![]() Pilus biogenesis proceeds via the strand complementation mechanism, the mechanism functions as a direct consequence of the structures of both the chaperones and pilus subunits. In vivo pilus subunits function to form the main body of the helical pilus (approximately 3.28 subunits per turn), and the adhesive head. Particular subunits may have additional domains e.g. Because this N-terminal extension is the same β strand that normally completes the Ig fold, a large hydrophobic cleft is formed in the mature subunit (P1-P5), this cleft is both essential to pilus biogenesis but also necessitates a chaperone to stabilise the structure. The pilus subunit structure also has an Ig-like fold but with an essential N-terminal extension rather than a C-terminal extension. PAP pilin major protein, PapA homo13mer, top view, E.Coli. The chaperone functions to stop aggregation and degradation of pilus subunits by binding their interfacing regions and facilitating correct protein folding before bringing them to the usher/fimbriae complex. Long looped chaperones are FGL and short looped chaperones are FGS. Chaperones belong to one of two families based on the length of the loop connecting the beta strands F1 and G1. This fold is formed by 13 β strands (A1-G1) and 4 short α helices. The periplasmic chaperone (PapD) has a 'boomerang' structure formed by an immunoglobulin (Ig) like fold with an essential C-terminal extension (G1). In the active dimeric usher one of the usher has the plug located periplasmically and the other has it located inside the translocation pore. The plug domain acts to stop premature formation of a pilus and can either be located inside the pore or on the periplasmic side of the membrane. The NTD and CTDs function to bring chaperone/subunit complexes to the growing pilus and facilitate translocation across the pore. The usher pore (PapC) is formed by a 24 stranded beta barrel with 4 additional domains: N-terminal domain (NTD), Plug domain, and two C-terminal domains (CTD1 and CTD2 respectively). The usher forms the outer-membrane pore and functions in vivo as a dimer, though only one of the ushers is active at any one time. PapD is the chaperone PapC is the usher PapA and the remaining subunits form the fimbria itself Usher structure and function ![]() Ī schematic overview of the pap chaperone-usher system showing all subunits and their organisation. The P-pilus (pap) system is one of the best characterised and is shown below. The Pilus subunits polymerise via a non- covalent interaction to form the mature pilus which consists of an adhesive tip, helical body and an usher bound base. Overall the system includes periplasmic chaperones, periplasmic and extracellular pilus subunits, dimeric usher outer membrane pore and associated Sec machinery. Chaperone-usher secreted fimbriae are important pathogenicity factors facilitating host colonisation, localisation and biofilm formation in clinically important species such as uropathogenic Escherichia coli and Pseudomonas aeruginosa.Īll chaperone/usher systems are found within gene clusters consisting of at least an usher, a chaperone and one or more fimbriae subunits. These fimbriae are built up out of modular pilus subunits, which are transported into the periplasm in a Sec dependent manner. Chaperone-usher fimbriae (CU) are linear, unbranching, outer-membrane pili secreted by gram-negative bacteria through the chaperone-usher system rather than through type IV secretion or extracellular nucleation systems. ![]()
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