Characterization of mutants of Caulobacter crescentus defective in surface attachment of the paracrystalline surface layer.

Strains of Caulobacter crescentus express a paracrystalline surface layer (S-layer) consisting of the protein RsaA. Mutants of C. crescentus NA1000 and CB2, isolated for their ability to grow in the absence of calcium ions, uniformly no longer had the S-layer attached to the cell surface. However, RsaA was still produced, and when colonies grown on calcium-sufficient medium were examined, large two-dimensional arrays of S-layer were found intermixed with the cells. Such arrays were not found in calcium-deficient medium even when high levels of magnesium ions were provided. The arrays could be disrupted with divalent ion chelators and more readily with the calcium-selective ethylene glycol-bis (beta-aminoethyl ether)N,N,N',N'-tetraacetic acid (EGTA). Thus, the outer membrane surface was not needed as a template for self-assembly, but calcium likely was. The cell surface and S-layer gene of assembly-defective mutants of NA1000 were examined to determine the basis of the S-layer surface attachment defect. Mutants had no detectable alteration in the rough lipopolysaccharide (LPS) or a characterized capsular polysaccharide, but another polysaccharide molecule was greatly reduced or absent in all calcium-independent mutants. The molecule was shown to be a smooth LPS with a core sugar and fatty acid complement identical to those of the rough LPS and an O polysaccharide of homogeneous length, tentatively considered to be composed of 4,6-dideoxy-4-amino hexose, 3,6-dideoxy-3-amino hexose, and glycerol in equal proportions. This molecule (termed SLPS) was detectable by surface labeling with a specific antiserum only when the S-layer was not present. The rsaA genes from three calcium-independent mutants were cloned and expressed in an S-layer-negative, SLPS-positive strain. A normal S-layer was produced, ruling out defects in rsaA in these cases. It is proposed that SLPS is required for S-layer surface attachment, possibly via calcium bridging. The data support the possibility that calcium binding is required to prevent an otherwise lethal effect of SLPS. If true, mutations that eliminate the O polysaccharide of SLPS eliminate the lethal effects of calcium-deprived SLPS, at the expense of S-layer attachment.     

Isolation and comparison of the paracrystalline surface layer proteins of freshwater caulobacters.

Several methods for isolation of the paracrystalline surface (S) layer protein (RsaA) of Caulobacter crescentus CB15A were evaluated. Treatment of cells with HEPES (N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid) buffer at pH 2 was the most effective means of selectively removing RsaA from cells, and after neutralization, the protein was capable of reassembling into a paracrystalline structure. Ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid treatment could also be used to extract RsaA and yielded protein capable of reassembly. The success of the methods was likely related to disruption of calcium-mediated bonding; calcium was required for recrystallization, while magnesium and strontium ions were ineffective. Antibody was raised against purified RsaA and, along with the S-layer extraction techniques, was used to evaluate 42 strains of caulobacters isolated from a variety of aquatic and wastewater treatment locations. A single characteristic protein could be isolated from the 35 strains that produced an S layer; with one exception, no proteins were extracted from strains that had no S layer. The presumed S-layer proteins ranged in size from 100 to 193 kDa. All of these proteins specifically reacted with anti-RsaA serum by Western immunoblot analysis. In strain CB15A, a specific S-layer-associated oligosaccharide has been proposed to be involved in a calcium-mediated attachment of the S layer to the cell surface. This molecule was detected by Western immunoblotting with a specific antiserum and on polyacrylamide gels stained for polysaccharides. A comparable band was found in all S-layer-producing strains and for most, S-layer-associated oligosaccharide-specific antibody reacted with them in Western analysis. Overall, in freshwater caulobacters at least portions of their S-layer structures appear to be strongly conserved entities, as well as the means of attachment to the cell surface.     

S-layer anchoring and localization of an S-layer-associated protease in Caulobacter crescentus

The S-layer of the gram-negative bacterium Caulobacter crescentus is composed of a single protein, RsaA, that is secreted and assembled into a hexagonal crystalline array that covers the organism. Despite the widespread occurrence of comparable bacterial S-layers, little is known about S-layer attachment to cell surfaces, especially for gram-negative organisms. Having preliminary indications that the N terminus of RsaA anchors the monomer to the cell surface, we developed an assay to distinguish direct surface attachment from subunit-subunit interactions where small RsaA fragments are incubated with S-layer-negative cells to assess the ability of the fragments to reattach. In doing so, we found that the RsaA anchoring region lies in the first approximately 225 amino acids and that this RsaA anchoring region requires a smooth lipopolysaccharide species found in the outer membrane. By making mutations at six semirandom sites, we learned that relatively minor perturbations within the first approximately 225 amino acids of RsaA caused loss of anchoring. In other studies, we confirmed that only this N-terminal region has a direct role in S-layer anchoring. As a by-product of the anchoring studies, we discovered that Sap, the C. crescentus S-layer-associated protease, recognized a cleavage site in the truncated RsaA fragments that is not detected by Sap in full-length RsaA. This, in turn, led to the discovery that Sap was an extracellular membrane-bound protease, rather than intracellular, as previously proposed. Moreover, Sap was secreted to the cell surface primarily by the S-layer type I secretion apparatus.     

Nucleotide sequence analysis of the gene encoding the Caulobacter crescentus paracrystalline surface layer protein.

The entire nucleotide sequence of the rsaA gene, encoding the paracrystalline surface (S) layer protein (RsaA) of Caulobacter crescentus CB15A, was determined. The rsaA gene encoded a protein of 1026 amino acids, with a predicted molecular weight of 98,132. Protease cleavage of mature RsaA protein and amino acid sequencing of retrievable peptides yielded two peptides: one aligned with a region approximately two-thirds the way into the predicted amino acid sequence and the second peptide corresponded to the predicted carboxy terminus. Thus, no cleavage processing of the carboxy portion of the RsaA protein occurred during export, and with the exception of the removal of the initial methionine residue, the protein was not processed by cleavage to produce the mature protein. The predicted RsaA amino acid profile was unusual, with small neutral residues predominating. Excepting aspartate, charged amino acids were in relatively low proportion, resulting in an especially acidic protein, with a predicted pI of 3.46. As with most other sequenced S-layer proteins, RsaA contained no cysteine residues. A homology scan of the Swiss Protein Bank 17 produced no close matches to the predicted RsaA sequence. However, RsaA protein shared measurable homology with some exported proteins of other bacteria, including the hemolysins. Of particular interest was a specific region of the RsaA protein that was homologous to the repeat regions of glycine and aspartate residues found in several proteases and hemolysins. These repeats are implicated in the binding of calcium for proper structure and biological activity of these proteins. Those present in the RsaA protein may perform a similar function, since S-layer assembly and surface attachment requires calcium. RsaA protein also shared some homology with 10 other S-layer proteins, with the Campylobacter fetus S-layer protein scoring highest.     

Evaluating secretion and surface attachment of SapA, an S-layer-associated metalloprotease of Caulobacter crescentus

Caulobacter crescentus is used to display foreign peptides at high density as insertions into the surface (S)-layer protein (RsaA). Many recombinant RsaA proteins, however, are cleaved by SapA, a 71-kDa metalloprotease, suggesting a role in maintaining S-layer integrity. When overexpressed on a multicopy plasmid SapA was detected on the surface by fluorescent antibody only if RsaA and the O-side chain of LPS that mediates S-layer attachment were removed by mutation, indicating an outer membrane location beneath the S-layer. Secretion was mediated by the RsaA type 1 transporter since secretion was eliminated in transporter deficient strains or by C-terminal deletions in SapA (the presumed location of type 1 secretion signals). Secretion was required to become an active protease; mass spectrometry suggested this might be due to N-terminal processing during secretion, a feature shared with other type 1-secreted proteases. Overexpression leads to additional processing C-terminal to the protease domain, producing a 45-kDa protein. This was demonstrated to be self-processing. Deletion analysis revealed the C-terminal 100 amino acids were sufficient for anchoring and secretion. When protein G was fused to the last 238 amino acids of SapA it was secreted, surface attached and bound immunoglobulin, indicating potential for foreign protein display.     

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.     

Comparison of S-layer secretion genes in freshwater caulobacters

Our freshwater caulobacter collection contains about 40 strains that are morphologically similar to Caulobacter crescentus. All elaborate a crystalline protein surface (S) layer made up of protein monomers 100-193 kDa in size. We conducted a comparative study of S-layer secretion in 6 strains representing 3 size groups of S-layer proteins: small (100-108 kDa), medium (122-151 kDa), and large (181-193 kDa). All contained genes predicted to encode ATP-binding cassette transporters and membrane fusion proteins highly similar to those of C. crescentus, indicating that the S-layer proteins were all secreted by a type I system. The S-layer proteins' C-termini showed unexpectedly low sequence similarity but contained conserved residues and predicted secondary structure features typical of type I secretion signals. Cross-expression studies showed that the 6 strains recognized secretion signals from C. crescentus and Pseudomonas aeruginosa and similarly that C. crescentus was able to secrete the S-layer protein C-terminus of 1 strain examined. Inactivation of the ATP-binding cassette transporter abolished S-layer protein secretion, indicating that the type I transporter is necessary for S-layer protein secretion. Finally, while all of the S-layer proteins of this subset of strains were secreted by type I mechanisms, there were significant differences in genome positions of the transporter genes that correlated with S-layer protein size.     

Cell-surface display of a Pseudomonas aeruginosa strain K pilin peptide within the paracrystalline S-layer of Caulobacter crescentus

The paracrystalline surface (S)-layer of Caulobacter crescentus is composed of a single secreted protein (RsaA) that interlocks in a hexagonal pattern to completely envelop the bacterium. Using a genetic approach, we inserted a 12 amino acid peptide from Pseudomonas aeruginosa strain K pilin at numerous semirandom positions in RsaA. We then used an immunological screen to identify those sites that presented the inserted pilin peptide on the C. crescentus cell surface as a part of the S-layer. Eleven such sites (widely separated in the primary sequence) were identified, demonstrating for the first time that S-layers can be readily exploited as carrier proteins to display ‘epitope-size’ heterologous peptides on bacterial cell surfaces. Whereas intact RsaA molecules carrying a pilin peptide could always be found on the surface of C. crescentus regardless of the particular insertion site, introduction of the pilin peptide at 9 of the 11 sites resulted in some proteolytic cleavage of RsaA. Two types of proteolytic phenomena were observed. The first was characterized by a single cleavage within the pilin peptide insert with both fragments of the S-layer protein remaining anchored to the outer membrane. The other proteolytic phenomenon was characterized by cleavage of the S-layer protein at a point distant from the site of the pilin peptide insertion. This cleavage always occurred at the same location in RsaA regardless of the particular insertion site, yielding a surface-anchored 26 kDa proteolytic fragment bearing the RsaA N-terminus; the C-terminal cleavage product carrying the pilin peptide was released into the growth medium. When the results of this work were combined with the results of a previous study, the RsaA primary sequence could be divided into three regions with respect to the location of a peptide insertion and its effect on S-layer biogenesis: (i) insertions in the extreme N-terminus of RsaA either produce no apparent effect on S-layer biogenesis or disrupt surface-anchoring of the protein; (ii) insertions in the extreme C-terminus either produce no apparent effect on S-layer biogenesis or disrupt protein secretion; and (iii) insertions more centrally located in the protein either have no apparent effect on S-layer biogenesis or result in proteolytic cleavage of RsaA. These data are discussed in relation to our previous assignment of the RsaA N- and C-terminus as regions that are important for surface anchoring and secretion respectively.     

Caulobacter crescentus synthesizes an S-layer-editing metalloprotease possessing a domain sharing sequence similarity with its paracrystalline S-layer protein

Strains of Caulobacter crescentus elaborate an S-layer, a two-dimensional protein latticework which covers the cell surface. The S-layer protein (RsaA) is secreted by a type I mechanism (relying on a C-terminal signal) and is unusual among type I secreted proteins because high levels of protein are produced continuously. In efforts to adapt the S-layer for display of foreign peptides and proteins, we noted a proteolytic activity that affected S-layer monomers with foreign inserts. The cleavage was precise, resulting in fragments with an unambiguous N-terminal sequence. We developed an assay to screen for loss of this activity (i.e., presentation of foreign peptides without degradation), using transposon and traditional mutagenesis. A metalloprotease gene designated sap (S-layer-associated protease) was identified which could complement the protease-negative mutants. The N-terminal half of Sap possessed significant similarity to other type I secreted proteases (e.g., alkaline protease of Pseudomonas aeruginosa), including the characteristic RTX repeat sequences, but the C-terminal half which normally includes the type I secretion signal exhibited no such similarity. Instead, there was a region of significant similarity to the N-terminal region of RsaA. We hypothesize that Sap evolved by combining the catalytic portion of a type I secreted protease with an S-layer-like protein, perhaps to associate with nascent S-layer monomers to "scan" for modifications.     

Factors controlling in vitro recrystallization of the Caulobacter crescentus paracrystalline S-layer.

The S-layer of Caulobacter is a two-dimensional paracrystalline array on the cell surface composed of a single protein, RsaA. We have established conditions for preparation of stable, soluble protein and then efficient in vitro recrystallization of the purified protein. Efficient recrystallization and long range order could not be obtained with pure protein only, though it was apparent that calcium was required for crystallization. Recrystallization was obtained when lipid vesicles were provided, but only when the vesicles contained the specific species of Caulobacter smooth lipopolysaccharide (SLPS) that previous studies implicated as a requirement for attaching the S-layer to the cell surface. The specific type of phospholipids did not appear critical; phospholipids rather different from those present in Caulobacter membranes or archaebacterial tetraether lipids worked equally well. The source of LPS was critical; rough and smooth variants of Salmonella typhimurium LPS as well as the rough form of Caulobacter LPS were ineffective. The requirement for calcium ions for recrystallization was further evaluated; strontium ions could substitute for calcium, and to a lesser extent, cobalt, barium, manganese and magnesium ions also stimulated crystallization. On the other hand, nickel and cadmium provided only weak crystallization stimulation, and zinc, copper, iron, aluminum ions, and the monovalent potassium, sodium, and lithium ions were ineffective. The recrystallization could also be reproduced with Langmuir-Blodgett lipid monolayers at an air-water interface. As with the vesicle experiments, this was only successful when SLPS was incorporated into the lipid mix. The best method for RsaA preparation, leading to apparently monomeric protein that was stable for many months, was an extraction with a low pH aqueous solution. We also achieved recrystallization, albeit at lower efficiency, using RsaA protein solubilized by 8 M urea, a method which allows retrieval of protein from inclusions, when expressed as heterologous protein in Escherichia coli or when retrieved as shed, precipitated protein from certain mutant caulobacters. In summary, the clarification of recrystallization methods has confirmed the requirement of SLPS as a surface attachment component and suggests that its presence in a membrane-like structure greatly stimulates the extent and quality of S-layer formation. The in vitro approach allowed the demonstration that specific ions are capable of participating in crystallization and now provides an assay for the crystallization potential of modified S-layer proteins, whether they were produced in or can be secreted by caulobacters.     

新月柄杆菌RsaA分泌系统用于大肠杆菌胞外递送重组蛋白的初步研究

目的：构建基于新月柄杆菌RsaA外运机制的以大肠杆菌为宿主的原核胞外分泌表达载体系统。方法：利用分子克隆手段,按RsaA分泌系统操纵子组织方式,将RsaA系统外运功能基因配合以异源调控序列克隆至pQE30骨架质粒。以绿色荧光蛋白（GFP）为报告分子、大肠杆菌M15为宿主茵,诱导表达后通过Western Blotting检测培养上清中GFP的表达。结果：获得了与设计完全一致的pQABPS载体,利用该载体系统,在培养上清中报告分子GFP的表达明显增加,且是通过特异的RsaA外运机制被分泌至胞外的,而非渗漏表达或简单的信号肽引导。结论：在大肠杆菌中重现了RsaA分泌系统的外运功能,为该系统在基因工程领域的应用研究打下了良好基础。     

Two outer membrane proteins are required for maximal type I secretion of the Caulobacter crescentus S-layer protein

Transport of RsaA, the crystalline S-layer subunit protein of Caulobacter crescentus, is mediated by a type I secretion mechanism. Two proteins have been identified that play the role of the outer membrane protein (OMP) component in the RsaA secretion machinery. The genes rsaF(a) and rsaF(b) were identified by similarity to the Escherichia coli hemolysin secretion OMP TolC by using the C. crescentus genome sequence. The rsaF(a) gene is located several kilobases downstream of the other transporter genes, while rsaF(b) is completely unlinked. An rsaF(a) knockout had approximately 56% secretion compared to wild-type levels, while the rsaF(b) knockout reduced secretion levels to approximately 79%. When expression of both proteins was eliminated, there was no RsaA secretion, but a residual level of approximately 9% remained inside the cell, suggesting posttranslational autoregulation. Complementation with either of the individual rsaF genes by use of a multicopy vector, which resulted in 8- to 10-fold overexpression of the proteins, did not restore RsaA secretion to wild-type levels, indicating that both rsaF genes were required for full-level secretion. However, overexpression of rsaF(a) (with normal rsaF(b) levels) in concert with overexpression of rsaA resulted in a 28% increase in RsaA secretion, indicating a potential for significantly increasing expression levels of an already highly expressing type I secretion system. This is the only known example of type I secretion requiring two OMPs to assemble a fully functional system.     

The S-layer protein of Corynebacterium glutamicum is anchored to the cell wall by its C-terminal hydrophobic domain

PS2 is the S-layer protein of Corynebacterium glutamicum . The S-layer may be detached from the cell as organized sheets by detergents at room temperature. The solubilization of PS2 in the form of monomers requires detergent treatment at high temperature (70°C), conditions under which the protein is denatured. Treatment of the cells with proteinase K or trypsin results in the detachment of the organized S-layer, which remains organized. Because we show that trypsin cleaves the C-terminal part of the protein, we conclude that this domain is involved in the association of the S-layer to the cell but is not essential in the interaction between individual PS2 proteins within the S-layer. A modified form of PS2, deleted of its C-terminal hydrophobic sequence, was constructed. The protein is almost unable to form an organized S-layer and is mainly released into the medium. We suggest that PS2 is anchored via its C-terminal hydrophobic sequence to a hydrophobic layer of the wall of the bacterium located some distance above the cytoplasmic membrane.     

Physical and functional S-layer reconstitution in Aeromonas salmonicida.

The various functions attributed to the S-layer of Aeromonas salmonicida have been previously identified by their conspicuous absence in S-layer-defective mutants. As a different approach to establish the multifunctional nature of this S-layer, we established methods for reconstitution of the S-layer of A. salmonicida. Then we investigated the functional competence of the reconstituted S-layer. S-layers were reconstituted in different systems: on inert membranes or immobilized lipopolysaccharide (LPS) from purified S-layer protein (A-protein) or on viable cells from either A-protein or preassembled S-layer sheets. In the absence of divalent cations and LPS, purified A-protein in solution spontaneously assembled into tetrameric oligomers and, upon concentration by ultrafiltration, into macroscopic, semicrystalline sheets formed by oligomers loosely organized in a tetragonal arrangement. In the presence of Ca2+, purified A-protein assembled into normal tetragonal arrays of interlocked subunits. A-protein bound with high affinity (Kd, 1.55 x 10(-7) M) and specificity to high-molecular-weight LPS from A. salmonicida but not to the LPSs of several other bacterial species. In vivo, A-protein could be reconstituted only on A. salmonicida cells which contained LPS, and Ca2+ affected both a regular tetragonal organization of the reattached A-protein and an enhanced reattachment of the A-protein to the cell surface. The reconstitution of preformed S-layer sheets (produced by an S-layer-secreting mutant) to an S-layer-negative mutant occurred consistently and efficiently when the two mutant strains were cocultured on calcium-replete solid media. Reattached A-protein (exposed on the surface of S-layer-negative mutants) was able to bind porphyrins and an S-layer-specific phage but largely lacked regular organization, as judged by its inability to bind immunoglobulins. Reattached S-layer sheets were regularly organized and imparted the properties of porphyrin binding, hydrophobicity, autoaggregation, adherence to and invasion of fish macrophages and epithelial cells, and resistance to macrophage cytotoxicity. However, cells with reconstituted S-layers were still sensitive to complement and insensitive to the antibiotics streptonigrin and chloramphenicol, indicating incomplete functional reconstitution.     

S-layer-mediated display of the immunoglobulin G-binding domain of streptococcal protein G on the surface of Caulobacter crescentus: development of an immunoactive reagent

The immunoglobulin G (IgG)-binding streptococcal protein G is often used for immunoprecipitation or immunoadsorption-based assays, as it exhibits binding to a broader spectrum of host species IgG and IgG subclasses than the alternative, Staphylococcus aureus protein A. Caulobacter crescentus produces a hexagonally arranged paracrystalline protein surface layer (S-layer) composed of a single secreted protein, RsaA, that is notably tolerant of heterologous peptide insertions while maintaining the surface-attached crystalline character. Here, a protein G IgG-binding domain, GB1, was expressed as an insertion into full-length RsaA on the cell surface to produce densely packed immunoreactive particles. GB1 insertions at five separate sites were expressed, and all bound rabbit and goat IgG, but expression levels were reduced compared to those of wild-type RsaA and poor binding to mouse IgG was noted. To remedy this, we used the 20-amino-acid Muc1 peptide derived from human mucins as a spacer, since insertions of multiple tandem repeats were well tolerated for RsaA secretion and assembly. This strategy worked remarkably well, and recombinant RsaA proteins, containing up to three GB1 domains, surrounded by Muc1 peptides, not only were secreted and assembled but did so at wild-type levels. The ability to bind IgG (including mouse IgG) increased as GB1 units were added, and those with three GB1 domains bound twice as much rabbit IgG per cell as S. aureus cells (Pansorbin). The ability of recombinant protein G-Caulobacter cells to function as immunoactive reagents was assessed in an immunoprecipitation assay using a FLAG-tagged protein and anti-FLAG mouse monoclonal antibody; their performance was comparable to that of protein G-Sepharose beads. This work demonstrates the potential for using cells expressing recombinant RsaA/GB1 in immunoassays, especially considering that protein G-Caulobacter cells are more cost-effective than protein G beads and exhibit a broader species and IgG isotype binding range than protein A.     

The Caulobacter crescentus Paracrystalline S-Layer Protein Is Secreted by an ABC Transporter (Type I) Secretion Apparatus

Caulobacter crescentus is a gram-negative bacterium that produces a two-dimensional crystalline array on its surface composed of a single 98-kDa protein, RsaA. Secretion of RsaA to the cell surface relies on an uncleaved C-terminal secretion signal. In this report, we identify two genes encoding components of the RsaA secretion apparatus. These components are part of a type I secretion system involving an ABC transporter protein. These genes, lying immediately 3′ of rsaA, were found by screening a Tn5 transposon library for the loss of RsaA transport and characterizing the transposon-interrupted genes. The two proteins presumably encoded by these genes were found to have significant sequence similarity to ABC transporter and membrane fusion proteins of other type I secretion systems. The greatest sequence similarity was found to the alkaline protease (AprA) transport system of Pseudomonas aeruginosa and the metalloprotease (PrtB) transport system of Erwinia chrysanthemi. The prtB and aprA genes were introduced into C. crescentus, and their products were secreted by the RsaA transport system. Further, defects in the S-layer protein transport system led to the loss of this heterologous secretion. This is the first report of an S-layer protein secreted by a type I secretion apparatus. Unlike other type I secretion systems, the RsaA transport system secretes large amounts of its substrate protein (it is estimated that RsaA accounts for 10 to 12% of the total cell protein). Such levels are expected for bacterial S-layer proteins but are higher than for any other known type I secretion system.     

A transducing bacteriophage for Caulobacter crescentus uses the paracrystalline surface layer protein as a receptor.

The bacteriophage phi Cr30, a transducing phage for Caulobacter crescentus strains, required the paracrystalline surface (S) layer for infectivity. Wild-type strains were phage resistant when rsaA, the gene for the 130K S-layer protein, was interrupted with an antibiotic resistance cassette. Strains that had lost the S layer by mutation were phage resistant, as were mutants that produce an S layer but which do not attach the structure to the cell surface. Phage sensitivity was restored to 130K-protein-deficient strains by introducing rsaA on a plasmid. Spontaneous phage-resistant strains produced expected phenotypes as follows (in order of decreasing frequency): S-layer cell attachment defects, no S layer, or an S layer that was wild type in appearance.     

Variation in the surface layer proteins of Clostridium difficile

Surface layers (S-layers) form regular crystalline structures on the outermost surface of many bacteria. Clostridium difficile possesses such an S-layer consisting of two protein subunits. Treatment of whole cells of C. difficile with 5 M guanidine hydrochloride revealed two major proteins of different molecular masses characteristic of the S-layer on SDS-PAGE. In this study 25 isolates were investigated. A high degree of variability in the molecular mass of the two S-layer proteins was evident. Molecular masses ranged from 48 to 56 kDa for the heavier protein and from 37 to 45 kDa for the lighter protein. A further protein component of 70 kDa was detectable in all isolates. No cross-reaction was seen between the two major proteins from isolates that produced different S-layer patterns, and most S-layer proteins from isolates with the same or similar banding patterns did not cross-react. The S-layer proteins, when detected by a combination of Coomassie blue staining and immunoblotting, are a useful marker for phenotyping.     

Evidence for an S-layer protein pool in the peptidoglycan of Bacillus stearothermophilus.

Intact cells of Bacillus stearothermophilus PV72 revealed, after conventional thin-sectioning procedures, the typical cell wall profile of S-layer-carrying gram-positive eubacteria consisting of a ca. 10-nm-thick peptidoglycan-containing layer and a ca. 10-nm-thick S layer. Cell wall preparations obtained by breaking the cells and removing the cytoplasmic membrane by treatment with Triton X-100 revealed a triple-layer structure, with an additional S layer on the inner surface of the peptidoglycan. This profile is characteristic for cell wall preparations of many S-layer-carrying gram-positive eubacteria. Among several variants of strain PV72 obtained upon single colony isolation, we investigated the variant PV72 86-I, which does not exhibit an inner S layer on isolated cell walls but instead possesses a profile identical to that observed for intact cells. In the course of a controlled mild autolysis of isolated cell walls, S-layer subunits were released from the peptidoglycan of the variant and assembled into an additional S layer on the inner surface of the walls, leading to a three-layer cell wall profile as observed for cell wall preparations of the parent strain. In comparison to conventionally processed bacteria, freeze-substituted cells of strain PV72 and the variant strain revealed in thin sections a ca. 18-nm-wide electron-dense peptidoglycan-containing layer closely associated with the S layer. The demonstration of a pool of S-layer subunits in such a thin peptidoglycan layer in an amount at least sufficient for generating one coherent lattice on the cell surface indicated that the subunits must have occupied much of the free space in the wall fabric of both the parent strain and the variant. It can even be speculated that the rate of synthesis and translation of the S-layer protein is influenced by the packing density of the S-layer subunits in the periplasm of the cell wall delineated by the outer S layer and the cytoplasmic membrane. Our data indicate that the matrix of the rigid wall layer inhibits the assembly of the S-layer subunits which are in transit to the outside.