![]() ![]() ![]() In the absence of an usher, chaperone–subunit complexes accumulate in the periplasm but cannot translocate across the OM for assembly into pili on the bacterial surface ( 11– 13). Dissociation of the chaperone uncaps the interactive surfaces of the subunits, which drives their assembly into pili ( 8). Chaperone–subunit complexes are targeted to the PapC usher in the OM ( 10), where the chaperone dissociates from the subunits. The chaperone caps interactive surfaces on the subunits, preventing their premature aggregation ( 8, 9). ![]() PapG recognition of the galabiose receptor is thought to be a prerequisite for pyelonephritis.Īssembly of P pili requires the PapD chaperone, which facilitates release of pilus subunits from the cytoplasmic membrane and likely provides a template for their folding in the periplasm ( 7). The PapG adhesin is situated at the distal end of the flexible tip fibrillum where it is capable of binding to the globo series of glycolipids in the human kidney ( 6). P pili are composite structures consisting of a thin flexible tip fibrillum connected to a rigid helical rod ( 4, 5). Assembly of P pili by uropathogenic Escherichia coli represents the prototype for the chaperone/usher pathway. Translocation of a superfamily of over 27 bacterial adhesive organelles across the outer membrane depends on OM proteins called ushers that work in concert with periplasmic chaperones as part of the chaperone/usher pathway ( 2, 3). Pili contain adhesins, typically located at their tips, that mediate the initial attachment of bacteria to their hosts ( 2), a key step in the pathogenesis of an organism. Pilin, one of the most abundant class of proteins produced by many bacterial pathogens ( 1), must cross the outer membrane (OM) to be assembled into thread-like fibers called pili. The usher complex appears to be similar to complexes formed by members of the PulD/pIV family of OM proteins, and thus these two protein families, previously thought to be unrelated, may share structural and functional homologies. Packaging of these fibers into their final helical structure would only occur outside the cell, a process that may drive outward growth of the pilus organelles. Thus, they are likely to pass through the usher in this unwound form. We show that P pilus rods can be unraveled into linear fibers by incubation in 50% glycerol. This is large enough to accommodate pilus subunits or the linear tip fibrillum of the pilus but not large enough to accommodate the final 6.8-nm-wide helical pilus rod. We present high-resolution electron micrographs of the OM usher PapC and show that it forms an oligomeric complex containing a channel approximately 2 nm in diameter. To elucidate this process we studied P pilus biogenesis in Escherichia coli. Protein translocation across the bacterial OM is not well understood. Bacterial virulence factors are typically surface-associated or secreted molecules that in Gram-negative bacteria must cross the outer membrane (OM). ![]()
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