The Binding of Factor H to a Complex of Physiological Polyanions and C3b on Cells Is Impaired in Atypical Hemolytic Uremic Syndrome

Department of Biochemistry, Center for Biomedical Research, University of Texas Health Science Center, Tyler, TX 75708, USA.
The Journal of Immunology (Impact Factor: 4.92). 07/2009; 182(11):7009-18. DOI: 10.4049/jimmunol.0804031
Source: PubMed

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.

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Available from: Dusan Uhrin, Jun 11, 2014
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    • "While direct binding to C3b occurs mainly through CCP 19 and the CCP 19–20 inter-modular junction, residues exposed on CCP 18 could nonetheless play a role in the encounter between the C terminus of FH and C3b and therefore modulate the ability of FH to control C3b amplification on host surfaces. In previous work reversal-of-charge mutations in CCPs 19 and 20 were found to influence affinity of FH19–20 for C3d/C3b even when they did not lie directly in the interface between these two molecules as visualized in the crystal structure of the FH19–20:C3d complex [17], [18], [59], [60]. Such observations were attributed to electrostatic steering. "
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    ABSTRACT: Factor H (FH) is a soluble regulator of the human complement system affording protection to host tissues. It selectively inhibits amplification of C3b, the activation-specific fragment of the abundant complement component C3, in fluid phase and on self-surfaces and accelerates the decay of the alternative pathway C3 convertase, C3bBb. We have determined the crystal structure of the three carboxyl-terminal complement control protein (CCP) modules of FH (FH18-20) that bind to C3b, and which additionally recognize polyanionic markers specific to self-surfaces. These CCPs harbour nearly 30 disease-linked missense mutations. We have also deployed small-angle X-ray scattering (SAXS) to investigate FH18-20 flexibility in solution using FH18-20 and FH19-20 constructs. In the crystal lattice FH18-20 adopts a "J"-shape: A ~122-degree tilt between the structurally highly similar modules 18 and 19 precedes an extended, linear arrangement of modules 19 and 20 as observed in previously determined structures of these two modules alone. However, under solution conditions FH18-20 adopts multiple conformations mediated by flexibility between CCPs 18 and 19. We also pinpoint the locations of disease-associated missense mutations on the module 18 surface and discuss our data in the context of the C3b:FH interaction.
    PLoS ONE 02/2012; 7(2):e32187. DOI:10.1371/journal.pone.0032187 · 3.23 Impact Factor
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    • "Molecules carrying these mutations in domain 20 have been shown to be defective in their ability to bind either C3b, polyanions, or both (Ferreira et al., 2009; Kavanagh et al., 2006; Lehtinen et al., 2009; Schmidt et al., 2008a), or surprisingly, in some cases to have an increased affinity for C3b, for both C3b and polyanions (Ferreira et al., 2009) or polyanions only (Lehtinen et al., 2009). In one study (Ferreira et al., 2009), the affinity for C3b, as measured by Biacore, correlated well with binding to C3b-coated zymosan (which lacks polyanions). Nevertheless, affinities for C3b and heparin (when measured separately) did not correlate consistently with the strength of binding to polyanion-rich human and sheep erythrocyte surfaces on which C3b has been deposited; nor did they correlate with the disease-risk phenotype. "
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