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J Immunol. 2009 Jun 1;182(11):7009-18. doi: 10.4049/jimmunol.0804031.

The binding of factor H to a complex of physiological polyanions and C3b on cells is impaired in atypical hemolytic uremic syndrome.

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  • 1Department of Biochemistry, Center for Biomedical Research, University of Texas Health Science Center, Tyler, TX 75708, USA.

Abstract

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.

PMID:
19454698
[PubMed - indexed for MEDLINE]
PMCID:
PMC2696619
Free PMC Article
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