Decay accelerating activity of complement receptor type 1 (CD35) - Two active sites are required for dissociating C5 convertases

The goal of this study was to identify the site(s) in CRI that mediate the dissociation of the C3 and C5 convertases. To that end, truncated derivativ es of CR1 whose extracellular part is composed of 30 tandem repealing modul es, termed complement control protein repeats (CCPs), were generated. Site 1 (CCPs 1-3) alone mediated the decay acceleration of the classical and alt ernative pathway C3 convertases, Site 2 (CCPs 8-10 or the nearly identical CCPs 15-17) had one-fifth the activity of site 1. In contrast, for the C5 c onvertase, site 1 had only 0.5% of the decay accelerating activity, while s ite 2 had no detectable activity. Efficient C5 decay accelerating activity was detected in recombinants that carried both site 1 and site 2, The activ ity was reduced if the intervening repeats between site 1 and site 2 were d eleted. The results indicate that, for the C5 convertases, decay accelerati ng activity is mediated primarily by site 1. A properly spaced site 2 has a n important auxiliary role, which may involve its C3b binding capacity. Mor eover, using homologous substitution mutagenesis, residues important in sit e 1 for dissociating activity were identified. Based on these results, we g enerated proteins one-fourth the size of CR1 but with enhanced decay accele rating activity for the C3 convertases.