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 system. 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.
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