The binding of factor H to a complex of physiological polyanions and C3b on cells is impaired in atypical hemolytic uremic syndrome.
Factor H (fH) is essential for complement homeostasis in fluid-phase and on surfaces. Its two C-terminal domains (CCP 19-20) anchor fH to self-surfaces where it prevents C3b amplification in a process requiring its N-terminal four domains. In atypical hemolytic uremic syndrome (aHUS), mutations clustering toward the C terminus of fH may disrupt interactions with surface-associated C3b or polyanions and thereby diminish the ability of fH to regulate complement. To test this, we compared a recombinant protein encompassing CCP 19-20 with 16 mutants. The mutations had only very limited and localized effects on protein structure. Although we found four aHUS-linked fH mutations that decreased binding to C3b and/or to heparin (a model compound for cell surface polyanionic carbohydrates), we identified five aHUS-associated mutants with increased affinity for either or both ligands. Strikingly, these variable affinities for the individual ligands did not correlate with the extent to which all the aHUS-associated mutants were found to be impaired in a more physiological assay that measured their ability to inhibit cell surface complement functions of full-length fH. Taken together, our data suggest that disruption of a complex fH-self-surface recognition process, involving a balance of affinities for protein and physiological carbohydrate ligands, predisposes to aHUS.
Ferreira
Viviana P
VP
Department of Biochemistry, Center for Biomedical Research, University of Texas Health Science Center, Tyler, TX 75708, USA.
Herbert
Andrew P
AP
Cortés
Claudio
C
McKee
Kristi A
KA
Blaum
Bärbel S
BS
Esswein
Stefan T
ST
Uhrín
Dusan
D
Barlow
Paul N
PN
Pangburn
Michael K
MK
Kavanagh
David
D
D000818:Animals; D002478:Cells, Cultured; D003179:Complement C3b / Q000378:metabolism*; D017242:Complement Factor H / Q000235:genetics / Q000378:metabolism*; D004912:Erythrocytes / Q000276:immunology* / Q000473:pathology; D020022:Genetic Predisposition to Disease; D006463:Hemolytic-Uremic Syndrome / Q000235:genetics* / Q000276:immunology / Q000473:pathology; D006493:Heparin / Q000378:metabolism*; D006801:Humans; D009154:Mutation; D000071228:Polyelectrolytes; D011108:Polymers / Q000378:metabolism; D011485:Protein Binding / Q000235:genetics; D012756:Sheep
D003179:Complement C3b; D017242:Complement Factor H; D006493:Heparin; D000071228:Polyelectrolytes; D011108:Polymers