The PapC usher forms an oligomeric channel: implications for pilus biogenesis across the outer membrane.
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.
Thanassi
D G
DG
Department of Molecular Microbiology, Box 8230, 660 South Euclid Avenue, Washington University School of Medicine, St. Louis, MO 63110, USA.
Saulino
E T
ET
Lombardo
M J
MJ
Roth
R
R
Heuser
J
J
Hultgren
S J
SJ
D001425:Bacterial Outer Membrane Proteins / Q000378:metabolism* / Q000648:ultrastructure; D001692:Biological Transport; D002462:Cell Membrane / Q000378:metabolism* / Q000648:ultrastructure; D004254:DNA Nucleotidyltransferases; D004926:Escherichia coli / Q000378:metabolism* / Q000648:ultrastructure; D029968:Escherichia coli Proteins; D036781:Fimbriae Proteins; D019426:Integrases; D018272:Porins / Q000737:chemistry / Q000378:metabolism / Q000648:ultrastructure*; D011487:Protein Conformation; D011489:Protein Denaturation; D017510:Protein Folding; D045522:Recombinases
D001425:Bacterial Outer Membrane Proteins; D029968:Escherichia coli Proteins; D018272:Porins; D045522:Recombinases; C111243:atpG protein, E coli; C061377:PulD protein, Klebsiella pneumoniae; D036781:Fimbriae Proteins; D004254:DNA Nucleotidyltransferases; D019426:Integrases; C115006:integron integrase IntI1