Ig-binding bacterial proteins also bind proteinase inhibitors.

Area of Science:

• Microbiology
• Immunology
• Biochemistry

Background:

• Protein G, a streptococcal cell wall protein, possesses distinct binding sites for immunoglobulin G (IgG) and human serum albumin (HSA).
• The functional significance of Protein G's interactions beyond IgG and HSA remains an area of exploration.

Purpose of the Study:

• To investigate the binding interactions of Protein G with other human plasma proteins.
• To determine if Protein G interacts with proteinase inhibitors and to characterize the nature of these interactions.

Main Methods:

• Affinity assays were performed to assess the binding of Protein G to various human plasma proteins, including alpha 2-macroglobulin (alpha 2M) and kininogen.
• Competitive binding experiments and synthetic peptide inhibition were used to map the binding sites.
• Protein G binding to different forms of alpha 2M and its effect on proteinase inhibitory activity were evaluated.

Main Results:

• Protein G specifically bound to alpha 2-macroglobulin (alpha 2M) and kininogen, but not to 23 other tested human proteins.
• alpha 2M binding occurred at the IgG-binding domains of Protein G, with mutual inhibition observed when both were in excess.
• A synthetic peptide mimicking an IgG-binding domain blocked Protein G-alpha 2M interaction. Protein G also bound Protein A and Protein L to alpha 2M and kininogen.

Conclusions:

• Protein G exhibits specific affinity for human plasma proteinase inhibitors alpha 2M and kininogen.
• These interactions do not impede the proteinase inhibitory functions of alpha 2M and kininogen.
• The findings extend to other Ig-binding bacterial proteins, suggesting a broader role in proteolytic processes.