Conformational changes of the subunits Clq, Clr and Cls of human complement component Cl demonstrated by 125I labeling

C1s and C1r proenzymes and enzymes (C1s, C1r) and C1q were labeled with ¹²⁵I. The distribution of the ¹²⁵I label between H- and L-chain of C1s was only slightly dependent on the state of activation of C1s, and approx. 90% of the label was found in the H-chain. In the Clr proenzyme molecules 50% of the label was incorporated into the H-chain. The C1r H-chain label was reduced to 10% on activation of C1r to C1r, while the L-chain label increased to 90% of the total label. The presence of either C1s, C1q or C1qs during labeling reduced the C1r H-chain label, although C1r remained in the proenzyme form. The presence of C1s or C1rs enhanced the ¹²⁵I uptake of C1q in Ca²⁺ or EDTA medium. This was unexpected because one would have anticipated a diminution of the C1q label due to the apposition of C1r and C1s, similarly as it occurs during C1rs complex and C1s dimer formation for the H-chain label of C1s. The results show that C1r and C1q after their conformation during activation and C1 complex formation.