Cleavage of C2 by C1̄s̄ into the antigenically distinct fragments C2a and C2b: Demonstration of binding of C2b to C4b

Abstract

The activation of C2 [complement component 2] by .**GRAPHIC**. is a major reaction step leading to the assembly of 2 related macromolecular enzymes in the classical complement pathway C3 convertase and C5 convertase. The present studies document the smaller fragment, C2b, that results when human C2 reacts with .**GRAPHIC**. C2b (34,000 daltons) is a single protein on disc electrophoresis and immunoelectrophoresis. C2a (73,000 daltons), the larger fragment from this reaction, has a more acidic nature and C2b is more basic. These fragments can be detected by their different antigenic determinants. When the C2-C4b complex is activated in the fluid phase by .**GRAPHIC**. and allowed to decay, it dissociates into C2a and the C2b-C4b complex. When C2 is bound to C4b-Sepharose and then reacted with .**GRAPHIC**. only the C2a fragment is released from the solid phase C2-C4b-Sepharose into the fluid phase, and the C2b fragment remains noncovalently bound to C4b-Sepharose. The C2b portion of C2 apparently contains a stable binding site for C4b and, after the decay release of C2a from this C3 convertase, the C2b fragment probably remains bound. The decay release of C2a may represent a temperature-dependent dissociation from C2b.