Structure and assembly of P-pili: A protruding hinge region used for assembly of a bacterial adhesion filament

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Abstract
High-resolution structures of macromolecular complexes offer unparalleled insight into the workings of biological systems and hence the interplay of these systems in health and disease. We have adopted a multifaceted approach to understanding the pathogenically important structure of P-pili, the class I adhesion pili from pyelonephritic Escherichia coli. Our approach combines electron cryomicroscopy, site-directed mutagenesis, homology modeling, and energy calculations, resulting in a high-resolution model of PapA, the major structural element of these pili. Fitting of the modeled PapA subunit into the electron cryomicroscopy data provides a detailed view of these pilins within the supramolecular architecture of the pilus filament. A structural hinge in the N-terminal region of the subunit is located at the site of a newly resolved electron density that protrudes from the P-pilus surface. The structural flexibility provided by this hinge is necessary for assembly of P-pili, illustrating one solution to construction of large macromolecular complexes from small repeating units. These data support our hypothesis that domain-swapped pilin subunits transit the outer cell membrane vertically and rotate about the hinge for final positioning into the pilus filament. Our data confirm and supply a structural basis for much previous genetic, biochemical, and structural data. This model of the P-pilus filament provides an insight into the mechanism of assembly of a macromolecular complex essential for initiation of kidney infection by these bacteria.