Structural basis of chaperone function and pilus biogenesis

Many Gram-negative pathogens assemble architecturally and functionally dive rse adhesive pill on their surfaces by the chaperone-usher pathway, Immunog lobulin-like periplasmic chaperones escort pilus subunits to the usher, a l arge protein complex that facilitates the translocation and assembly of sub units across the outer membrane. The crystal structure of the PapD-PapK cha perone-subunit complex, determined at 2.4 angstrom resolution, reveals that the chaperone functions by donating its G(1) beta strand to complete the i mmunoglobulin-like fold of the subunit via a mechanism termed donor strand complementation. The structure of the PapD-PapK complex also suggests that during pilus biogenesis, every subunit completes the immunoglobulin-like fo ld of its neighboring subunit via a mechanism termed donor strand exchange.