The Cryoelectron Microscopy Structure of the Type 1 Chaperone-Usher Pilus Rod

Abstract

SummaryAdhesive chaperone-usher pili are long, supramolecular protein fibers displayed on the surface of many bacterial pathogens. The type 1 and P pili of uropathogenic Escherichia coli (UPEC) play important roles during urinary tract colonization, mediating attachment to the bladder and kidney, respectively. The biomechanical properties of the helical pilus rods allow them to reversibly uncoil in response to flow-induced forces, allowing UPEC to retain a foothold in the unique and hostile environment of the urinary tract. Here we provide the 4.2-Å resolution cryo-EM structure of the type 1 pilus rod, which together with the previous P pilus rod structure rationalizes the remarkable “spring-like” properties of chaperone-usher pili. The cryo-EM structure of the type 1 pilus rod differs in its helical parameters from the structure determined previously by a hybrid approach. We provide evidence that these structural differences originate from different quaternary structures of pili assembled in vivo and in vitro.

The Remarkable Biomechanical Properties of the Type 1 Chaperone-Usher Pilus: A Structural and Molecular Perspective

“…The tip fibrillum of P pili is longer, as it contains 5 to 10 copies of PapE, while all other subunits are present as a single copy (7,8). The largest section of CU pili, the rod, is composed of thousands of copies of a single pilin (FimA for type 1 pili; PapA for P pili) (2,9,10). All CU pilins are composed of C-terminally truncated and thus incomplete immunoglobulin (Ig)-like folds, lacking the 7th β-strand ( Fig.…”

“…Once assembled into a pilus, the 10-to 20-residue-long N-terminal extension (Nte) of each subunit complements the preceding pilin's groove, stabilizing the structure and linking the subunits in the pilin polymer. This is referred to as donor strand exchange (DSE) (PDB code 5OH0 [10]) (right side). The Nte of FimA in the surface model on the right has been removed for clarity.…”

“…3c). It was suggested that the increased resistance against unwinding of type 1 pili could be explained, in part, by the larger stacking interface (10). The physiological relevance of these biomechanical properties was demonstrated when UPEC strains expressing type 1 pilus rods with weaker stackto-stack interactions were significantly attenuated in their ability to cause intestinal colonization and bladder infection in mice (77).…”

“…Interestingly, the stability of type 1 pili seems to depend on the route of their assembly. Type 1 pili assembled in vivo, by the CU machinery, are significantly more stable against dissociation and unfolding than pilus rods assembled from FimA alone in vitro (10). In fact, the unfolding rate constants differed by 3 to 4 orders of magnitude, raising the intriguing question of whether the usher, or indeed another factor, can guide and influence the assembly of an optimally stable structure in vivo.…”

“…Future experiments will hopefully FIGURE 3 The structure of the rod. (a) Surface models showing the type 1 pilus rod structure (PDB code 5OH0 [10]) in a side view and a top view, which are 90°rotated with respect to each other. The Nte of the uppermost FimA molecule is removed in the top view for illustrative purposes.…”