Structural requirements for the complement regulatory activities of C4BP.

Area of Science:

• Immunology
• Molecular Biology
• Structural Biology

Background:

• C4b-binding protein (C4BP) is a key regulator of the classical complement pathway.
• C4BP functions by inhibiting the C3 convertase (C4bC2a complex).
• C4BP is a multimeric protein composed of alpha- and beta-chains, featuring multiple complement control protein (CCP) domains.

Purpose of the Study:

• To investigate the structural requirements for C4BP's interaction with C4b.
• To determine the role of C4BP's polymeric nature in its function.
• To elucidate the specific contributions of individual CCP domains and their spatial arrangement to C4BP activity.

Main Methods:

• Creation and functional characterization of 19 recombinant C4BP variants.
• Construction of truncated monomeric and polymeric variants with deleted CCP domains.
• Introduction of alanine substitutions to assess the importance of spatial arrangements between CCPs.

Main Results:

• The smallest active C4BP variant included CCP1-3.
• Monomeric C4BP variants showed reduced efficiency in C4b degradation compared to polymeric forms.
• All three N-terminal CCP domains (CCP1-3) are important for C4b binding and function, with CCP2 and CCP3 being most critical.
• Altering the spatial arrangement between CCPs impaired C4BP function.

Conclusions:

• The polymeric structure of C4BP is essential for optimal C4b degradation on cell surfaces.
• Specific N-terminal CCP domains (CCP1-3) and their precise spatial orientation are critical for C4BP's regulatory activity.
• Understanding these structural determinants provides insight into complement system regulation.