groove on
its surface that exposes its hydrophobic core. In a process termed donor strand
complementation (DSC), the G1 strand of the chaperone is donated in trans in a
non-canonical, parallel fashion to the exposed hydrophobic groove of the subunit
to facilitate its proper folding ( Sauer et al., 1999 ; Barnhart et al., 2000 ; Bann
et al., 2004 ) ( Figure 12.2 B). The solvent-exposed set of alternating hydrophobic
residues on the chaperone's G1 strand directly interact with subunit pockets
The ribbon diagram on the right also depicts DSE, in which the PapKNte (blue) completes the fold
of PapE (red). [PDB code: 1N12]. (D) The P1-P5 pockets of PapE. The surface representation of
PapE (red) emphasizes the pockets that allow the PapKNte (blue) and the PapD G1 strand (green)
to mediate DSE and DSC, respectively. PapD residues L103, I105, and L107 (yellow, ball-and-stick
representation) project into the P1-P3 pockets and are correspondingly referred to as P1-P3 resi-
dues. PapKNte residues (cyan, ball-and-stick representation) project into the P2-P5 pockets and are
correspondingly referred to as P2-P5 residues. The N-terminal ends of the Nte and G1 strand are
labeled in the corresponding color. Note the shallow nature of the P4 pocket, which only accommodates
a glycine residue in the Nte sequence of Pap subunits, as shown in (E). (E) Sequence alignment of the
PapD G1 strand and Nte's of all Pap subunits (except PapG, which lacks an Nte). Pap subunits have
conserved P2-P5 residues at their Nte. P2, P3, and P5 residues are hydrophobic (blue), while the P4
residue is strictly glycine (yellow) to prevent steric clashes with the shallow P4 binding groove. PapD
has three hydrophobic residues that correspond to the P1-P3 residues. The arrows indicate the N- to
C-terminal direction for the sequences of Pap subunits (blue) and PapD (green).