Nature of the interaction between the C1q and C1r 2s 2 subunits of the first component of human complement

Abstract

The strength of interaction between the C1q and C1r 2s 2 subunits of C1 was studied as a function of temp. During centrifugation through sucrose density gradients at 4°C, macromolecular C1 readily dissociated as it sedimented away from its free subunits. In contrast, at 20°C, C1 remained associated as the 16S complex throughout centrifugation, thus indicating a stronger interaction between C1q and C1r 2s 2 at the higher temp. C1-inhibitor (C1-In) or nitrophenylguanidinobenzoate was present during centrifugation to prevent C1 activation. That native C1 was in fact the species being studied was confirmed by SDS-PAGE analysis. To investigate this temp dependence without using inhibitors, an alternative approach was used. Trace amounts of 125I-C1q were centrifuged through numerous sucrose density gradients, each of which contained a different concn of native C1r 2s 2 throughout the gradient. The s-rate of 125I-C1q increased with increasing C1r 2s 2 input. An association constant of 4.9 × 10 7 M −1 was calculated for this reversible interaction at 4°C. However, at 20°C, the data indicated a much higher affinity reaction since the addition of far less C1r 2s 2 was required for the s-rate of 125I-C1q to reach the 16S plateau. The presence of C1-In did not affect these results. We have demonstrated that the association of C1q with C1r 2s 2 increases with increasing temp, a finding suggestive of a hydrophobic interaction. However, since we also show that C1 readily dissociates with increasing NaCI concn, the C1q-C1r 2s 2 interaction must, in fact, be ionic in nature. We therefore conclude that the temp dependence of the inter-subunit interaction is the result of a conformational change(s) within one of the subunits, and propose that this change may be similar to that occurring during C1 activation.