The C1̄ inhibitor of the complement cascade forms stoichiometric complexes with C1̄r and C1̄s and controls the activation of first component C1 of complement. Literature sedimentation coefficients s°20.w for the complexes formed between C1̄ inhibitor, C1̄r and C1̄s were analysed using frictional ratios and the hydrodynamic sphere approach. A head-and-tail two-domain model for C1 inhibitor was combined with cylindrical hydrodynamic models for the six-domain structures of C1̄r and C1̄s. The hydrodynamic data show that the heavily glycosylated N-terminal domain of C1̄ inhibitor is positioned close to the two complement ‘short consensus repeat’ domains found in the centre of C1̄r and C1̄s.
