Biomedical subjects

U Sjöbring

Publications and source records attributed to U Sjöbring.

40 records · Page 3Linked to original sources

PubMed1989

Protein G genes: structure and distribution of IgG-binding and albumin-binding domains.

Protein G (also designated Fc receptor type III) is the IgG-binding protein of group C and G streptococci. Protein G has also been shown to bind human serum albumin but at a site that is structurally separated from the IgG-binding region. From the known gene sequence of protein G, two synthetic oligonucleotides were constructed for use as probes in DNA-hybridization experiments to study the structure and distribution of the albumin- and IgG-binding regions in bacterial strains belonging to different species. Thus, one of the probes corresponded to repeats within the IgG-binding region (I) and the other corresponded to repeats in the albumin-binding encoding region (II). Probe I showed strong hybridization to DNA isolated from 31 human group C and G strains, whereas hybridization to probe II was variable. With the three restriction endonucleases used, three restriction patterns were found in Southern blot experiments. No fundamental difference could be detected in hybridization experiments, either between strains of group C and G streptococci, or between isolates of different clinical origin. No hybridization to DNA from other bacterial species was found.

Antigens, Bacterial↗

PubMed1988

Isolation and characterization of a 14-kDa albumin-binding fragment of streptococcal protein G.

Protein G, a streptococcal cell wall protein, has separate binding sites for human albumin and IgG. Streptococci expressing protein G were treated with the bacteriolytic agent mutanolysin. Several IgG- and human serum albumin (HSA)-binding peptides were identified in the material thus solubilized and one of these, a 14-kDa peptide, was found to bind HSA but not IgG in Western blot experiments. This molecule was purified by affinity chromatography on Sepharose coupled with HSA followed by gel filtration on Sepharose 6B and a final affinity chromatography on IgG-Sepharose, by which low Mr W(15 to 20 kDa)IgG-binding peptides were removed. In different binding experiments the purified 14-kDa peptide bound exclusively HSA and the equilibrium constant between the peptide and HSA was determined to be 3.4 X 10(8) M-1. The relation between the 14-kDa molecule and protein G was studied by analyzing the N-terminal amino acid sequence of the peptide and comparing it with the previously determined protein G sequence. The 40 N-terminal amino acids were found to be identical with an amino acid sequence starting at position 62 in the protein G molecule. These and previous data enabled us to locate the albumin binding to the repetitively arranged domains in the N-terminal half of the protein G molecule.

Bacterial Proteins↗

PubMed1988

Streptococcal protein G has affinity for both Fab- and Fc-fragments of human IgG.

We have analyzed the binding of IgG and fragments of IgG [Fc, F(ab')2, and Fab] to group C and G streptococci and to protein G, the IgG binding cell wall protein of these bacteria. A direct correlation (r = 0.87, P less than 0.0001) was observed when the binding of radiolabelled, polyclonal IgG F(ab')2- and Fc-fragments to 23 group C and G streptococcal strains was compared. One strain (G-148) was treated with increasing amounts of pepsin, trypsin or papain and the Fab-binding structure was found to be much more sensitive to the enzymes as compared to the Fc-binding. A 35 K fragment of protein G was coupled to Sepharose, and both radiolabelled IgG F(ab')2- and Fc-fragments bound to the Sepharose beads. Binding of IgG fragments was inhibited by intact IgG or by the homologous IgG fragment, whereas Fc-fragments did not inhibit Fab binding or vice versa. Two radiolabelled protein G-fragments (28 and 35 K) showed different binding to polyclonal IgG, IgG F(ab')2-, IgG Fab- and IgG Fc-fragments. Thus, in a dot binding assay the 35 K fragment bound all IgG fragments tested, whereas the 28 K protein G fragment bound only intact IgG and IgG Fc-fragments. These results indicate two independent and separate binding sites for Fab- and Fc-fragments on protein G. Different binding sites on protein G were also indicated by Western blot analysis of four different protein G-fragments (28, 35, 42 and 65 K). In these experiments the 28 K fragment showed affinity only for Fc-fragments, while the higher mol. wt protein G preparations bound both IgG Fab- and Fc-fragments.

Antigen-Antibody Reactions↗

PubMed1988

Non-immune Fab- and Fc- mediated interactions of avian Ig with S. aureus and group C and G streptococci.

Serum samples from 19 avian species representing 8 orders were tested for their capacity to inhibit the Fab- and Fc-mediated immunoglobulin binding to protein A-carrying S. aureus and protein G-carrying group C and G streptococci. Four species (mallard, dunlin, starling and blackbird) belonging to three different orders showed a high degree of Fc-mediated protein A- and protein G-reactivity. Five species demonstrated a high level and nine species exhibited a low level of Fab-mediated protein A-reactivity. The four species identified as Fc-reactive were capable of Fab-mediated immunoglobulin binding with streptococcal surface proteins but incapable of Fab-mediated protein A binding. SDS-PAGE analysis confirmed that the protein A-Sepharose affinity purified material contained proteins corresponding to immunoglobulin chains. Inhibition results by avian sera were confirmed by direct binding of protein A-reactive proteins to bacteria, by precipitation in gel and by Western blot analysis of binding to protein A and protein G, respectively.

Animals↗