Caulobacter crescentus pilin. Purification, chemical characterization, and NH2-terminal amino acid sequence of a structural protein regulated during development

AFM Unravels the Unique Adhesion Properties of the Caulobacter Type IVc Pilus Nanomachine

Bacterial pili are proteinaceous motorized nanomachines that play various functional roles including surface adherence, bacterial motion, and virulence. The surface-contact sensor type IVc (or Tad) pilus is widely distributed in both Gram-positive and Gram-negative bacteria. In Caulobacter crescentus, this nanofilament, though crucial for surface colonization, has never been thoroughly investigated at the molecular level. As Caulobacter assembles several surface appendages at specific stages of the cell cycle, we designed a fluorescence-based screen to selectively study single piliated cells and combined it with atomic force microscopy and genetic manipulation to quantify the nanoscale adhesion of the type IVc pilus to hydrophobic substrates. We demonstrate that this nanofilament exhibits high stickiness compared to the canonical type IVa/b pili, resulting mostly from multiple hydrophobic interactions along the fiber length, and that it features nanospring mechanical properties. Our findings may be helpful to better understand the structure-function relationship of bacterial pilus nanomachines.

Exopolysaccharide production in Caulobacter crescentus: A resource allocation trade-off between protection and proliferation

Complex and interacting selective pressures can produce bacterial communities with a range of phenotypes. One measure of bacterial success is the ability of cells or populations to proliferate while avoiding lytic phage infection. Resistance against bacteriophage infection can occur in the form of a metabolically expensive exopolysaccharide capsule. Here, we show that in Caulobacter crescentus, presence of an exopolysaccharide capsule provides measurable protection against infection from a lytic paracrystalline S-layer bacteriophage (CR30), but at a metabolic cost that reduces success in a phage-free environment. Carbon flux through GDP-mannose 4,6 dehydratase in different catabolic and anabolic pathways appears to mediate this trade-off. Together, our data support a model in which diversity in bacterial communities may be maintained through variable selection on phenotypes utilizing the same metabolic pathway.

OmpW of Caulobacter crescentus Functions as an Outer Membrane Channel for Cations

Caulobacter crescentus is an oligotrophic bacterium that lives in dilute organic environments such as soil and freshwater. This bacterium represents an interesting model for cellular differentiation and regulation because daughter cells after division have different forms: one is motile while the other is non-motile and can adhere to surfaces. Interestingly, the known genome of C. crescentus does not contain genes predicted to code for outer membrane porins of the OmpF/C general diffusion type present in enteric bacteria or those coding for specific porins selective for classes of substrates. Instead, genes coding for 67 TonB-dependent outer membrane receptors have been identified, suggesting that active transport of specific nutrients may be the norm. Here, we report that high channel-forming activity was observed with crude outer membrane extracts of C. crescentus in lipid bilayer experiments, indicating that the outer membrane of C. crescentus contained an ion-permeable channel with a single-channel conductance of about 120 pS in 1M KCl. The channel-forming protein with an apparent molecular mass of about 20 kDa was purified to homogeneity. Partial protein sequencing of the protein indicated it was a member of the OmpW family of outer membrane proteins from Gram-negative bacteria. This channel was not observed in reconstitution experiments with crude outer membrane extracts of an OmpW deficient C. crescentus mutant. Biophysical analysis of the C. crescentus OmpW suggested that it has features that are special for general diffusion porins of Gram-negative outer membranes because it was not a wide aqueous channel. Furthermore, OmpW of C. crescentus seems to be different to known OmpW porins and has a preference for ions, in particular cations. A putative model for OmpW of C. crescentus was built on the basis of the known 3D-structures of OmpW of Escherichia coli and OprG of Pseudomonas aeruginosa using homology modeling. A comparison of the two known structures with the model of OmpW of C. crescentus suggested that it has a more hydrophilic interior and possibly a larger diameter.

The core and O-polysaccharide structure of the Caulobacter crescentus lipopolysaccharide

Here we describe the analysis of the structure of the lipopolysaccharide (LPS) from Caulobacter crescentus strain JS1025, a derivative of C. crescentus CB15 NA1000 with an engineered amber mutation in rsaA, leading to the loss of the protein S-layer and gene CCNA_00471 encoding a putative GDP-L-fucose synthase. LPS was isolated using an aqueous membrane disruption method. Polysaccharide and core oligosaccharide were produced by mild acid hydrolysis and analyzed by nuclear magnetic resonance spectroscopy and chemical methods. Spectra revealed the presence of two polysaccharides, one of them, a rhamnan, could be removed using periodate oxidation. Another polymer, built from 4-amino-4-deoxy-D-rhamnose (perosamine), mannose, and 3-O-methyl-glucose, should be the O-chain of the LPS according to genetic data. The attribution of the rhamnan as a part of LPS or a separate polymer was not possible.

Analysis of high-level S-layer protein secretion inCaulobacter crescentus

Caulobacter crescentus exhibits a hexagonally arranged protein layer on its outermost surface. RsaA, the sole protein of this “S-layer”, is secreted by a type I (ABC) transporter. Few type I transporters show high-level secretion, and few bacterial S-layers have been carefully examined for the amount of protein synthesis capacity needed to maintain cell coverage. Here we determined RsaA levels by quantitative immunoblotting methods, learned that very stable mRNA is a key factor in high-level secretion, and found that the transporter was capable of still higher secretion. A propensity for RsaA to aggregate was a barrier to quantitation, but with the use of S-layer shedding mutants and methods to keep RsaA soluble, we learned that ~31% of cell protein is RsaA. When multiple copies of rsaA were introduced, the level increased to ~51% of cell protein, a higher level than we are aware of for any protein in any bacterium. Unexpectedly, in comparing normal and S-layer shedding strains, an assembled S-layer was not a significant barrier to elevated secretion. The rsaA mRNA half-life was determined by real-time PCR to be 36 min, ranking with the most stable known in bacteria. A modification of the 5′ region resulted in a shorter half-life and a reduction in maximum protein synthesis levels. If secretion was prevented by knockout of type I transporter genes, RsaA levels dropped to 10% or less of normal, but with no significant reduction in rsaA mRNA. Overall, normal levels of RsaA were unexpectedly high, and still higher levels were not limited by transporter capability, the presence of an assembled S-layer, or the capacity of the cell’s physiology to produce large amounts of one protein. The normal upper limit of RsaA production appears to be controlled only by the level of an unusually stable message. Significant down-regulation is possible and is accomplished posttranscriptionally.

Expression and testing of Pseudomonas aeruginosa vaccine candidate proteins prepared with the Caulobacter crescentus S-layer protein expression system

A novel bacterial protein secretion system was used to produce vaccine candidates against Pseudomonas aeruginosa. The surface protein (RsaA) of Caulobacter crescentus was adapted to produce recombinant vaccine proteins based on the pilus tip epitope ('adhesintope') of P. aeruginosa. Two versions of the adhesintope, with (PCK) or without (PE) the cysteine residues that flank the epitope were investigated, fused to the C-terminus or inserted into full-length RsaA. When expressed in caulobacter the fusion proteins were secreted as aggregates. Full length RsaA-containing adhesintopes assembled on the cell surface as an S-layer; these were recovered by low pH extraction. Vaccine candidates were evaluated in a mouse challenge model. PCK-containing proteins produced at least 1000-fold higher antibody titers against the adhesintope, compared to the PE version, exceeding titers achievable with any other vaccine preparation method. Immunoglobulin isotyping indicated a balanced IgG1/IgG2 response, though when challenged with P. aeruginosa, the PE- and PCK-containing proteins did not afford mice a significant level of protection. Overall, we describe a new system for vaccine production that shows promise as a fast, economical way to construct, evaluate and produce vaccine proteins.

A family of six flagellin genes contributes to the Caulobacter crescentus flagellar filament

The Caulobacter crescentus flagellar filament is assembled from multiple flagellin proteins that are encoded by six genes. The amino acid sequences of the FljJ and FljL flagellins are divergent from those of the other four flagellins. Since these flagellins are the first to be assembled in the flagellar filament, one or both might have specialized to facilitate the initiation of filament assembly.

Secretion of the Caulobacter crescentus S-layer protein: further localization of the C-terminal secretion signal and its use for secretion of recombinant proteins

The secretion signal of the Caulobacter crescentus S-layer protein (RsaA) was localized to the C-terminal 82 amino acids of the molecule. Protein yield studies showed that 336 or 242 C-terminal residues of RsaA mediated secretion of >50 mg of a cellulase passenger protein per liter to the culture fluids.

Regulation of cellular differentiation in Caulobacter crescentus

In Caulobacter crescentus, asymmetry is generated in the predivisional cell, resulting in the formation of two distinct cell types upon cell division: a motile swarmer cell and a sessile stalked cell. These progeny cell types differ in their relative programs of gene expression and DNA replication. In progeny swarmer cells, DNA replication is silenced for a defined period, but stalked cells reinitiate chromosomal DNA replication immediately following cell division. The establishment of these differential programs of DNA replication may be due to the polar localization of DNA replication proteins, differences in chromosome higher-order structure, or pole-specific transcription. The best-understood aspect of Caulobacter development is biogenesis of the polar flagellum. The genes encoding the flagellum are expressed under cell cycle control predominantly in the predivisional cell type. Transcription of flagellar genes is regulated by a trans-acting hierarchy that responds to both flagellar assembly and cell cycle cues. As the flagellar genes are expressed, their products are targeted to the swarmer pole of the predivisional cell, where assembly occurs. Specific protein targeting and compartmentalized transcription are two mechanisms that contribute to the positioning of flagellar gene products at the swarmer pole of the predivisional cell.

Characterization of the cspB gene encoding PS2, an ordered surface-layer protein in Corynebacterium glutamicum

PS2 is one of two major proteins detected in the culture media of various Corynebacterium glutamicum strains. The coding and promoter regions of the cspB gene encoding PS2 were cloned in lambda gt11 using polyclonal antibodies raised against PS2 for screening. Expression of the cspB gene in Escherichia coli led to the production of a major anti-PS2 labelled peptide of 63,000 Da, corresponding presumably to the mature form of PS2. It was detected in the cytoplasm, periplasm and surrounding medium of E. coli. Three other slower migrating bands of 65,000 68,000 and 72,000 Da were detected. The largest one probably corresponds to the precursor form of PS2 in E. coli. Analysis of the nucleotide sequence revealed an open reading frame (ORF) of 1533 nucleotides. The deduced 510-amino-acid polypeptide had a calculated molecular mass of 55,426 Da. According to the predicted amino acid sequence, PS2 is synthesized with a N-terminal segment of 30-amino-acid residues reminiscent of eukaryotic and prokaryotic signal peptides, and a hydrophobic domain of 21 residues near the C-terminus. Although no significant homologies were found with other proteins, it appears that some characteristics and the amino acid composition of PS2 share several common features with surface-layer proteins. The cspB gene was then disrupted in C. glutamicum by gene replacement. Freeze-etching electron microscopy performed on the wild-type strain indicated that the cell wall of C. glutamicum is covered with an ordered surface of proteins (surface layer, S-layer) which is in very close contact with other cell-wall components. These structures are absent from the cspB-disrupted strain but are present after reintroduction of the cspB gene on a plasmid into this mutant. Thus we demonstrate that the S-layer protein is the product of the cspB gene.

Attachment of the adhesive holdfast organelle to the cellular stalk of Caulobacter crescentus

Caulobacters attach to surfaces in the environment via their holdfasts, attachment organelles located at the base of the flagellum in swarmer cells and later at the end of the cellular stalk in the stalked cells which develop from the swarmer cells. There seems to be little specificity with respect to the types of surfaces to which holdfasts adhere. A notable exception is that the holdfast of one cell does not adhere to the cell surface of another caulobacter, except by joining holdfasts, typically forming "rosettes" of stalked cells. Thus, the localized adhesion of the holdfasts to the cells is in some way a specialized attachment. We investigated this holdfast-cell attachment by developing an adhesion screening assay and analyzing several mutants of Caulobacter crescentus CB2A selected to be defective in adhesion. One class of mutants made a normal holdfast by all available criteria, yet the attachment to the cell was very weak, such that the holdfast was readily shed. Another class of mutants made no holdfast at all, but when mixed with a wild-type strain, a mutant of this class participated in rosette formation. The mutant could also attach to the discarded holdfast produced by a shedding mutant. In addition, when rosettes composed of holdfast-defective and wild-type cells were examined, an increase in the number of holdfast-defective cells was correlated with a decrease in the ability of the holdfast material at the center of the rosette to bind colloidal gold particles. Gold particles are one type of surface to which holdfasts adhere well, suggesting that the stalk end and the colloidal gold particles occupy the same sites on the holdfast substance. Taken together, the data support the interpretation that there is a specialized attachment site for the holdfast at the base of the flagellum which later becomes the end of the stalk, but not a specialized region of the holdfast for attachment to this site. Also, attachment to the cell is accomplished by bond formations that occur not only at the time of holdfast production. Thus, we propose that the attachment of the holdfast to the cell is a true adhesion process and that the stalk tip and base of the flagellum must have compositions distinctly different from that of the remainder of the caulobacter cell surface.

Cloning of the major protein of the Caulobacter crescentus periodic surface layer: detection and characterization of the cloned peptide by protein expression assays

A precisely ordered crystalline array is found on the surface of the bacterium Caulobacter crescentus CB15. Using an immunological assay, we identified recombinant bacteriophage clones expressing the predominant protein of this structure from a lambda 1059 library of C. crescentus CB15 DNA. A single 4.4-kilobase HindIII fragment encoded a polypeptide whose antigenic determinants, molecular weight, and peculiar solubilization properties were identical with those of the authentic predominant polypeptide (130K) of the surface array. The 130K protein was produced as a discrete product as a result of gene transcription initiated from a lambda promoter; several experiments suggested that the Caulobacter promoter for this gene is not efficiently recognized by the Escherichia coli transcription machinery. Genomic Southern analysis revealed a single copy of the 130K protein gene per genome. The 130K protein gene was hybridized with DNA of two closely related laboratory strains of C. crescentus which have lost their ability to produce a surface array. One of these strains, CB2, possesses an homologous copy of the 130K gene, whereas DNA from the other strain, CB13B1a, showed a lesser degree of hybridization to the 130K gene probe; genomic fragments which did hybridize were of different sizes in CB13 as compared with those of CB15. These findings are discussed in relation to studies of the surface array function and its role in cellular morphogenesis in this stalk-forming bacterium.

Proteinaceous bacterial adhesins and their receptors

The adhesion of bacteria to surfaces is an ecologically important property which enables them to colonize their natural habitats. Adhesion between bacteria mediated by sex pili and aggregation substances may also promote gene transfers. In this review, we describe the adhesive properties of bacteria (to eukaryotic and prokaryotic cells, and inert surfaces) and emphasize the characteristics of adhesins (structure, function, genetics, and morphology) and their cognate receptors on target surfaces. The physiochemical interactions between bacteria and surfaces can be described by the DLVO theory, but the interaction between bacterial adhesins and their receptor is better described as a ligand receptor interaction. The DLVO theory predicts that no physical contact can occur between bacteria and surface and, hence, predicts that adhesins must be filamentous in order to bridge the space between the two bodies and allow attachment of the bacteria. Adhesins are primarily proteinaceous, although adhesins of streptococci may involve dextrans or lipoteichoic acids. The cognate receptors for adhesins all appear to contain carbohydrates and as such as likely to be glycoconjugates with carbohydrate moieties acting as the receptor sites.