Structural Basis of Chaperone Function and Pilus Biogenesis

Abstract

Many Gram-negative pathogens assemble architecturally and functionally diverse adhesive pili on their surfaces by the chaperone-usher pathway. Immunoglobulin-like periplasmic chaperones escort pilus subunits to the usher, a large protein complex that facilitates the translocation and assembly of subunits across the outer membrane. The crystal structure of the PapD-PapK chaperone-subunit complex, determined at 2.4 angstrom resolution, reveals that the chaperone functions by donating its G(1) beta strand to complete the immunoglobulin-like fold of the subunit via a mechanism termed donor strand complementation. The structure of the PapD-PapK complex also suggests that during pilus biogenesis, every subunit completes the immunoglobulin-like fold of its neighboring subunit via a mechanism termed donor strand exchange.

Protein dynamics and function from solution state NMR spectroscopy

“…The final result, i.e. a β-strand donation from the ligand to the protein, is fundamentally related to the 'donor strand complementation' model that was discovered for chaperone-adhesin complexes in bacterial pili (Choudhury et al 1999;Sauer et al 1999). In a broader sense, it seems that cooperative folding reactions are fundamental in molecular recognition and that several flavors of folding are utilized, as exemplified by systems making use of 'coupled folding-binding', 'coupled unfoldingbinding' and 'cracking' mechanisms for recognition of their respective targets.…”

Quality control of disulfide bond formation in pilus subunits by the chaperone FimC

“…Each subunit has an incomplete immunoglobulin-like pilin fold 1 lacking the C-terminal β-strand. In addition, each subunit (except for FimH) has an N-terminal extension ('donor strand'), which is donated to the preceding subunit in the pilus, thereby completing its fold 1,6,7 .…”

Molecular mechanism of bacterial type 1 and P pili assembly

“…However, even with a correct disulfide bond, pilins are still intrinsically unstable because they have an incomplete immunoglobulin (Ig)-like fold that lacks its seventh, C-terminal β-strand, resulting in a deep hydrophobic groove on the subunit's surface. The pilin is stable only when the cognate chaperone completes the pilin's Ig-like fold by occupying the pilin's groove with part of its own G1 strand, a process called donor strand complementation (DSC) [31][32][33] (figure 2). DSC stabilizes pilus subunits as they emerge from the Sec translocon in the IM, promotes their folding, and also prevents their premature self-polymerization in the periplasm [12,34].…”

“…(d) Subunit-subunit assembly Every subunit, with the exception of the tip adhesins FimH and PapG, has an N-terminal extension (Nte) of 10-18 residues. Subunit-subunit interaction occurs via a process termed donor strand exchange (DSE) [32] (figure 2): in a chaperone-subunit complex, the chaperone's G1 strand is inserted into the groove of the subunit it complements (also termed the 'receiving' groove); this G1 strand is substituted with the Nte of the subunit next in assembly. Insertion of the Nte is, in this case, antiparallel to the F strand of the complemented subunit, and, thus, a canonical Ig-fold is reconstituted, resulting in an energetically more favourable state that drives the DSE process [13].…”