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Human C4b-binding protein, C4bp. Chymotryptic cleavage and location of the 48 kDa active fragment within C4bp
Insights
Complement C4bp, a classical pathway regulator, is cleaved by chymotrypsin. This process releases a 48 kDa active fragment, revealing insights into C4bp
Area of Science:
- Biochemistry
- Immunology
- Proteomics
Background:
- The complement system regulates innate and adaptive immunity.
- C4bp (C4b-binding protein) is a key regulator of the classical complement pathway.
- Understanding C4bp structure is crucial for its functional role in immunity.
Purpose of the Study:
- To investigate the cleavage products of C4bp upon chymotrypsin treatment.
- To identify the active fragment of C4bp and its origin.
- To propose a structural model for C4bp based on experimental findings.
Main Methods:
- Incubation of purified C4bp with chymotrypsin.
- Analysis of cleavage fragments using SDS-PAGE and N-terminal sequencing.
- Characterization of the active fragment's origin.
Main Results:
- Chymotrypsin rapidly cleaves C4bp into 48 kDa and 27 kDa fragments.
- A subsequent slow cleavage liberates the 48 kDa fragment containing the active site.
- N-terminal sequencing confirms the 48 kDa fragment originates from the N-terminus of the parent C4bp subunit.
Conclusions:
- The 48 kDa fragment represents the N-terminal active domain of C4bp.
- The cleavage pattern provides evidence for a proposed gross structure of C4bp.
- These findings contribute to understanding complement regulation mechanisms.
Abstract:
C4bp, a regulator of the classical pathway of complement system, is composed of 6-8 disulfide-linked subunit chains of 75 kDa. Upon incubation with chymotrypsin, C4bp was rapidly cleaved into a nicked C4bp, composed of disulfide-linked 48 kDa and 27 kDa fragments. Subsequent slow cleavage on the 27 kDa fragment resulted in the liberation of the active site-containing 48 kDa fragment from the nicked C4bp. The N-terminal amino acid sequence of the 48 kDa fragment was identical to that of the parent subunit chain of C4bp, indicating that the 48 kDa active fragment was released from the N-terminal side of the parent subunit chain. Based on these results, a possible gross structure of C4bp is proposed.