The assembly of C1, the first component of human complement, involves interactions between various domains of each of its three subcomponents, C1q, C1r, and C1s. The isolation, assignment of function, and structural characterization of the individual domains of C1r and C1s are critical for a thorough understanding of this complex assembly. The present study describes a 27-kDa plasmin-generated fragment derived from the NH2-terminal half of the heavy A chain of C1s-, the activated form of C1s. This fragment, C1s-alpha, was shown in the presence of Ca2+ to mimic the ability of whole C1s- to self-associate, bind to C1r-, and facilitate the binding of C1r to C1q. These results directly prove that the Ca2(+)-binding sites of C1s as well as all of the determinants necessary for binding of C1s- to C1r- and C1q are located in the NH2-terminal 27-kDa alpha region of the A chain.

The rate of formation of human chorionic gonadotropin from its alpha and beta subunits has been measured at neutral pH and 37 degrees C as a function of subunit concentration, using the fluorescence probe, 1,8-anilinoaphthalene-sulfonate (ANS), to monitor the reaction. The subunits were prepared by acid dissociation of the intact hormone (pH less than or equal to 2, 37 degrees C). Following neutralization, the rate of appearance of ANS fluorescence was identical with the rate of recovery of receptor binding activity and both of these properties were completely recovered. Kinetic data obtained over a 100-fold range of subunit concentrations (1.5 to 146 muM) were not compatible with a simple second-order reaction scheme, but required at least one additional step. The data were best fit by a model in which the subunits reversibly form an intermediate complex (alpha + beta in equilibrium alphabeta) which then undergoes a conformational rearrangement to form the native structure (alphabeta leads to H). Ultraviolet difference absorption measurements suggest that most of the change in the environment of the tyrosyl residues occurs during this second step.