Biochemical analysis of CR1 derivatives. (A) Schematic representation of CR1 truncations. The top diagram depicts the full-length CR1 protein consisting of a signal peptide followed by tandemly arranged 30 SCRs (shown as joined boxes).^15,19  sCR1 lacks the transmembrane and cytoplasmic tails; LHR-A through LHR-D are truncations of 7 SCRs each and LHR-D+ consists of LHR-D and the last 2 SCRs. (B) Pull-down assays with C4b and C3b and CR1 derivatives: LHR-A, LHR-B, LHR-C, LHR-D, LHR-D+, sCR1, and mock as a control. The input (first lane) C4b (upper blot) and C3b (lower blot) are indicated by arrows. Both C3b and C4b were eluted from sCR1, LHR-B, and LHR-C. C4b was eluted, but C3b was not eluted from LHR-A. Although similar amounts of LHR-D and LHR-D+ were used in the pull-down assays, neither one bound to C3b or C4b. (C-D) Cofactor activity of LHRs for cleavage of C4b (C) and C3b (D). Purified C4b or C3b was incubated with factor I alone (first lane, In C4b or In C3b, respectively), or in the presence of 2 different amounts (100 ng or 10 ng, indicated as a gradient) of CR1 derivatives: LHR-A, LHR-B, LHR-C, LHR-D, and mock as a control. The position of degradation products of C3b and C4b are indicated by arrows.