Abstract
P pili are important adhesive fibres involved in kidney infection by uropathogenic Escherichia coli strains. P pili are assembled by the conserved chaperone–usher pathway, which involves the PapD chaperone and the PapC usher. During pilus assembly, subunits are incorporated into the growing fiber via the donor–strand exchange (DSE) mechanism, whereby the chaperone's G1 β-strand that complements the incomplete immunoglobulin-fold of each subunit is displaced by the N-terminal extension (Nte) of an incoming subunit. P pili comprise a helical rod, a tip fibrillum, and an adhesin at the distal end. PapA is the rod subunit and is assembled into a superhelical right-handed structure. Here, we have solved the structure of a ternary complex of PapD bound to PapA through donor–strand complementation, itself bound to another PapA subunit through DSE. This structure provides insight into the structural basis of the DSE reaction involving this important pilus subunit. Using gel filtration chromatography and electron microscopy on a number of PapA Nte mutants, we establish that PapA differs in its mode of assembly compared with other Pap subunits, involving a much larger Nte that encompasses not only the DSE region of the Nte but also the region N-terminal to it.
Highlights
Urinary tract infections, which include infections of the bladder and kidney, are some of the most common bacterial infections
P pili consist of six subunits making up a composite fiber with a short tip fibrillum composed of the PapE subunit joined to a more rigid helical rod composed of the PapA subunit [6,7]
This process occurs through a zip-in–zip-out process whereby the donor–strand exchange (DSE) reaction is initiated by the insertion of the P5 residue of the N-terminal extension (Nte) of one subunit into the P5 pocket of the groove of the other [21]
Summary
Urinary tract infections, which include infections of the bladder (cystitis) and kidney (pyelonephritis), are some of the most common bacterial infections. As chaperone– subunit complexes are differentially targeted to the usher [16,17], each subunit donates its Nte to complete the Ig-fold of the subunit previously assembled by inserting its P2–P5 residues into the corresponding P2–P5 binding pockets, first displacing and replacing the chaperone G1 strand in the groove of the previously assembled pilus subunit [18,19,20,21] This process occurs through a zip-in–zip-out process whereby the DSE reaction is initiated by the insertion of the P5 residue of the Nte of one subunit into the P5 pocket of the groove of the other [21].
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