Fourth component of human complement: description of a three polypeptide chain structure

Area of Science:

• Biochemistry
• Immunology
• Molecular Biology

Background:

• The fourth component of human complement (C4) is a crucial part of the immune system.
• Understanding C4's structure and activation is vital for immunology research.

Purpose of the Study:

• To elucidate the subunit composition and molecular weight of human C4.
• To investigate the effect of C1s enzyme on C4 structure.
• To characterize the resulting fragments of C4 activation.

Main Methods:

• Sodium dodecyl sulfate (SDS) polyacrylamide gel electrophoresis was used to analyze C4.
• Periodic acid-Schiff staining identified carbohydrate moieties.
• Molecular weight determination of polypeptide chains.

Main Results:

• Human C4 consists of alpha (93,000 Da), beta (78,000 Da), and gamma (33,000 Da) chains.
• C1s action on C4 reduced the alpha-chain to 87,000 Da, yielding C4a (6,000 Da) and C4b (198,000 Da).
• Carbohydrate is associated with all three C4 polypeptide chains.

Conclusions:

• The study details the polypeptide chain composition and molecular weights of human C4.
• C1s specifically cleaves the alpha-chain, producing defined activation fragments.
• The findings contribute to a better understanding of complement system activation pathways.