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

• Published in: European journal of immunology 1999-12, Vol.29 (12), p.3837-3844
• Main Authors: Johnson, Anna Ansaba, Mirowski Rosengard, Ariella, Skjødt, Karsten, Ahearn, Joseph Michael, Leslie, Robert Graham Quinton
• Format: Article
• Language: English
• Subjects: AC3 protein; Alternative pathway activation; Animals; Antigens, Viral - genetics; Antigens, Viral - immunology; CD21 antigen; Complement; Complement Activation - immunology; complement control protein; Complement receptor 2; Complement receptor type 2; complement receptors; COS Cells; Receptors, Complement 3d - genetics; Receptors, Complement 3d - immunology; Recombinant Fusion Proteins - genetics; Recombinant Fusion Proteins - immunology; Sequence Deletion; Short consensus repeats; Signal Transduction - immunology; Structure-Activity Relationship; vaccinia virus; Vaccinia virus - genetics; Vaccinia virus - immunology
• ISSN: 0014-2980; 1521-4141
• DOI: 10.1002/(SICI)1521-4141(199912)29:12<3837::AID-IMMU3837>3.0.CO;2-K

The role of complement receptor 2 (CR2) short consensus repeats (SCR) in binding of hydrolyzed C3 (iC3) to form an alternative pathway (AP) convertase, and promoting C3 fragment deposition following AP activation, was examined. We used (1) K562 cells transfected with CR2 constructs, where the C3d‐binding site of CR2 (SCR1+2) was replaced with the four‐SCR vaccinia virus complement 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 AP activity of 5.5 %, the truncation mutants being inactive. Of the CR2 mutants, only EK (ΔSCR10 – 11) had AP activity similar to wtCR2. NN (ΔSCR6 – 8) and NOP (ΔSCR6‐mid14) had reduced AP activity, but near normal iC3 binding. XB (ΔSCR3 – 6) and PP (ΔSCR3‐mid14) were inactive in both assays. We conclude that, whilst iC3 binding to CR2 via SCR1 – 4 is essential for AP activation, the efficiency of C3 deposition also depends on the midportion of CR2.