ACTIVATION OF HUMAN-COMPLEMENT SERINE-PROTEINASE C1R IS DOWN-REGULATED BY A CA2-DEPENDENT INTRAMOLECULAR CONTROL THAT IS RELEASED IN THE C1COMPLEX THROUGH A SIGNAL TRANSMITTED BY C1Q()

The activation of human C1, a Ca2+-dependent complex proteinase compri sing a non-enzymic protein, C1q, and two serine proteinases, C1r and C 1s, is based primarily on the intrinsic property of C1r to autoactivat e. The aim of the present study was to investigate the mechanisms invo lved in the regulation of C1r autoactivation, with particular attentio n to the role of Ca2+ ions. Spontaneous activation of proenzyme C1r wa s observed upon incubation in the presence of EDTA, whereas Ca2+ ions reduced markedly the activation process. Several lines of evidence ind icated that Ca2+ inhibited the intramolecular activation reaction but had little or no effect on the intermolecular activation reaction. C1q caused partial release of this inhibitory effect of Ca2+. Complete st abilization of C1r in its proenzyme form was obtained upon incorporati on within the Ca2+-dependent C1s-C1r-C1r-C1s tetramer, and a comparabl e effect was observed when C1s was replaced by its Ca2+-binding alpha- fragment. Both tetramers, C1s-C1r-C1r-C1s and C1s alpha-C1r-C1r-C1s al pha, readily associated with C1q to form 16.0 S and 14.7 S complexes r espectively in which C1r fully recovered its activation potential. Bot h complexes showed indistinguishable activation kinetics, indicating t hat the gamma B catalytic region of C1s plays no role in the mechanism that triggers C1r activation in C1. The collagen-like fragments of C1 q retained the ability to bind to C1s-C1r-C1r-C1s, but, in contrast wi th intact C1q, failed to induce C1r activation in the resulting comple x at temperatures above 25 degrees C. On the basis of these observatio ns it is proposed that activation of the serine-proteinase domain of C 1r is controlled by a Ca2+-dependent intramolecular mechanism involvin g the Ca2+-binding alpha-region, and that this control is released in C1 by a signal originating in C1q and transmitted through the C1q/C1r interface.