Periplasmic pilus chaperone protein

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. 

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. 

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