Cell wall-associated protein A as a tool for immunolocalization of Staphylococcus aureus in infected human airway epithelium.

Staphylococcus aureus is a common human pathogen involved in non-bronchial diseases and in genetic and acquired bronchial diseases. In this study, we applied an immunolabeling approach for in vivo and in vitro detection of S. aureus, based on the affinity of staphylococcal protein A (SpA) for the Fc region of immunoglobulins, especially IgG. Most strains of S. aureus, including clinical strains, can be detected with this labeling technique. The approach can be used for detection and localization with transmission electron microscopy or light-fluorescence microscopy of S. aureus in infected tissues such as human bronchial tissue from cystic fibrosis (CF) patients. The methodology can also be applied to cell culture models with the aim of characterizing bacterial adherence to epithelial cells in backscattered electron imaging with scanning electron microscopy. Application to the study of S. aureus adherence to airway epithelium showed that the bacteria did not adhere in vivo to intact airway epithelium. In contrast, bacteria adhered to the basolateral plasma membrane of columnar cells, to basal cells, to the basement membrane and were identified beneath the lamina propria when the epithelium was injured and remodeled, or in vitro when the epithelial cells were dedifferentiated.     

A protein component of the cell surface of Staphylococcus aureus

1. Treatment with trypsin of cells of Staphylococcus aureus strain Cowan I, known to carry a protein component that includes the Jensen protein A, results in an increase in the negative mobility of the cells at pH values greater than 6. 2. Similar treatment of cells of strain Wood 46, which has no surface protein component, causes no change in the electrophoretic mobility. 3. Electrokinetically heterogeneous populations are observed in two strains of S. aureus, one of human and the other of animal origin. Evidence is presented ascribing this to the presence of varying amounts of protein components on the surface of different cells.     

The elastin-binding protein of Staphylococcus aureus (EbpS) is expressed at the cell surface as an integral membrane protein and not as a cell wall-associated protein.

The elastin-binding proteins EbpS of Staphylococcus aureus strains Cowan and 8325-4 were predicted from sequence analysis to comprise 486 residues. Specific antibodies were raised against an N-terminal domain (residues 1-267) and a C-terminal domain (residues 343-486) expressed as recombinant proteins in Escherichia coli. Western blotting of lysates of wild-type 8325-4 and Newman and the corresponding ebpS mutants showed that EbpS migrated with an apparent molecular mass of 83 kDa. The protein was found exclusively in cytoplasmic membrane fractions purified from protoplasts or lysed cells, in contrast to the clumping factor ClfA, which was cell-wall-associated. EbpS was predicted to have three hydrophobic domains H1-(205-224), H2-(265-280), and H3-(315-342). A series of hybrid proteins was formed between EbpS at the N terminus and either alkaline phosphatase or beta-galactosidase at the C terminus (EbpS-PhoA, EbpS-LacZ). PhoA and LacZ were fused to EbpS between hydrophobic domains H1-H2 and H2-H3, and distal to H3. Expression of enzymatic activity in E. coli showed that EbpS is an integral membrane protein with two membrane-spanning domains H1 and H3. N-terminal residues 1-205 and C-terminal residues 343-486 were predicted to be exposed on the outer face of the cytoplasmic membrane. The ligand-binding domain of EbpS is known from previous studies to be present in the N terminus between residues 14-34 and probing whole cells with anti-EbpS1-267 antibodies indicated that this region is exposed on the surface of intact cells. This was also confirmed by the observation that wild-type S. aureus Newman cells bound labeled tropoelastin whereas the ebpS mutant bound 72% less. In contrast, the C terminus, which carries a putative LysM peptidoglycan-binding domain, is not exposed on the surface of intact cells and presumably remains buried within the peptidoglycan. Finally, expression of EbpS was correlated with the ability of cells to grow to a higher density in liquid culture, suggesting that EbpS may have a role in regulating cell growth.     

Inhibition of breast cancer cell adhesion and bone metastasis by the extracellular adherence protein of Staphylococcus aureus

Bone metastasis is a common sequelae of breast cancer and the interaction of alpha v beta3-integrin with osteopontin (OPN) found in the extracellular matrix of mineralized tissues is implicated in this process. The integrin-dependent proadhesive and promigratory functions of OPN are particularly attributed to the 40 kD N-terminal fragment that derives upon matrix metalloproteinase (MMP) cleavage. Based on the broad repertoire of interactions between Staphylococcus aureus extracellular adherence protein (Eap) and host components, we here characterized Eap to specifically interact with recombinant full-length OPN and the 40 kD N-terminal MMP cleavage fragment, but not with the 32 kD or the 25 kD C-terminal fragments of OPN. Eap thereby prevented the OPN/alpha v beta3-integrin interaction, as well as the alpha v beta3-integrin-dependent adhesion of MDA-MB-231 breast cancer cells to full-length OPN or to the 40 kD fragment and the migration of these cells towards OPN. Furthermore, Eap treatment markedly impaired the development of osseous metastasis of human MDA-MB-231 cells in vivo. Taken together, Eap may represent an attractive novel treatment for the prevention of breast cancer bone metastasis.     

Isolation of the fifth IgG-binding domain from Staphylococcus aureus protein A

Using affinity chromatography on IgG-Sepharose at pH 5.0, a new fragment capable of binding to IgG (domain E) was isolated from trypsin hydrolysate of protein A. Trypsinolysis of protein A was performed at low temperatures. Thus, the intact structure of protein A was found to include six domains, of which five interact with IgG.     

Extracellular protein A from a methicillin-resistant strain of Staphylococcus aureus

Protein A has been isolated from a methicillin-resistant strain of Staphylococcus aureus, A676, which produces only extracellular protein A. The material is homogeneous after a one-step separation and has a molecular weight of 41000. Its physicochemical and immunochemical properties have been studied.     

Characterization of the h1 protein antigen from Staphylococcus aureus 17A

The h1 protein antigen from Staphylococcus aureus 17 A was found to be present associated with the cell wall as well as freely in the growth medium. The two forms did not differ in size or immunological properties. Only lytic or proteolytic enzymes could release h1 from the isolated cell wall. The molecule was readily broken down by trypsin to discrete fragments which all had common antigenic determinants. The results suggest that the antigen is covalently linked in the cell wall and that its structure might be similar to that of protein A.     

Fibronectin binding protein A of Staphylococcus aureus can mediate human T lymphocyte adhesion and coactivation

The extracellular matrix protein fibronectin (FN) mediates the adhesion of bacteria as well as T lymphocytes. Mammalian cells express integrins alpha(4)beta(1) and alpha(5)beta(1) as the major FN-binding cell surface receptors. Bacteria such as Staphylococcus aureus, also express FN-binding receptors that are important for adherence to host tissue and initiation of infection. The S. aureus FN-binding protein, FnbpA, has been previously identified, and recombinant proteins that correspond to distinct functional regions of this protein have been made. Three recombinant truncated forms of FnbpA, rFnbpA(37-881), rFnbpA(37-605), and rFnbpA(620-881), were examined for effects on in vitro adhesion and coactivation of human T lymphocytes. These proteins, when coimmobilized with anti-CD3 mAb, activated T lymphocyte proliferation. The coactivation signal generated by the rFnbpA proteins required medium containing serum with FN. Furthermore, the costimulatory signal could be restored in FN-depleted serum when the rFnbpAs were preloaded with soluble FN. Monoclonal Ab blocking studies revealed that integrin alpha(5)beta(1) is the major receptor responsible for the rFnbpA costimulatory signal. Shear flow cell detachment assays confirmed that lymphocytes can bind to FN captured by the rFnbpA proteins. These results suggest that the S. aureus rFnbpA can interact with integrin alpha(5)beta(1) via an FN bridge to mediate adhesion and costimulatory signals to T lymphocytes.     

Inhibition of antibody-dependent cellular cytotoxicity by protein A from Staphylococcus aureus.

Protein A, a cell wall constituent of several strains of Staphylococcus aureus, binds strongly to the Fc portion of immunoglobulins. This investigation demonstrated that such binding can inhibit antibody-dependent cellular cytotoxicity (ADCC). The degree to which ADCC was inhibited depended upon the relative concentrations of protein A and anti-target cell antiserum. Protein A also inhibited the formation of rosettes between antibody-coated sheep red blood cells and lymphoid cells with Fc receptors. We, therefore, conclude that protein A inhibits ADCC by preventing the binding of antibody-coated target cells to Fc receptors on cytotoxic effector cells.     

Distribution of protein A on the surface of Staphylococcus aureus

Surface proteins of Staphylococcus aureus fulfill many important roles during the pathogenesis of human infections and are anchored to the cell wall envelope by sortases. Although the chemical linkage of proteins to cell wall cross bridges is known, the mechanisms whereby polypeptides are distributed on the staphylococcal surface have not been revealed. We show here that protein A, the ligand of immunoglobulin, is unevenly distributed over the staphylococcal surface. Upon removal with trypsin, newly synthesized polypeptide is deposited at two to four discrete foci. During subsequent growth, protein A appears to be slowly distributed from these sites. When viewed through multiple focal planes by laser scanning microscopy, protein A foci are arranged in a circle surrounding the bacterial cell. This pattern of distribution requires the LPXTG sorting signal of protein A as well as sortase A, the transpeptidase that anchors polypeptides to cell wall cross bridges. A model is presented whereby protein A deposition at discrete sites coupled with cell wall synthesis enables distribution of protein A on the staphylococcal surface.     

Selective inhibition of biotin protein ligase from Staphylococcus aureus

There is a well documented need to replenish the antibiotic pipeline with new agents to combat the rise of drug resistant bacteria. One strategy to combat resistance is to discover new chemical classes immune to current resistance mechanisms that inhibit essential metabolic enzymes. Many of the obvious drug targets that have no homologous isozyme in the human host have now been investigated. Bacterial drug targets that have a closely related human homologue represent a new frontier in antibiotic discovery. However, to avoid potential toxicity to the host, these inhibitors must have very high selectivity for the bacterial enzyme over the human homolog. We have demonstrated that the essential enzyme biotin protein ligase (BPL) from the clinically important pathogen Staphylococcus aureus could be selectively inhibited. Linking biotin to adenosine via a 1,2,3 triazole yielded the first BPL inhibitor selective for S. aureus BPL over the human equivalent. The synthesis of new biotin 1,2,3-triazole analogues using click chemistry yielded our most potent structure (K(i) 90 nM) with a >1100-fold selectivity for the S. aureus BPL over the human homologue. X-ray crystallography confirmed the mechanism of inhibitor binding. Importantly, the inhibitor showed cytotoxicity against S. aureus but not cultured mammalian cells. The biotin 1,2,3-triazole provides a novel pharmacophore for future medicinal chemistry programs to develop this new antibiotic class.     

Inhibition of sortase-mediated Staphylococcus aureus adhesion to fibronectin via fibronectin-binding protein by sortase inhibitors

The sortase enzymes are a family of Gram-positive transpeptidases responsible for anchoring surface protein virulence factors to the peptidoglycan cell wall layer. In Staphylococcus aureus, deletion of the sortase isoforms results in marked reduction in virulence and infection potential, making it an important antivirulence target. Recombinant sortase A (SrtA) and sortase B (SrtB) were incubated with peptide substrate containing either the LPETG or NPQTN motifs. (Z)-3-(2,5-dimethoxyphenyl)-2-(4-methoxyphenyl) acrylonitrile, β-sitosterol-3-O-glucopyranoside, berberine chloride, and psammaplin A1 showed potent inhibitory activity against SrtA and SrtB. These compounds also exhibited potent inhibitory activity against S. aureus cell adhesion to fibronectin. The fibronectin-binding activity data highlight the potential of these compounds for the treatment of S. aureus infections via inhibition of sortase activity.     

Staphylococcus aureus isolated from Kawasaki disease patients hyper-releases extracellular protein A.

S. aureus isolates from patients with Kawasaki disease (KD) release high levels of extracellular protein A (SpA), as compared to S. aureus in other diseases. The molecular weight of this released protein A is about 70 kDa. Extracellular KD SpA purified by affinity chromatography possessed the same amino acid sequence at the NH2-terminal IgG binding region and the same antigenic specificity as recombinant and cell-wall-bound SpA preparations. The size of DNA fragments containing the spa gene from S. aureus KD strains was 160-165 kb. All of these DNA fragments contained the igb portion encoding the IgG-binding region of KD SpA. Significantly higher molecular size of the SpA molecules hyper-released in the stationary-phase culture and the lack of production of other exo-proteins allow us to speculate that S. aureus isolated from patients with KD have mutations occurring in the agr locus.     

Proteomic profiling of cell envelope-associated proteins from Staphylococcus aureus

The emergence of highly virulent community acquired Staphylococcus aureus and continued progression of resistance to multiple antimicrobials, including methicillin and vancomycin, marks the reemergence of S. aureus as a serious health care threat. Investigation of proteins localized to the cell surface could help to elucidate mechanisms of virulence and antibiotic resistance in S. aureus. In this study, proteomic profiling methods were developed to solubilize, display, and evaluate abundance levels of proteins present in the supernatants of the lysostaphin-digested cell envelope from cultured vancomycin-intermediate S. aureus (VISA) cells. Combining approaches of 2-DE or chromatographic separation of proteins with MS analyses resulted in the identification of 144 proteins of particular interest. Of these proteins, 48 contained predicted cell wall localization or export signal motifs, including 14 with distinct covalent peptidoglycan-anchor sites, four of which are uncharacterized to date. One of the two most abundant cell envelope proteins, which showed remarkably high variations in MW and pI in the 2-DE gel display, was the S. aureus surface protein G. The display of numerous secreted proteins that are not covalently cell wall-anchored, suggests that, in the exponential growth phase, secreted proteins can be retained physiologically in the cell envelope and may interact with cell wall-anchored proteins and carbohydrate structures in a manner yet to be determined. The remaining 96 proteins, devoid of recognizable motifs, were repeatedly profiled in the VISA cell envelope fractions. We describe a novel semiquantitative method to determine abundance factors of such proteins in 2-DE gels of cell envelope fractions relative to whole cell lysates and discuss these data in the context of true cell envelope localization versus experimentally caused cell lysis.