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 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 (DeltaSCR10 - 11) had AP activity similar to wtCR2. NN (DeltaSCR6 - 8) and NOP (DeltaSCR6-mid14) had reduced AP activity, but near normal iC3 binding. XB (DeltaSCR3 - 6) and PP (DeltaSCR3-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.
Johnson
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AA
Department of Immunology and Microbiology, University of Southern Denmark, Odense, Denmark.
Mirowski Rosengard
A
A
Skjodt
K
K
Ahearn
J M
JM
Leslie
R G
RG
D000818:Animals; D000956:Antigens, Viral / Q000235:genetics / Q000276:immunology; D019556:COS Cells; D003167:Complement Activation / Q000276:immunology*; D017464:Receptors, Complement 3d / Q000235:genetics / Q000276:immunology*; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000276:immunology*; D017384:Sequence Deletion; D015398:Signal Transduction / Q000276:immunology; D013329:Structure-Activity Relationship; D014616:Vaccinia virus / Q000235:genetics / Q000276:immunology*
D000956:Antigens, Viral; D017464:Receptors, Complement 3d; D011993:Recombinant Fusion Proteins