Mutants of complement component C3 cleaved by the C4-specific C1-s protease.

Area of Science:

• Immunology
• Molecular Biology
• Protein Chemistry

Background:

• The complement system is crucial for innate immunity, involving complex proteolytic cascades.
• Complement components C3 and C4 are central to these cascades, with distinct cleavage specificities.
• Understanding protease recognition mechanisms is key to deciphering complement regulation.

Purpose of the Study:

• To elucidate the structural determinants governing the specific recognition of complement components C3 and C4 by proteases.
• To investigate how alterations in cleavage sites affect protease susceptibility in C3 and C4.

Main Methods:

• Site-specific mutagenesis was employed to create mutant murine C3 (mC3) proteins.
• Mutant mC3 proteins were designed to be cleaved by the C4-specific protease, C1-s.
• Cleavage efficiencies were assessed for both C1-s and the alternative-pathway C3 convertase.

Main Results:

• Inserting C4-specific residues into the C3 cleavage site rendered mC3 susceptible to C1-s cleavage.
• Both insertions and substitutions at the cleavage site influenced C1-s recognition, with varying efficiencies.
• Modifications at the mC3 cleavage site partially inhibited cleavage by the C3 convertase, suggesting distal interaction sites.

Conclusions:

• C1-s protease recognition of complement components is primarily dictated by steric accessibility and specific amino acid residues at the cleavage site.
• C3 convertase cleavage relies more on interactions distal to the cleavage site, with weaker, non-specific interactions at the site itself.
• These findings provide insights into the molecular basis of complement component specificity and regulation.