Chaperone-assisted self-assembly of pili independent of cellular energy

Assembly of P pili on the surface of pyelonephritic Escherichia coli proceeds from periplasmic chaperone-subunit complexes. The outer membrane protein PapC, which has been termed a molecular usher, is thought to be the site of assembly, where the chaperone dissociates from the subunits as they are incorporated into the pilus across the outer membrane. The kinetics of assembly and the energy requirements of the 'secretion' events at the outer membrane were investigated using a pulse-chase analysis in which preformed labeled periplasmic chaperone-subunit complexes were assembled into pili in synchrony by the induction of PapC. Provided that a sufficient amount of PapC was present and functional in the outer membrane, the incorporation of the major PapA subunit into pili was shown to be completed in less than 5 min. Our results also indicated that the targeting of PapC to the outer membrane may be a rate-limiting factor for pilus assembly. Following the arrival of PapC, the formation of pili seemed to proceed spontaneously and was not sensitive to a pH shift or an inhibitor of the electrochemical gradient across the cytoplasmic membrane. We suggest that the secretion of pili across the outer membrane may be independent of cellular energy and thermodynamically driven.