Active Disassembly of the First Complement Component, C1̄, by C1̄ Inactivator

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Abstract
In order to clarify its mechanism of disassembly, the first component of human complement, C1, was reconstituted from C1q, 125I-C1r, and 131I-C1s. This radiolabeled C1 bound tightly to aggregated IgG covalently linked to Sepharose 4B. The subsequent addition of C1̄ inactivator (C1̄-In) led to the rapid release of equimolar quantitites of 125I-C1r̅ and 131I-C1s̅ from the agg IgG. SDS-PAGE analyses of such eluates showed that one C1̄-In molecule was associated with each C1r̅ polypeptide chain and another C1̄-In molecule was bound to each C1s̅ polypeptide chain. The released C1r̅, C1s̅, and C1̄-In co-sedimented in sucrose density gradients with a rate of 9S and monospecific anti-C1s̅ pelleted all three proteins. Therefore, C1r̅, C1s̅, and C1̄-In, in a molar ratio of 1:1:2, respectively, are released from C1̄ as a complex. An analogous 9S complex containing C1r̅, C1s̅, and C1̄-In was generated in normal human serum after incubation with activators of the classical complement pathway. The C1r̅C1s̅(C1̄-In) complexes generated in both serum and the purified system were stable in the presence of EDTA. A diffusion coefficient of 2.3 × 10-7 cm2/sec was determined for the C1r̅C1s̅(C1̄-In) complex from its behavior on gel filtration. An m.w. of 330,000 was then calculated from its sedimentation and diffusion coefficients. This m.w. is consistent with the value of 382,000 for a molecule of composition C1r̅C1s̅(C1̄-In)2 obtained by summing the weights of the subunits. These results indicate that C1-In efficiently disassembles C1̄, thereby releasing two C1r̅C1s̅(C1̄-In)2 complexes per C1̄ molecule.