PapD-like chaperones provide the missing information for folding of pilin proteins

Abstract

A fundamental question in molecular biology is how proteins fold into domains that can serve as assembly modules for building up large macromolecular structures. The biogenesis of pili on the surface of Gram-negative bacteria requires the orchestration of a complex process that includes protein synthesis, folding via small chaperones, secretion, and assembly. The results presented here support the hypothesis that pilus subunit folding and biogenesis proceed via mechanisms termed donor strand complementation and donor strand exchange. Here we show that the steric information necessary for pilus subunit folding is not contained in one polypeptide sequence. Rather, the missing information is transiently donated by a strand of a small chaperone to allow folding. Providing the missing information for folding, via a 13-amino acid peptide extension to the C-terminal end of a pilus subunit, resulted in the production of a protein that no longer required the chaperone to fold. This mechanism of small periplasmic chaperone function described here deviates from classical hsp60 chaperoneassisted folding.

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

“…5b). This fast binding to FimC in vivo may be responsible for the reported protection against degradation of pilus subunits by pilus chaperones 36 .…”

Rationally designed small compounds inhibit pilus biogenesis in uropathogenic bacteria

“…Pilus subunits consist of a single, incomplete Ig-like fold that lacks its C-terminal seventh (G) ␤-strand (12,13). In a process termed donor-strand complementation, the chaperone's G1 ␤-strand provides in trans the pilin's seventh strand (12)(13)(14)(15). The mechanism of action of PapD depends on the formation of an ion pair between the C-terminal carboxyl group of each subunit and the conserved chaperone cleft residues R8 and K112 (7,8).…”

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].…”