Interaction of C1s and C4. A binding phenomenon

Area of Science:

• Biochemistry
• Immunology
• Complement System

Background:

• The complement system is crucial for innate and adaptive immunity.
• The first component of complement (C1) initiates the classical pathway.
• C1s is the enzymatic subunit of C1 responsible for cleaving C4 and C2.

Purpose of the Study:

• To investigate the interaction between the first component of complement (C1s) and the fourth component of complement (C4).
• To determine the roles of enzymatic and binding activities of C1s in C4 interaction.
• To elucidate the mechanism of C4 activation within the complement cascade.

Main Methods:

• Sucrose density gradient ultracentrifugation to detect C1s-C4 binding.
• Use of enzymatically inactive C1s (DFP-treated) and C1s fragments to assess functional requirements.
• Protease treatment (leukocyte lysosomal enzymes, trypsin, plasmin) to study C1s activity.
• Hemolytic assays to confirm C4 activity.

Main Results:

• Enzymatically active 125-I-labeled C1s binds to purified C4.
• C1s-C4 binding is demonstrated by sucrose density gradient ultracentrifugation.
• Binding requires hemolytically active C4 but not enzymatically active C1s.
• Protease treatment inactivates C1s enzymatic activity but at a slower rate than functional activity loss.
• Esteratic activity decline is slower than functional activity decline.

Conclusions:

• C1s possesses distinct enzymatic and binding sites.
• The binding site of C1s is crucial for positioning C4 for proteolytic cleavage.
• This interaction is essential for initiating the complement cascade and preparing C4 for subsequent steps.