Structural transitions of complement component C3 and its activation products.
Noritaka Nishida1, Thomas Walz, Timothy A Springer
1CBR Institute for Biomedical Research and Department of Pathology, Harvard Medical School, 200 Longwood Avenue, Boston, MA 02115, USA.
Summary
Structural changes in complement protein C3 activation were visualized using electron microscopy. This reveals how C3b and iC3b fragments facilitate pathogen binding and immune responses.
Area of Science:
- Immunology
- Structural Biology
- Biochemistry
Background:
- The complement system is crucial for innate and adaptive immunity.
- Complement protein C3 is central to all complement activation pathways.
- Understanding C3 structural dynamics is key to deciphering immune responses.
Purpose of the Study:
- To visualize the structural transitions of complement protein C3 during activation.
- To elucidate the molecular mechanisms underlying C3b and iC3b formation and function.
- To identify structural intermediates and final products of C3 activation.
Main Methods:
- High-resolution electron microscopy was employed.
- Structural analysis of C3 cleavage products (C3b, iC3b) was performed.
- Nucleophile addition assays were used to capture activation intermediates.
Main Results:
- Significant structural rearrangement of the thioester domain in C3b was observed (~100 Angstrom movement).
- Cleavage in iC3b generates a flexible linker, altering domain interactions.
- A structural intermediate and a final product with significant anaphylatoxin domain movement were identified.
Conclusions:
- Complement protein C3 undergoes substantial conformational changes during activation.
- These dynamic structural transitions are critical for C3's function in pathogen recognition and immune signaling.
- The findings provide atomic-level insights into complement-mediated immunity.
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