Subunit interactions in the first component of complement, C1.
Interactions between C1q and other subunits of C1 were analyzed by sucrose gradient ultracentrifugation. A zone of dilute, radioiodine labelled C1q was sedimented through uniform concentrations of either C1r2C1s2, C1r2, C1r2 or C1s(2). The dissociation constants were found to be 3 x 10(-9) M and 6 x 10(-9) M for C1r2C1s2 and C1r2 binding respectively. Hill coefficients of 1 indicated no cooperativity in these bindings. Positive cooperativity was found in binding of C1s to C1q. Dissociation constants of 2 x 10(-6) M and 5 x 10(-8) M were obtained form computer modelling of a two step binding mechanism. No interaction was detected between C1q and activated C1r2. The data indicate that most of the interactions between C1q and C1r2C1s2 originates from a strong binding to the C1r2 moiety of the zymogen complex. This interaction is lost upon activation of C1r2.
Lakatos
S
S
Institute of Enzymology, Biological Research Center, Hungarian Academy of Sciences, Budapest.
D001665:Binding Sites; D002499:Centrifugation, Density Gradient; D003166:Complement Activating Enzymes / Q000032:analysis; D003172:Complement C1 / Q000032:analysis* / Q000378:metabolism; D015922:Complement C1q; D015923:Complement C1r; D003173:Complement C1s / Q000032:analysis
D003172:Complement C1; D015922:Complement C1q; D003166:Complement Activating Enzymes; D015923:Complement C1r; D003173:Complement C1s