Purification and some properties of rabbit C1r.

C1r, an activated subcomponent of the first component of the complement system, was highly purified from rabbit serum by affinity chromatography on IgG-Sepharose 6B followed by column chromatography on CM-Sephadex C-50. The C1r thus purified had a molecular weight of 105,000, consisting of two polypeptide chains connected by disulfide bonds; the molecular weights of the chains were 60,000 and 45,000. The C1r was found to reconstitute C1 complex when it reacted with rabbit C1q and C1s in the presence of Ca2+, since C1s was able to bind to C1q bound on sensitized sheep erythrocytes only in the presence of C1r. On the other hand, and active C1s fragment derived by hydrolysis of the H chain without any loss of C1s activity [J. Biochem. 80, 1423--1427 (1976)] could not bind to C1q even in the presence of C1r. This result indicates that a part of the H chain of C1s not contributing to the structural integrity of an active site may be involved in the binding of C1s to C1r.