The structural basis for complement receptor type 2 (CR2, CD21)-mediated alternative pathway activation of complement: studies with CR2 deletion mutants and vaccinia virus complement-control protein-CR2 chimeras

The role of complement receptor 2 (CR2) short consensus repeats (SCR) in bi nding of hydrolyzed C3 (iC3) to form an alternative pathway (AP) convertase , and promoting C3 fragment deposition following AP activation, was examine d. We used (1) K562 cells transfected with CR2 constructs, where the C3d-bi nding site of CR2 (SCR1+2) was replaced with the four-SCR vaccinia virus co mplement control protein (VCP), or truncation mutants thereof, and (2) COS cells transfected with wild-type (wt) CR2, or deletion mutants thereof. AP activation required iC3 binding in both systems. Thus, the VCP-CR2 chimera had an iC3 binding efficiency of 11.4%, compared to wtCR2, and a relative A P activity of 5.5%, the truncation mutants being inactive. Of the CR2 mutan ts, only EK (Delta SCR10-11) had AP activity similar to wtCR2. NN (Delta SC R6-8) and NOP (Delta SCR6-mid14) had reduced AP activity, but near normal i C3 binding. XB (Delta SCR3-6) and PP (Delta SCR3-mid14) were inactive in bo th assays. We conclude that, whilst iC3 binding to CR2 via SCR1-4 is essent ial for AP activation, the efficiency of C3 deposition also depends on the midportion of CR2.