We thank Drs Bitter and Tommassen for their comments on our recent work on the role of ushers in pilus assembly and its relationship to Gram-negative bacterial secretion systems. The chaperone/usher pathway shares no homology at the genetic or amino acid level with the type II and type III secretion systems. Despite this, these systems have adopted a similar strategy for secreting proteins across the outer membrane: the use of oligomeric ring-shaped complexes as apparent transport channels. The similarities among the outer membrane components of these secretion systems is striking, although not entirely surprising. The goal of each system is (probably) to transport folded proteins across the outer membrane. Furthermore, as with the chaperone/usher pathway, the type II (type IV pill) and type III pathways can both assemble macromolecular structures on the cell surface. This raises the question, as posed by Bitter and Tommassen, of why the chaperone/usher pathway has not been used for secretion of soluble proteins. The chaperone/usher system lacks any obvious components for harnessing energy available at the cytoplasm membrane and has been shown to function independently of cellular energy. Winding of the pilus rod on the cell surface probably helps drive pilus export, but this cannot be the whole story. Pilus tips can be assembled on the cell surface in the absence of the pilus rod, and the chaperone/usher pathway also assembles several non-pilus adhesive structures. Part of the energy for secretion might come from the favorable protein-protein interactions involved in assembling protein subunits into a fiber. By definition, secretion of soluble proteins will lack these interactions. Thus, the chaperone/usher system might not be able to adapt for secretion of soluble proteins.
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