Immunocytochemical localization of the elongation factor Tu in E. coli cells

Surface proteome of "Mycobacterium avium subsp. hominissuis" during the early stages of macrophage infection

"Mycobacterium avium subsp. hominissuis" is a robust and pervasive environmental bacterium that can cause opportunistic infections in humans. The bacterium overcomes the host immune response and is capable of surviving and replicating within host macrophages. Little is known about the bacterial mechanisms that facilitate these processes, but it can be expected that surface-exposed proteins play an important role. In this study, the selective biotinylation of surface-exposed proteins, streptavidin affinity purification, and shotgun mass spectrometry were used to characterize the surface-exposed proteome of M. avium subsp. hominissuis. This analysis detected more than 100 proteins exposed at the bacterial surface of M. avium subsp. hominissuis. Comparisons of surface-exposed proteins between conditions simulating early infection identified several groups of proteins whose presence on the bacterial surface was either constitutive or appeared to be unique to specific culture conditions. This proteomic profile facilitates an improved understanding of M. avium subsp. hominissuis and how it establishes infection. Additionally, surface-exposed proteins are excellent targets for the host adaptive immune system, and their identification can inform the development of novel treatments, diagnostic tools, and vaccines for mycobacterial disease.

A novel receptor - ligand pathway for entry of Francisella tularensis in monocyte-like THP-1 cells: interaction between surface nucleolin and bacterial elongation factor Tu

BACKGROUND

Francisella tularensis, the causative agent of tularemia, is one of the most infectious human bacterial pathogens. It is phagocytosed by immune cells, such as monocytes and macrophages. The precise mechanisms that initiate bacterial uptake have not yet been elucidated. Participation of C3, CR3, class A scavenger receptors and mannose receptor in bacterial uptake have been already reported. However, contribution of an additional, as-yet-unidentified receptor for F. tularensis internalization has been suggested.

RESULTS

We show here that cell-surface expressed nucleolin is a receptor for Francisella tularensis Live Vaccine Strain (LVS) and promotes LVS binding and infection of human monocyte-like THP-1 cells. The HB-19 pseudopeptide that binds specifically carboxy-terminal RGG domain of nucleolin inhibits LVS binding and infection of monocyte-like THP-1 cells. In a pull-down assay, elongation factor Tu (EF-Tu), a GTP-binding protein involved in protein translation, usually found in cytoplasm, was recovered among LVS bacterial membrane proteins bound on RGG domain of nucleolin. A specific polyclonal murine antibody was raised against recombinant LVS EF-Tu. By fluorescence and electron microscopy experiments, we found that a fraction of EF-Tu could be detected at the bacterial surface. Anti-EF-Tu antibodies reduced LVS binding to monocyte-like THP-1 cells and impaired infection, even in absence of complement and complement receptors. Interaction between EF-Tu and nucleolin was illustrated by two different pull-down assays using recombinant EF-Tu proteins and either RGG domain of nucleolin or cell solubilized nucleolin.

DISCUSSION

Altogether, our results demonstrate that the interaction between surface nucleolin and its bacterial ligand EF-Tu plays an important role in Francisella tularensis adhesion and entry process and may therefore facilitate invasion of host tissues. Since phagosomal escape and intra-cytosolic multiplication of LVS in infected monocytes are very similar to those of human pathogenic F. tularensis ssp tularensis, the mechanism of entry into monocyte-like THP-1 cells, involving interaction between EF-Tu and nucleolin, might be similar in the two subspecies. Thus, the use of either nucleolin-specific pseudopeptide HB-19 or recombinant EF-Tu could provide attractive therapeutic approaches for modulating F. tularensis infection.

A surface-focused biotinylation procedure identifies the Yersinia pestis catalase KatY as a membrane-associated but non-surface-located protein

This study identified major surface proteins of the plague bacterium Yersinia pestis. We applied a novel surface biotinylation method, followed by NeutrAvidin (NA) bead capture, on-bead digestion, and identification by liquid chromatography-tandem mass spectrometry (LC-MS-MS). The use of stachyose during biotinylation focused the reaction to the surface. Coupled with NA pulldown and immunoblot analysis, this method determined whether a protein was accessible to the surface. We applied the method to test the hypothesis that the catalase KatY is a surface protein of the plague bacterium Y. pestis. A rabbit serum recognized the catalase KatY as a major putative outer membrane-associated antigen expressed by Y. pestis cells grown at 37 degrees C. Similar findings by other groups had led to speculations that this protein might be exposed to the surface and might be a candidate for evaluation as a protective antigen for an improved plague vaccine. KatY was obtained only in the total membrane fraction, and stachyose greatly reduced its biotinylation as well as that of the periplasmic maltose binding protein, indicating that KatY is not on the bacterial surface. LC-MS-MS analysis of on-bead digests representing ca. 10(9) cells identified highly abundant species, including KatY, Pal, and OmpA, as well as the lipoprotein Pcp, all of which bound in a biotin-specific manner. Pla, Lpp, and OmpX (Ail) bound to the NA beads in a non-biotin-specific manner. There was no contamination from abundant cytoplasmic proteins. We hypothesize that OmpX and Pcp are highly abundant and likely to be important for the Y. pestis pathogenic process. We speculate that a portion of KatY associates with the outer membrane in intact cells but that it is located on the periplasmic side. Consistent with this idea, it did not protect C57BL/6 mice against bubonic plague.

The prokaryotic cytoskeleton: a putative target for inhibitors and antibiotics?

In the recent decade, our view on the organization of the bacterial cell has been revolutionized by the identification of cytoskeletal elements. Most bacterial species have structural homologs of actin and tubulin that assemble into dynamic, filamentous structures at precisely defined sub-cellular locations. The essential cell division protein FtsZ forms a dynamic ring at mid-cell and is similar in its structure to tubulin. Proteins of the MreB family, which are structural homologs of actin, assemble into helical or straight filaments in the bacterial cytoplasm. As in eukaryotic cells, the bacterial cytoskeleton drives essential cellular processes such as cell division, cell wall growth, DNA movement, protein targeting, and alignment of organelles. Different high-throughput assays have been developed to search for inhibitors of components of the bacterial cytoskeleton. Cell-based assays for the detection of cell division inhibitors as well as FtsZ GTPase assays led to the identification of several compounds that inhibit the polymerization of FtsZ, by this blocking bacterial cell division. Such inhibitors might not only be valuable tools for basic research, but might also lead to novel therapeutic agents against pathogenic bacteria. For example, the polyphenol dichamanetin, the 2-alkoxycarbonylaminopyridine SRI-3072, and the benzophenanthridine alkaloid sanguinarine inhibit the GTPase activity of FtsZ and exhibit antimicrobial activity.

Dynamics and morphology of the in vitro polymeric form of elongation factor Tu from Escherichia coli

Elongation factor Tu from Escherichia coli is known to polymerize at slightly acidic pH and low ionic strength. The structure and dynamics of these aggregates have been examined using imaging and spectroscopic methodologies. Electron microscopy provides evidence for two-dimensional sheets and bundled filaments of EF-Tu, whereas fluorescence microscopy of EF-Tu covalently labeled with tetramethylrhodamine isothiocyanate showed highly branched polymers of EF-Tu several microns in diameter. These polymers were studied using quasi-elastic light scattering to determine the evolution of the translational diffusion coefficient during the polymerization process. The rotational dynamics of the aggregate were investigated using phosphorescence anisotropy of EF-Tu covalently labeled with erythrosin isothiocyanate. A high infinite-time anisotropy was observed, suggesting a lack of motion or entanglement of EF-Tu polymers. A sub-microsecond motion which was slowed in the presence of glycerol may be due to local flexibility of the polymers. The possible relevance of polymeric EF-Tu to its function in vivo is discussed.

Production of enzymatically active rat protein disulfide isomerase in Escherichia coli

We report the development of a bacterial expression system allowing high-level synthesis of enzymatically active rat protein disulfide isomerase (rPDI). After expression of the rpdi gene under control of the inducible trc promoter (Ptrc), a significant amount of soluble, active rPDI was detected in the periplasmic contents, which were released from the cells by cold osmotic shock. However, the exported molecules were incompletely or improperly processed, while the major amount of synthesized rPDI was in fact detected in the soluble cellular fraction. Substitution of the autologous eukaryotic export signal with the nucleotide (nt) sequence encoding the signal peptide (sOmpA) of the bacterial outer membrane protein A, and expression of the sompA::rpdi fusion gene under control of both the lpp promoter (Plpp) and the lac promoter-operator (POlac), resulted in high-level production of rPDI. Furthermore, the latter was efficiently exported into the periplasmic compartment, from where it was recovered as a soluble, fully active form with the sOmpA precisely removed. The synthesis of a small 21-kDa peptide accompanying the production of rPDI was also observed. This rPDI-related peptide (rPDIf), which represented a C-terminal fragment of rPDI including the second active site, arose by internal translation initiation within rpdi. Replacement of the presumed internal start codon by CTC completely eliminated the aforementioned phenomenon and resulted in the production of a slightly mutated, enzymatically active enzyme (rPDIm).

Heterologous protein export in Escherichia coli: influence of bacterial signal peptides on the export of human interleukin 1 beta

Expression plasmids carrying the coding sequence of mature human interleukin 1 beta (IL 1 beta) linked either to a Met start codon, or fused to different efficient Escherichia coli secretion signal sequences, have been constructed. In the latter case, we used signal peptides derived either from an outer membrane protein (OmpA) or from a periplasmic protein (PhoA). The synthesis of IL1 beta from these fusions was investigated in an otherwise strictly isogenic context using identical conditions of derepression and culture media. The Met-IL1 beta fusion produced a soluble cytoplasmic protein which could be released from the cells by osmotic shock whereas the OmpA and PhoA fusions were always insoluble. The extent of sOmpA-IL1 beta maturation was found to vary from 50 to 100%, mainly depending on the medium used, whereas no significant maturation of the signal peptide could be detected in the case of the sPhoA-IL1 beta fusion. Immuno-electron microscopy revealed that the sOmpA-IL1 beta fusion was targeted to the inner membrane, whereas the sPhoA-IL1 beta fusion remained within the cytoplasm and thus did not appear to enter the secretion pathway. Amplifying the E. coli signal peptidase lep gene on a multicopy plasmid did not improve signal peptide removal from sOmpA-IL1 beta. Moreover, these E. coli secretion vectors allowed us to produce, in high levels, IL1 beta fragments which otherwise could not be stably accumulated within the cytoplasmic compartment.

Control of the tRNA-tufB operon in Escherichia coli. 3. Feedback inhibition of tufB expression by an EF-Tu with a deletion in the guanine-nucleotide-binding domain

The expression of tufB, one of the two EF-Tu-encoding genes in Escherichia coli, is under autogenous control. Feedback inhibition of tufB expression by plasmid-borne EF-Tu has been used to answer the question of whether or not the integrity of the guanine-nucleotide-binding domain of EF-Tu is required for the autoregulatory role of the factor protein. We show that a large deletion of tufB, causing the elimination of an 81-amino-acid segment from the plasmid-borne EF-Tu, does not abolish tufB repression. We conclude that the autoregulation of the cellular EF-Tu level is not dependent on an intact guanine-nucleotide-binding domain and does not require binding of GTP to EF-Tu. The repressor activity of the deletion derivative of EF-Tu can be measured despite a rapid disappearance of the (altered) mutant protein from the soluble cytoplasmic fraction of the cell. Degradation and assembly in larger complexes are responsible for this disappearance.

Development of structural organization of protein-synthesizing machinery from prokaryotes to eukaryotes

Though the mechanisms of protein biosynthesis are similar in the cells of prokaryotes and eukaryotes, the eukaryotic translational machinery in the cell is arranged more intricately. One of the most striking characteristic features of the eukaryotic translational machinery is that the eukaryotic proteins involved in the translational process, such as initiation factors, elongation factors and aminoacyl-tRNA synthetases, in contrast to their prokaryotic analogs, possess a non-specific affinity for RNA. Due to the RNA-binding ability, these eukaryotic proteins can be compartmentalized on polyribosomes. In addition to the proteins of the translational apparatus, several other eukaryotic RNA-binding proteins can be also compartmentalized on polyribosomes; these proteins include glycolytic enzymes, steroid hormone receptors and intermediate filament proteins. Thus, the eukaryotic polyribosome is an element of the cytoplasmic labile structure on which various proteins can be compartmentalized and, consequently, different biochemical pathways can be integrated.

Overproduction of the proFhuA outer membrane receptor protein of Escherichia coli K-12: isolation, properties, and immunocytochemical localization at the inner side of the cytoplasmic membrane

The fhu operon of Escherichia coli K-12 comprises four genes, termed fhuA,C,D,B, which are involved in the uptake of iron-hydroxamate compounds. The fhuA gene encodes the outer membrane receptor protein. Cells that contained three copies of the fhuACD fragment on the thermoamplifiable plasmid pHK232 accumulated at 37 degrees C large amounts of the proFhuA protein. Most of the overproduced proFhuA protein was not translocated into the outer membrane but instead precipitated at the cytoplasmic side of the inner membrane, presumably at the sites of synthesis. Despite inhibition of export proFhuA synthesis continued. The precipitate formed was sedimented by centrifugation at 8,000 x g. The proFhuA protein could be solubilized in 1% sodium dodecyl sulfate. Replacement of sodium dodecyl sulfate by Triton X-100 resulted in a proFhuA protein which exhibited 10% of the phage T5 binding activity of renatured mature FhuA protein. Binding of the phage T5 was inhibited by the FhuA-specific ligands ferrichrome, albomycin and colicin M. Limited proteolysis of the isolated pro- and mature form of the FhuA protein with trypsin yielded similar oligopeptide patterns. Addition of ferrichrome affected trypsin cleavage of both proteins in the same way. The common proteolytic intermediates together with phage inactivation indicate a similar conformation of the pro- and mature form.

Negative staining and immunoelectron microscopy of adhesion-deficient mutants of Streptococcus salivarius reveal that the adhesive protein antigens are separate classes of cell surface fibril

The subcellular distribution of the cell wall-associated protein antigens of Streptococcus salivarius HB, which are involved in specific adhesive properties of the cells, was studied. Mutants which had lost the adhesive properties and lacked the antigens at the cell surface were compared with the parent strain. Immunoelectron microscopy of cryosections of cells labeled with affinity-purified, specific antisera and colloidal gold-protein A complexes was used to locate the antigens. Antigen C (AgC), a glycoprotein involved in attachment to host surfaces, was mainly located in the fibrillar layer outside the cell wall. A smaller amount of label was also found throughout the cytoplasmic area in the form of small clusters of gold particles, which suggests a macromolecular association. Mutant HB-7, which lacks the wall-associated AgC, accumulated AgC reactivity intracellularly. Intracellular AgC was often found associated with isolated areas of increased electron density, but sometimes seemed to fill the entire interior of the cell. Antigen B (AgB), a protein responsible for interbacterial coaggregation, was also located in the fibrillar layer, although its distribution differed from that of the wall-associated AgC since AgB was found predominantly in the peripheral areas. A very small amount of label was also found in the cytoplasmic area as discrete gold particles. Mutant HB-V5, which lacks wall-associated AgB, was not labeled in the fibrillar coat, but showed the same weak intracellular label as the parent strain. Immunolabeling with serum against AgD, another wall-associated protein but of unknown function, demonstrated its presence in the fibrillar layer of strain HB. Negatively stained preparations of whole cells of wild-type S. salivarius and mutants that had lost wall-associated AgB or AgC revealed that two classes of short fibrils are carried on the cell surface at the same time. AgB and AgC are probably located on separate classes of short, protease-sensitive fibrils 91 and 72 nm in length, respectively. A third class of only very sparsely distributed short fibrils (63 nm) was observed on mutant HB-V51, which lacks both wall-associated AgB and AgC antigens. The identity of these fibrils and whether they are present on the wild type are not clear. The function of long, protease-resistant fibrils of 178 nm, which are also present on the wild-type strain, remains unknown.

Precursor for elongation factor Tu from Escherichia coli

The tufA gene, one of two genes in Escherichia coli encoding elongation factor Tu (EF-Tu), was cloned into a ColE1-derived plasmid downstream of the lac promoter-operator. In cells carrying this plasmid, the synthesis of EF-Tu was increased four- to fivefold upon the addition of isopropyl-beta-D-thiogalactopyranoside (an inducer of the lac promoter). This condition led to the synthesis of a novel protein, called pTu, which comigrated with EF-Tu on a sodium dodecyl sulfate-polyacrylamide gel but could be separated on an isoelectric focusing gel, since pTu is slightly more basic than EF-Tu. The synthesis of pTu could also be induced by the synthesis of a hybrid protein containing just the amino-terminal half of the EF-Tu protein. Genetic data suggest that pTu is the product of the tufA and tufB genes. The pTu protein was shown to be related to EF-Tu by gel electrophoresis of tryptic peptides. Pulse-chase experiments suggest that pTu is a precursor of EF-Tu. Interestingly, in a classic membrane fractionation procedure, EF-Tu was found in the cytosolic fraction, whereas pTu was partitioned with the outer membrane.