C1 Complex: An Adaptable Proteolytic Module for Complement and Non-Complement Functions.

Area of Science:

• Immunology
• Molecular Biology
• Cellular Biology

Background:

• The complement C1 complex is central to the classical complement pathway.
• C1q acts as a scaffold, binding ligands and activating serine proteases C1r and C1s.
• Activated C1s initiates the complement cascade by cleaving C4 and C2.

Purpose of the Study:

• To explore the non-classical functions of the complement C1 complex.
• To investigate novel ligands and substrates of C1q and C1s.
• To understand the broader molecular and cellular roles of C1.

Main Methods:

• Literature review of recent studies on C1 complex functions.
• Analysis of C1q interactions with Frizzled receptors and apoptotic cells.
• Examination of C1s protease activity on non-complement targets like LRP6 and MHC class I.

Main Results:

• C1q binds Frizzled receptors, activating C1s to cleave LRP6, impacting Wnt signaling.
• C1q binds apoptotic cells, leading to cleavage of nuclear antigens by activated C1 proteases.
• C1s demonstrates broad substrate specificity, including MHC class I molecules.

Conclusions:

• The C1 complex exhibits significant functions beyond the complement system.
• C1q's diverse ligand binding and C1 protease's broad substrate specificity confer modularity.
• C1 complex adaptability allows participation in multiple molecular and cellular processes.