PapD-like chaperones and pilus biogenesis.

The assembly of adhesive pili from individual subunits by periplasmic PapD-like chaperones in Gram-negative bacteria offers insight into the complex process of organelle biogenesis. PapD-like chaperones bind, stabilize, and cap interactive surfaces of subunits until they are assembled into the pilus. Subunits lack the seventh *gb-strand necessary to complete their immunoglobulin-like folds; the chaperone supplies this missing strand. Indeed, the chaperone may act as a template, providing steric information to facilitate subunit folding. In the mature pilus, each subunit is thought to supply the missing strand to complete the fold of its neighbor. Thus, one general function of chaperones in organelle biogenesis may be to cap highly interactive surfaces of subunits until they reach the proper assembly site.