InterPro domain: IPR008962

The PapD-like superfamily of periplasmic chaperones directs the assembly of over 30 diverse adhesive surface organelles that mediate the attachment of many different pathogenic bacteria to host tissues, a critical early step in the development of disease. PapD, the prototypical chaperone, is necessary for the assembly of P pili. P pili contain the adhesin PapG, which mediates the attachment of uropathogenic Escherichia coli to Gal(alpha) Gal receptors present on kidney cells and are critical for the initiation of pyelonephritis. The PapD-like chaperones consist of two Ig-like domains oriented toward each other, forming L-shaped molecules. In the chaperone-subunit complex, the G1beta strand of the chaperone completes an atypical Ig fold in the subunit by occupying the groove and running parallel to the subunit C-terminal F strand. This donor strand complementation interaction simultaneously stabilises pilus subunits and caps their interactive surfaces, preventing their premature oligomerisation in the periplasm. During pilus biogenesis, the highly conserved N-terminal extension of one subunit has been proposed to displace the chaperone G1beta strand from its neighbouring subunit in a mechanism termed donor strand exchange [ 1 ].

This entry represents the immunoglobulin (Ig)-like beta-sandwich domain found in PapD, as well as in other periplasmic chaperone proteins that include FimC and SfaE from E. coli, and Caf1m from Yersinia pestis [ 2 ]. In addition, major sperm proteins (MSP) and other related sperm proteins (such as WR4 and SSP-19) contain an Ig-like domain with a similar structural fold to PapD [ 3 , 4 ]. Major sperm proteins are central components in molecular interactions underlying sperm motility, with many isoforms existing in Caenorhabditis elegans.

1. PapD-like chaperones provide the missing information for folding of pilin proteins. Proc. Natl. Acad. Sci. U.S.A. 97, 7709-14
2. Pilus chaperones represent a new type of protein-folding catalyst. Nature 431, 329-33
3. 2.6 A resolution crystal structure of helices of the motile major sperm protein (MSP) of Caenorhabditis elegans. J. Mol. Biol. 319, 491-9
4. Structure of sperm-specific protein SSP-19 from Caenorhabditis elegans. Acta Crystallogr. D Biol. Crystallogr. 60, 1840-5