Structure of the Neisseria meningitidis Outer Membrane PilQ Secretin Complex at 12 Å Resolution

Structural insights into the secretin PulD and its trypsin-resistant core

Limited proteolysis, secondary structure and biochemical analyses, mass spectrometry, and mass measurements by scanning transmission electron microscopy were combined with cryo-electron microscopy to generate a three-dimensional model of the homomultimeric complex formed by the outer membrane secretin PulD, an essential channel-forming component of the type II secretion system from Klebsiella oxytoca. The complex is a dodecameric structure composed of two rings that sandwich a closed disc. The two rings form chambers on either side of a central plug that is part of the middle disc. The PulD polypeptide comprises two major, structurally quite distinct domains; an N domain, which forms the walls of one of the chambers, and a trypsin-resistant C domain, which contributes to the outer chamber, the central disc, and the plug. The C domain contains a lower proportion of potentially transmembrane beta-structure than classical outer membrane proteins, suggesting that only a small part of it is embedded within the outer membrane. Indeed, the C domain probably extends well beyond the confines of the outer membrane bilayer, forming a centrally plugged channel that penetrates both the peptidoglycan on the periplasmic side and the lipopolysaccharide and capsule layers on the cell surface. The inner chamber is proposed to constitute a docking site for the secreted exoprotein pullulanase, whereas the outer chamber could allow displacement of the plug to open the channel and permit the exoprotein to escape.

Resistance of neisseria meningitidis to the toxic effects of heme iron and other hydrophobic agents requires expression of ght

Several genetic systems that allow the use of iron-protoporphyrin IX (heme) have been described for the pathogenic bacterium Neisseria meningitidis. However, many questions about the process of heme acquisition and utilization remain to be answered. To isolate and analyze unidentified genes that play a role in heme iron uptake and utilization, a Himar1 transposon mutant library was screened in N. meningitidis serogroup A strain IR4162. One locus identified by transposon mutagenesis conferred protection against heme toxicity. A mutant with a deletion in a gene termed ght (gene of hydrophobic agent tolerance) within this locus was susceptible to heme and other hydrophobic agents compared to the parental strain. Transcriptional analysis indicated that ght is cotranscribed with an upstream open reading frame NMA2149. Uncharacterized orthologues of ght were identified in many other gram-negative bacteria. We present genetic evidence for the importance of ght in resistance to hydrophobic agents and its potential role in interaction with other hydrophobic agent resistance mechanisms within N. meningitidis.

Identification and characterization of genes required for competence in Neisseria meningitidis

We have identified genes required for competence of Neisseria meningitidis, a naturally transformable human pathogen. Although not comprehensive, our analysis identified competence-defective mutants with transposon insertions in genes not previously implicated in this process in Neisseria.

Structure and biochemical analysis of a secretin pilot protein

The ability to translocate virulence proteins into host cells through a type III secretion apparatus (TTSS) is a hallmark of several Gram-negative pathogens including Shigella, Salmonella, Yersinia, Pseudomonas, and enteropathogenic Escherichia coli. In common with other types of bacterial secretion apparatus, the assembly of the TTSS complex requires the preceding formation of its integral outer membrane secretin ring component. We have determined at 1.5 A the structure of MxiM28-142, the Shigella pilot protein that is essential for the assembly and membrane association of the Shigella secretin, MxiD. This represents the first atomic structure of a secretin pilot protein from the several bacterial secretion systems containing an orthologous secretin component. A deep hydrophobic cavity is observed in the novel 'cracked barrel' structure of MxiM, providing a specific binding domain for the acyl chains of bacterial lipids, a proposal that is supported by our various lipid/MxiM complex structures. Isothermal titration analysis shows that the C-terminal domain of the secretin, MxiD525-570, hinders lipid binding to MxiM.

Interaction with type IV pili induces structural changes in the bacterial outer membrane secretin PilQ

Type IV pili are cell surface organelles found on many Gram-negative bacteria. They mediate a variety of functions, including adhesion, twitching motility, and competence for DNA uptake. The type IV pilus is a helical polymer of pilin protein subunits and is capable of rapid polymerization or depolymerization, generating large motor forces in the process. Here we show that a specific interaction between the outer membrane secretin PilQ and the type IV pilus fiber can be detected by far-Western analysis and sucrose density gradient centrifugation. Transmission electron microscopy of preparations of purified pili, to which the purified PilQ oligomer had been added, showed that PilQ was uniquely located at one end of the pilus fiber, effectively forming a "mallet-type" structure. Determination of the three-dimensional structure of the PilQ-type IV pilus complex at 26-angstroms resolution showed that the cavity within the protein complex was filled. Comparison with a previously determined structure of PilQ at 12-angstroms resolution indicated that binding of the pilus fiber induced a dissociation of the "cap" feature and lateral movement of the "arms" of the PilQ oligomer. The results demonstrate that the PilQ structure exhibits a dynamic response to the binding of its transported substrate and suggest that the secretin could play an active role in type IV pilus assembly as well as secretion.