The C-terminus of complement regulator Factor H mediates target recognition: evidence for a compact conformation of the native protein.
The complement inhibitor Factor H has three distinct binding sites for C3b and for heparin, but in solution uses specifically the most C-terminal domain, i.e. short consensus repeats (SCR) 20 for ligand interaction. Two novel monoclonal antibodies (mABs C14 and C18) that bind to the most C-terminal domain SCR 20 completely blocked interaction of Factor H with the ligands C3b, C3d, heparin and binding to endothelial cells. In contrast, several mAbs that bind to the N-terminus and to the middle regions of the molecule showed no or minor inhibitory effects when assayed by enzyme-linked immunosorbent assay (ELISA) and ligand interaction assays. This paradox between a single functional binding site identified for native Factor H versus multiple interaction sites reported for deletion constructs is explained by a compact conformation of the fluid phase protein with one accessible binding site. On zymosan particles mAbs C14 and C18 blocked alternative pathway activation completely. Thus demonstrating that native Factor H makes the first and initial contact with the C terminus, which is followed by N terminally mediated complement regulation. These results are explained by a conformational hypothetical model: the native Factor H protein has a compact structure and only one binding site accessible. Upon the first contact the protein unfolds and exposes the additional binding sites. This model does explain how Factor H mediates recognition functions during complement control and the clustering of disease associated mutations in patients with haemolytic uraemic syndrome that have been reported in the C-terminal recognition domain of Factor H.
Oppermann
M
M
Department of Immunology, University of Göttingen, Göttingen, Germany.
Manuelian
T
T
Józsi
M
M
Brandt
E
E
Jokiranta
T S
TS
Heinen
S
S
Meri
S
S
Skerka
C
C
Götze
O
O
Zipfel
P F
PF
D000911:Antibodies, Monoclonal / Q000276:immunology; D001666:Binding Sites, Antibody / Q000276:immunology; D003179:Complement C3b / Q000276:immunology; D015933:Complement C3d / Q000276:immunology; D017242:Complement Factor H / Q000235:genetics / Q000276:immunology; D003170:Complement Pathway, Alternative / Q000276:immunology; D042783:Endothelial Cells / Q000276:immunology; D000939:Epitopes / Q000276:immunology; D006493:Heparin / Q000276:immunology; D006801:Humans; D008024:Ligands; D008954:Models, Biological; D009154:Mutation; D011487:Protein Conformation; D015054:Zymosan / Q000276:immunology
D000911:Antibodies, Monoclonal; C493495:CFH protein, human; D000939:Epitopes; D008024:Ligands; D003179:Complement C3b; D015933:Complement C3d; D017242:Complement Factor H; D006493:Heparin; D015054:Zymosan