Human C4b‐binding protein, C4bp

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Abstract
C4bp, a regulator of the classical pathway of complement system, is composed of 6–8 disulfide-linked subunit chains of 75 kDa. Upon incubation with chymotrypsin, C4bp was rapidly cleaved into a nicked C4bp, composed of disulfide-linked 48 kDa and 27 kDa fragments. Subsequent slow cleavage on the 27 kDa fragment resulted in the liberation of the active site-containing 48 kDa fragment from the nicked C4bp. The N-terminal amino acid sequence of the 48 kDa fragment was identical to that of the parent subunit chain of C4bp, indicating that the 48 kDa active fragment was released from the N-terminal side of the parent subunit chain. Based on these results, a possible gross structure of C4bp is proposed.