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Novel subunit in C4b-binding protein required for protein S binding.
1Department of Clinical Chemistry, University of Lund, Malmö General Hospital, Sweden.
The Journal of Biological Chemistry
|September 5, 1988
Summary
C4b-binding protein (C4BP) has a newly discovered 45-kDa subunit. This subunit is crucial for binding protein S, regulating both complement and coagulation systems.
Area of Science:
- Biochemistry
- Immunology
- Molecular Biology
Background:
- C4b-binding protein (C4BP) is a key regulator of the complement system.
- C4BP interacts with protein S, involved in coagulation regulation.
- Native C4BP comprises seven identical 70-kDa subunits in a spider-like structure.
Purpose of the Study:
- To identify and characterize a novel subunit of C4BP.
- To investigate the role of the new subunit in C4BP function, particularly protein S binding.
Main Methods:
- Sodium dodecyl sulfate-poly-acrylamide gel electrophoresis (SDS-PAGE) to detect the new subunit.
- Gel filtration in guanidine hydrochloride for subunit isolation.
- Amino-terminal sequencing to determine subunit identity.
- Chymotrypsin proteolysis assays to assess functional impact.
Main Results:
- A new, approximately 45-kDa subunit was identified in C4BP.
- The 45-kDa subunit is disulfide-linked to the central core of C4BP, with a 1:9 stoichiometry.
- The amino-terminal sequence of the 45-kDa subunit is unique.
- Cleavage of the 45-kDa subunit by chymotrypsin abolished C4BP's protein S binding ability.
- Protein S binding protected the 45-kDa subunit from degradation.
Conclusions:
- A novel 45-kDa subunit is an integral component of C4BP.
- This subunit is essential for the protein S binding activity of C4BP.
- The 45-kDa subunit plays a direct role in mediating the interaction between C4BP and protein S.