The PapC usher forms an oligomeric channel: Implications for pilus biogenesis across the outer membrane

David G. Thanassi, Evan T. Saulino, Mary Jane Lombardo, Robyn Roth, John Heuser, Scott J. Hultgren

Research output: Contribution to journalArticle

134 Scopus citations


Bacterial virulence factors are typically surface-associated or secreted molecules that in Gram-negative bacteria must cross the outer membrane (OM). Protein translocation across the bacterial OM is not well understood. To elucidate this process we studied P pilus biogenesis in Escherichia coli. 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. 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 show that P pilus rods can be unraveled into linear fibers by incubation in 50% glycerol. Thus, they are likely to pass through the usher in this unwound form. 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. 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.

Original languageEnglish
Pages (from-to)3146-3151
Number of pages6
JournalProceedings of the National Academy of Sciences of the United States of America
Issue number6
StatePublished - Mar 17 1998

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