Chaperones pilus

The production of adhesive P pili requires PapC, which is an 88-kDa outer membrane protein. Genetic lesions in PapC result in a block in the assembly pathway, leading to an accumulation of chaperone-pilus protein complexes in the periplasm (Norgren et al., 1987). Thus, in the absence of PapC it seems that the chaperone preassembly complexes are no longer targeted to outer membrane assembly sites. As a consequence, the periplasmic chaperone remains bound to the subunits, preventing their assembly into pili. 

PapO AND SUPERFAMILY OF PERIPLASMIC IMMUNOGLOBULIN-LIKE PILUS CHAPERONES... 

B. Conserved Structural Features in Periplasmic Pilus Chaperone... 

G. Ushering of Pilus Subunits into Pili from Chaperone Complexes... 118... 

The three-dimensional structure of the PapD periplasmic chaperone that forms transient complexes with pilus subunit proteins has been solved by Holmgren and Branden (1989). PapD consists of two globular domains oriented in the shape of a boomerang (Fig. 2). Each domain is a /3-barrel structure formed by two antiparallel /8-pleated sheets that have a topology similar to an immunoglobulin fold. The relationship between PapD and other immunoglobulin-like proteins is discussed in Section IV,C. 

Chaperone Length (amino acids) Organism Type of pilus... 

Recent studies have shown that the recognition of pilus subunit proteins by PapD involves the conserved cleft of the chaperone (Slonim et... 

Fig. 3. Periplasmic pilus chaperone consensus sequence. Amino acid sequence of the PapD chaperone (top line) and consensus sequence derived from the comparison of twelve chaperones (second line). Amino acids are indicated using the one-letter code. In the consensus sequence, a letter shows a residue that is present in at least eight out of twelve sequences, an asterisk designates an invariant residue, and a box shows a position with a hydrophobic residue in all twelve periplasmic chaperones. The arrows underneath the sequence represent the /3 strands found in the PapD structure.

PapD and PapC appear to act as molecular escorts, regulating the interactions of each pilus protein. As a molecular chaperone, PapD prevents nonproductive interactions of the subunits and allows the subunits to fold properly. We propose that PapC is a member of a new class of proteins that we have named molecular ushers. The PapC usher acts as... 

The ability of the PapD chaperone to bind transiently to pilus subunits and cap interactive surfaces allows the subunits to pass through the periplasm without aggregation. The uncapping of PapD at the outer membrane seems to expose polymerization sites and drive assembly. The mechanism of chaperone uncapping is unknown but is seemingly ATP independent and may involve PapC. 

PapD and Superfamily of Periplasmic Immunoglobulin-like Pilus Chaperones... 

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