Identification of a catalytic exosite for complement component C4 on the serine protease domain of C1s

Area of Science:

• Immunology
• Molecular Biology
• Biochemistry

Background:

• The classical complement pathway is vital for immunity but implicated in inflammatory diseases when dysregulated.
• Activation involves the C1 complex (C1r, C1s) cleaving C4 and C2.
• Molecular details of C1s-C4 interaction, particularly exosite involvement, remain unclear.

Purpose of the Study:

• To elucidate the molecular mechanisms underlying the interaction between C1s and its substrate C4.
• To identify key residues and factors involved in the catalytic activity of C1s towards C4.

Main Methods:

• Structural analysis of the C1s serine protease domain.
• Identification of positively charged amino acids forming a potential catalytic exosite.
• Co-crystallization studies involving C1s and sulfate ions.
• Interaction studies with a C4-derived peptide containing sulfotyrosine residues.
• Molecular modeling of the C1s-C4 complex.

Main Results:

• Four positively charged amino acids on the C1s protease domain were identified as forming a catalytic exosite essential for C4 cleavage.
• These residues were found to coordinate sulfate ions in the crystal structure.
• Evidence suggests sulfate ions mediate the interaction between C1s and C4.
• C1s was shown to interact with a C4 peptide featuring sulfotyrosine residues.

Conclusions:

• A catalytic exosite involving specific amino acids and potentially sulfate ions is critical for C1s-mediated C4 cleavage.
• This study provides a molecular model for C1s-C4 interaction, offering insights into complement activation.
• Further research is warranted to fully understand the role of these findings in complement activation and disease.