Alternative complement pathway activation fragment Ba binds to C3b. Evidence that formation of the factor B-C3b complex involves two discrete points of contact.

Alternative complement pathway C3 convertase formation involves the cleavage of C3b-associated factor B into fragments Ba and Bb. Whereas Bb, in complex with C3b, has proteolytic specificity toward native C3, the function of the Ba moiety in the formation and/or decay of alternative complement pathway C3 convertase is uncertain. Therefore, we have examined the effect of purified Ba fragment on both fluid-phase and surface-bound enzymatic activity and showed that whereas Ba could inhibit the rate of C3 convertase formation, the rate of intrinsic decay remained unaffected. A specific, metal ion-independent interaction between Ba and C3b was subsequently demonstrated by use of the cross-linking reagent dithiobis(succinimidyl propionate). When cell-associated 125I-B was activated by D, the dissociation of Bb fragment displayed simple first-order kinetics with a half-time of 2.4 min, this value being in reasonable agreement with the hemolytically determined decay rate of 1.8 min. In contrast, most of the Ba fragment undergoes rapid dissociation, but there is also evidence to suggest the establishment of a new equilibrium due to the ability of Ba to rebind to C3b. Cumulatively, these data are consistent with a model in which the attachment of intact B to C3b is mediated by two points of contact, one being in the Ba domain and the other in the Bb domain. Due to avidity effects, each of these interactions could be of relatively low intrinsic affinity, and the characteristic unidirectionality of alternative complement pathway C3 convertase decay may simply result from the low intrinsic association of "univalent" Bb for the C3b subunit.