THE PAPC USHER FORMS AN OLIGOMERIC CHANNEL - IMPLICATIONS FOR PILUS BIOGENESIS ACROSS THE OUTER-MEMBRANE

Bacterial virulence factors are typically surface-associated or secret ed molecules that in Gram-negative bacteria must cross the outer membr ane (OM). Protein translocation across the bacterial OM is not well un derstood. To elucidate this process we studied P pilus biogenesis in E scherichia coli. We present high-resolution electron micrographs of th e OM usher PapC and show that it forms an oligomeric complex containin g a channel approximately 2 nm in diameter. This is large enough to ac commodate pilus subunits or the linear tip fibrillum of the pilus but not large enough to accommodate the final 6.8-nm-wide helical pilus ro d, We show that P pilus rods can be unraveled into linear fibers by in cubation in 50% glycerol. Thus, they are likely to pass through the us her in this unwound form. Packaging of these fibers into their final h elical structure would only occur outside the cell, a process that may drive outward growth of the pilus organelles. The usher complex appea rs to be similar to complexes formed by members of the PuID/pIV family of OM proteins, and thus these two protein families, previously thoug ht to be unrelated, may share structural and functional homologies.