The cleavage of two C1s subunits by a single active clr reveals substantial flexibility of the C1s-C1r-C1r-C1s tetramer in the C1 complex

The activation of the C1s-C1r-C1r-C1s tetramer in the C1 complex, which inv olves the cleavage of an Arg-Ile bond in the catalytic domains of the subco mponents, is a two-step process. First, the autolytic activation of C1r tak es place, then activated C1r cleaves zymogen C1s. The Arg(463)Gln mutant of C1r (C1r(QI)) is stabilized in the zymogen form. This mutant was used to f orm a C1q(C1s-C1r(QI)-C1r-C1s) heteropentamer to study the relative positio n of the C1r and C1s subunits in the C1 complex. After triggering the C1 by IgG-Sepharose, both C1s subunits are cleaved by the single proteolytically active C1r subunit in the C1s-C1r(QI) C1r-C1s tetramer, This finding indic ates that the tetramer is flexible enough to adopt different conformations within the C1 complex during the activation process, enabling the single ac tive C1r to cleave both C1s, the neighboring and the sequentially distant o ne.