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Protein D of Haemophilus influenzae. A novel bacterial surface protein with affinity for human IgD
M R Ruan1, M Akkoyunlu, A Grubb
1Department of Medical Microbiology, University of Lund, Malmö, Sweden.
Abstract:
Protein D, a novel surface protein of the bacterial species Haemophilus influenzae with affinity for human IgD, was isolated after solubilization with sonication and Sarcosyl-extraction by a single SDS-PAGE step. From 1 ml of packed bacteria was prepared 0.25 mg of purified protein D. The apparent m.w. of protein D was estimated to 42,000 by SDS-PAGE and gel chromatography. Edman degradation cycles of protein D produced no amino acid phenylthiohydantoin derivatives and the amino-terminal end of the single protein D polypeptide chain is thus probably blocked. Protein D differs from all previously described outer membrane proteins (protein 1 to 6) of H. influenzae. Thus, protein D did not react with antibodies against protein 1 or protein 2 and the latter proteins did not bind IgD. Protein D was found to exhibit unique Ig-binding properties. Thus, in dot blots protein D bound four different human IgD myeloma proteins but not IgG, IgM, IgA, IgE, or some additional proteins. On the IgD molecule, constant parts of the H chains both in the Fab and Fc fragments appear responsible for the interaction with protein D. This novel Ig-binding reagent promises to be of theoretical and practical interest in immunologic and microbiologic research.
Insights
Researchers isolated Protein D, a novel surface protein from Haemophilus influenzae, which specifically binds human immunoglobulin D (IgD). This unique Ig-binding protein has potential applications in immunologic and microbiologic research.
Area of Science:
- Microbiology
- Immunology
- Biochemistry
Background:
- Haemophilus influenzae is a significant bacterial pathogen.
- Outer membrane proteins of H. influenzae are crucial for bacterial structure and function.
- Understanding bacterial surface proteins can reveal novel targets for research and therapeutics.
Purpose of the Study:
- To isolate and characterize a novel surface protein from Haemophilus influenzae.
- To investigate the binding properties of this protein with human immunoglobulins.
- To assess the potential utility of this protein in scientific research.
Main Methods:
- Protein D was isolated using sonication and Sarcosyl-extraction followed by SDS-PAGE.
- Molecular weight was determined using SDS-PAGE and gel chromatography.
- Amino-terminal sequencing was performed using Edman degradation.
- Immunoglobulin-binding assays were conducted using dot blots.
Main Results:
- Purified Protein D (0.25 mg from 1 ml packed bacteria) has an apparent molecular weight of 42,000 Da.
- The amino-terminal end of Protein D is blocked.
- Protein D is distinct from previously described H. influenzae outer membrane proteins.
- Protein D specifically binds human IgD, including myeloma proteins, but not other immunoglobulin classes (IgG, IgM, IgA, IgE).
- Binding involves constant regions of IgD heavy chains in both Fab and Fc fragments.
Conclusions:
- Protein D is a novel surface protein of Haemophilus influenzae with specific affinity for human IgD.
- Its unique Ig-binding properties make it a valuable reagent for immunologic and microbiologic research.
- Further investigation into Protein D could lead to new diagnostic or therapeutic strategies.