Kinetic analysis of the synthesis and assembly of type 1 fimbriae of Escherichia coli

Stochastic chain termination in bacterial pilus assembly

Adhesive type 1 pili from uropathogenic Escherichia coli strains are filamentous, supramolecular protein complexes consisting of a short tip fibrillum and a long, helical rod formed by up to several thousand copies of the major pilus subunit FimA. Here, we reconstituted the entire type 1 pilus rod assembly reaction in vitro, using all constituent protein subunits in the presence of the assembly platform FimD, and identified the so-far uncharacterized subunit FimI as an irreversible assembly terminator. We provide a complete, quantitative model of pilus rod assembly kinetics based on the measured rate constants of FimD-catalyzed subunit incorporation. The model reliably predicts the length distribution of assembled pilus rods as a function of the ratio between FimI and the main pilus subunit FimA and is fully consistent with the length distribution of membrane-anchored pili assembled in vivo. The results show that the natural length distribution of adhesive pili formed via the chaperone-usher pathway results from a stochastic chain termination reaction. In addition, we demonstrate that FimI contributes to anchoring the pilus to the outer membrane and report the crystal structures of (i) FimI in complex with the assembly chaperone FimC, (ii) the FimI-FimC complex bound to the N-terminal domain of FimD, and (iii) a ternary complex between FimI, FimA and FimC that provides structural insights on pilus assembly termination and pilus anchoring by FimI.

Use of a combined cryo-EM and X-ray crystallography approach to reveal molecular details of bacterial pilus assembly by the chaperone/usher pathway

Many bacteria assemble hair-like fibers termed pili or fimbriae on their cell surface. These fibers mediate adhesion to various surfaces, including host cells, and play crucial roles in pathogenesis. Pili are polymers composed of thousands of individual subunit proteins. Understanding how these subunit proteins cross the bacterial envelope and correctly assemble at the cell surface is important not only for basic biology but also for the development of novel antimicrobial agents. The chaperone/usher pilus biogenesis pathway is one of the best-understood protein secretion systems, thanks largely to innovative efforts in biophysical techniques such as X-ray crystallography and cryo-electron microscopy. Such a combined approach holds promise for further elucidating remaining questions regarding the multi-step and highly dynamic pilus assembly process, as well as for studying other protein secretion and organelle biogenesis systems.

Aerobic and anaerobic growth modes and expression of type 1 fimbriae in Salmonella

The aim of this study was to clarify the growth rates of facultatively anaerobic Salmonella enterica serovar Enteritidis strain in aerobic and anaerobic conditions and the expression of type 1 fimbriae in relation to the growth phases. The cultivation was carried out in a Portable Microbe Enrichment Unit (PMEU) where in same conditions one can grow the cells in parallel by modifying, e.g. aerobiosis only. The results obtained show that although the anaerobic metabolism is generally believed to be a slower producer of biomass or metabolites, in these circumstances S. enterica serovar Enteritidis strain gave comparable growth rates in anaerobiosis with nitrogenation as in aerobic cultures with constant aeration. Fimbrial antigens were produced in the beginning of logarithmic phase of the growth cycle both in the aerobic and anaerobic conditions. The fimbria remained in the presence of oxygen. This capability is possibly used for the intrusion of oxygen containing tissues of host body by the invading pathogens. In conclusion S. enterica serovar Enteritidis strain suspensions grow equally well in constant nitrogenation and aeration, and fimbria were produced in both conditions, during the early logarithmic phase but they prevailed in the presence of aeration.

Mechanism of fibre assembly through the chaperone-usher pathway

The chaperone-usher pathway directs the formation of adhesive surface fibres in numerous pathogenic Gram-negative bacteria. The fibres or pili consist exclusively of protein subunits that, before assembly, form transient complexes with a chaperone in the periplasm. In these chaperone:subunit complexes, the chaperone donates one beta-strand to complete the imperfect immunoglobulin-like fold of the subunit. During pilus assembly, the chaperone is replaced by a polypeptide extension of another subunit in a process termed 'donor strand exchange' (DSE). Here we show that DSE occurs in a concerted reaction in which a chaperone-bound acceptor subunit is attacked by another chaperone-bound donor subunit. We provide evidence that efficient DSE requires interactions between the reacting subunits in addition to those involving the attacking donor strand. Our results indicate that the pilus assembly platforms in the outer membrane, referred to as ushers, catalyse fibre formation by increasing the effective concentrations of donor and acceptor subunits.

Biochemical Characterization and Agglutinating Properties of Xenorhabdus nematophilus F1 Fimbriae

Xenorhabdus spp., entomopathogenic bacteria symbiotically associated with nematodes of the family Steinernematidae, occur spontaneously in two phases. Only the phase I variants of Xenorhabdus nematophilus F1 expressed fimbriae when the bacteria were grown on a solid medium (nutrient agar; 24 and 48 h of growth). These appendages were purified and characterized. They were rigid, with a diameter of 6.4 (plusmn) 0.3 nm, and were composed of 16-kDa pilin subunits. The latter were synthesized and assembled during the first 24 h of growth. Phase II variants of X. nematophilus did not possess fimbriae and apparently did not synthesize pilin. Phase I variants of X. nematophilus have an agglutinating activity with sheep, rabbit, and human erythrocytes and with hemocytes of the insect Galleria mellonella. The purified fimbriae agglutinated sheep and rabbit erythrocytes. The hemagglutination by bacteria and purified fimbriae was mannose resistant and was inhibited by porcine gastric mucin and N-acetyl-lactosamine. The last sugar seems to be a specific inhibitor of hemagglutination by X. nematophilus.

The export systems of type 1 and F1C fimbriae are interchangeable but work in parental pairs

Type 1 and F1C fimbriae are surface organelles of Escherichia coli which mediate receptor-specific binding to different host surfaces. Such fimbriae are found, among others, on strains associated with urinary tract infections. Biosynthesis of type 1 and F1C fimbrial organelles requires individual, specialized two-component assembly systems. The organization of the fim and foc gene clusters encoding these fimbriae, as well as the structure of the organelles, is very similar; however, the actual sequence homology of the structural elements is not remarkable (34 to 60%). Both gene clusters encode a periplasmically located chaperone and an usher protein, located in the outer membrane, required for organelle biogenesis. Deletion of either element causes abolishment of fimbriation. The present report addresses the question of promiscuity in fimbrial biogenesis. Our data indicate that the two-component export systems of the two organelle systems are reciprocally interchangeable; however, they seem to function only in parental pairs.

Expression of type 1 fimbriae and mannose-sensitive hemagglutinin by recombinant plasmids

Deletions within the cloned genes (fimA) encoding the type 1 major fimbrial subunits of two isolates of Klebsiella pneumoniae resulted in a nonfimbriate but hemagglutinating phenotype after transformation of Escherichia coli HB101 or ORN103. Phenotypic expression of type 1 fimbriae could be restored by transformation with plasmids containing the fimA genes of the fimbrial gene clusters from different strains. The surface fimbriae expressed were serologically identical to those of the polymerized product of the introduced fimA gene. The fimA gene products of Salmonella typhimurium and Serratia marcescens could utilize the accessory fimbrial genes of K. pneumoniae to produce surface-associated, hemagglutinating fimbriae. The relatedness of the type 1 fimbrial gene clusters from multiple isolates of members of the family Enterobacteriaceae was examined by DNA hybridization techniques. These analyses demonstrated little nucleotide sequence agreement among distinct genera of the enteric bacteria.

Infection of a human leukemia K-562 cell line with Semliki Forest virus

Infection of the human leukemia hematopoietic stem cell line, K-562, with Semliki Forest virus (SFV) can be characterized by three stages: (1) an early virus-proliferating stage lasting 1 to 4 days post-infection (pi) in which infectious virus is produced in high titers (10(3)pfu/cell) but in which there is minimal cytopathic effect. All cells appear viable by trypan blue dye exclusion, although they do not proliferate, and DNA and cell protein synthesis decrease to less than 3% of uninfected controls within 24 hours; (2) an intermediate stage extending from day 5 to about day 24-30 pi in which the amount of infectious virus declines to low levels. During this stage, viral protein synthesis decreases to undetectable levels, although viral gylcoproteins are readily demonstrated by immunofluorescence and by immunoblot; however, capsid protein appears to degrade within 21 days pi. Cell numbers remain constant but the viability of the non-proliferating cells determined by trypan blue exclusion could not be determined with confidence; (3) a final long-term stage in which viral glycoproteins, E1 and E2, are detectable by immunoblots and immunofluorescence for many months but the cells are metabolically inactive and do not synthesize viral proteins. These non-viable cells do not lyse for as long as 2 years.

Immunoelectron microscopic analysis of elongation of type 1 fimbriae in Escherichia coli

Using 10- and 20-nm-diameter gold particles conjugated to an antifimbrial monoclonal antibody, we analyzed the location of assembly of newly formed subunits on growing type 1 fimbriae of Escherichia coli. Fimbriae were removed from an E. coli K-12-derived strain, CSH50, by blending. Blended cells were allowed to regenerate their fimbriae in growth medium for approximately 25 min, after which they were labeled with a 20-nm-gold-monoclonal antibody probe. Continued outgrowth of these labeled fimbriae was allowed for additional time intervals, after which they were labeled with a 10-nm-gold-monoclonal antibody probe. The resulting fimbriae, double labeled with 10- and 20-nm-diameter gold particles, were examined in an electron microscope. The pattern of labeling on individual fimbrial organelles indicated morphologically that newly synthesized subunits are added to a growing organelle at its base.

Role of Pili (Fimbriae) in Attachment of Bradyrhizobium japonicum to Soybean Roots

Pili (fimbriae) were observed on cells of each of the five strains of Bradyrhizobium japonicum and the one strain of Rhizobium trifolii examined. Pili on B. japonicum were about 4 nm in diameter and polarly expressed. Piliated cells were estimated by transmission electron microscopy and hydrophobic attachment to polystyrene to constitute only a small percentage of the total population. The proportion of piliated cells in these populations was dependent on culture age in some strains. Piliated B. japonicum cells were selectively and quantitatively removed from suspension when cultures were incubated with either soybean roots or hydrophobic plastic surfaces, indicating that pili were involved in the attachment of the bacteria to these surfaces. Pili from B. japonicum 110 ARS were purified and found to have a subunit molecular weight of approximately 21,000. Treatment of B. japonicum suspensions with antiserum against the isolated pili reduced attachment to soybean roots by about 90% and nodulation by about 80%. Pili appear to be important mediators of attachment of B. japonicum to soybean roots under the conditions examined.