The interaction of C5 with C3b in free solution: a sufficient condition for cleavage by a fluid phase C3/C5 convertase.

Abstract

We have measured the interactions of C3b with C5 in free solution under conditions that favor detecting weak binding interactions (high C3b and low C5 concentrations and low ionic strength). When a mixture of 125I-C5 (2 X 10(-8) M) and unlabeled C3b (3.8 x 10(-5) M) was ultracentrifuged in a sucrose gradient, virtually all of the C5 sedimented to the position of a 13 to 14S complex. In contrast, a sedimentation rate for C5 of 9S was obtained in the absence of C3b. The ability to bind C5 was observed to be a property of C3b since native C3 was unable to bind C5. It was also found that beta 1H by itself could inhibit the binding of C5 to cell-bound C3b. From inhibition studies, we estimate that the association constant for the C3b-C5 interaction is on the order of 2 x 10(6) M-1 in a low ionic strength buffer (mu = 0.06) and 5-fold weaker at physiologic ionic strength. C5 bound to C3b in free solution was cleaved by nephritic factor-stabilized fluid phase C3bB. C5 activation did not occur on omitting C3b. We conclude that the ability to bind C5 is a property of C3b molecules whether surface bound or in free solution and that when C5 is bound in either fashion, it can be cleaved by a fluid phase C3/C5 convertase.