Structural and energetic basis of folded protein transport by the FimD usher

Type 1 pili, produced by uropathogenic Escherichia coli (UPEC), are multisubunit fibers that play crucial roles in recognition of and adhesion to host tissues(1). During pilus biogenesis, subunits are recruited to an outer membrane assembly platform, the FimD usher, which catalyzes their polymerization and mediates pilus secretion(2). The recent crystal structure of an initiation complex provided insights into the initiation step of pilus biogenesis resulting in pore activation, but very little is known about the elongation steps that follow(3). To address this question, we determined the structure of an elongation complex whereby FimD is traversed by the tip complex assembly composed of FimC:FimF:FimG:FimH. This structure reveals the conformational changes required to prevent backsliding of the nascent pilus through the FimD pore and also reveals unexpected properties of the usher pore. We show that the usher pore is energetically designed to facilitate substrate passage within the pore and that the circular binding interface between the pore lumen and the folded substrate plays a role in transport by defining a low energy pathway along which the nascent pilus polymer is guided during secretion.