Keying Li

University College London, London, ENG, United Kingdom

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Publications (10)29.56 Total impact

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    ABSTRACT: The solution structure of complement C3b is crucial for the understanding of complement activation and regulation. C3b is generated by the removal of C3a from C3. Hydrolysis of the C3 thioester produces C3u, an analogue of C3b. C3b cleavage results in C3c and C3d (TED). To resolve functional questions in relation to C3b and C3u, analytical ultracentrifugation and X-ray and neutron scattering studies were used with C3, C3b, C3u, C3c and C3d, using the wild-type allotype with R102. In 50 mM NaCl buffer, atomistic scattering modelling showed that both C3b and C3u adopted a compact structure, similar to the C3b crystal structure in which its TED and MG1 domains (MG: macroglobulin) were connected through the R102-E1032 salt-bridge. In physiological 137 mM NaCl, scattering modelling showed that C3b and C3u were both extended in structure with the TED and MG1 domains now separated by up to 6 nm. The importance of the R102-E1032 salt-bridge was determined using surface plasmon resonance to monitor the binding of wild-type C3d(E1032) and mutant C3d(A1032) to immobilised C3c. The mutant did not bind while the wild-type form did. The high conformational variability of TED in C3b in physiological buffer showed that C3b is more reactive than previously thought. Because the R102-E1032 salt-bridge is essential for the C3b-Factor H complex during the regulatory control of C3b, the known clinical associations of the major C3S (R102) and disease-linked C3F (G102) allotypes of C3b were experimentally explained for the first time. Copyright © 2014, The American Society for Biochemistry and Molecular Biology.
    The Journal of biological chemistry. 12/2014;
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    ABSTRACT: A novel therapeutic reagent TT30 was designed to be effective in diseases of the alternative pathway of complement such as paroxysmal nocturnal hemoglobinuria and other diseases. TT30 is constructed from the first four short complement regulator (SCR) domains of complement receptor type 2 (CR2) that bind to complement C3d, followed by the first five SCR domains of complement factor H that bind to complement C3b. In order to assess how TT30 binds to C3d and C3b, we determined the TT30 solution structure by a combination of analytical ultracentrifugation, X-ray scattering and constrained modeling. The sedimentation coefficients and radius of gyration of TT30 were unaffected by citrate or phosphate-buffered saline buffers and indicate an elongated monomeric structure with a sedimentation coefficient of 3.1 S and a radius of gyration R(G) of 6.9 nm. Molecular modeling starting from 3000 randomized TT30 conformations showed that high-quality X-ray curve fits were obtained with extended SCR arrangements, showing that TT30 has a limited degree of inter-SCR flexibility in its solution structure. The best-fit TT30 structural models are readily merged with the crystal structure of C3b to show that the four CR2 domains extend freely into solution when the five complement factor H domains are bound within C3b. We reevaluated the solution structure of the CR2-C3d complex that confirmed its recent crystal structure. This recent CR2-C3d crystal structure showed that TT30 is able to interact readily with C3d ligands in many orientations when TT30 is bound to C3b.
    Journal of Molecular Biology 03/2012; 418(3-4):248-63. · 3.91 Impact Factor
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    ABSTRACT: Factor H (FH) is the major plasma regulator of the central complement protein C3b in the alternative pathway of complement activation. The elucidation of the FH interactions with five major ligands (below) is complicated by their weak μM dissociation constants K(D) and FH multivalency. We present the first survey of all the K(D) values for the major FH-ligand interactions and critically review their physiological significance. (i) FH self-association is presently well-established. We review multiple data sets that show that 5-14% of FH is self-associated in physiological conditions. FH self-association is significant for both laboratory investigations and physiological function.(ii) The FH-C3b complex shows low M affinity, meaning that the complex is not fully formed in plasma. In addition, C3, its hydrolysed form C3u, and its cleaved forms C3b and C3d show multimerisation. Current data favour a model when two C3b molecules bind independently to one FH molecule, as opposed to a1:1 stoichiometry where FH wraps itself around C3b.(iii) Heparin is often used as an analogue of the polyanionic host cell surface. The FH-heparin complex also shows a low M affinity, again meaning that complexes are not fully formed in vivo. The oligomeric FH-heparin complexes clarify a two-site interaction model of FH with host-cell surfaces.(iv) Reinvestigation of the FH and C-reactive protein (CRP) interaction revealed that this can only occur in plasma when CRP levels are elevated during acute-phase conditions. Given that CRP binds more weakly to the His402 allotype of FH than the Tyr402 allotype, this suggested a link with age-related macular degeneration (AMD).(v) FH activity is inhibited by zinc, which causes FH to aggregate strongly. High levels of bioavailable zinc occur in sub-retinal pigment epithelial deposits which lead to AMD. Excess zinc binds weakly to a central region of FH, explaining how zinc inhibits FH regulation of C3b.
    Immunobiology 10/2011; 217(2):281-97. · 2.81 Impact Factor
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    ABSTRACT: Analytical ultracentrifugation and solution scattering provide different multi-parameter structural and compositional information on proteins. The joint application of the two methods supplements high resolution structural studies by crystallography and NMR. We summarise the procedures required to obtain equivalent ultracentrifugation and X-ray and neutron scattering data. The constrained modelling of ultracentrifugation and scattering data is important to confirm the experimental data analysis and yields families of best-fit molecular models for comparison with crystallography and NMR structures. This modelling of ultracentrifugation and scattering data is described in terms of starting models, their conformational randomisation in trial-and-error fits, and the identification of the final best-fit models. Seven applications of these methods are described to illustrate the current state-of-the-art. These include the determination of antibody solution structures (the human IgG4 subclass, and oligomeric forms of human IgA and its secretory component), the solution structures of the complement proteins of innate immunity (Factor H and C3/C3u) and their interactions with macromolecular ligands (C-reactive protein), and anionic polysaccharides (heparin). Complementary features of joint ultracentrifugation and scattering experiments facilitate an improved understanding of crystal structures (illustrated for C3/C3u, C-reactive protein and heparin). If a large protein or its complex cannot be crystallised, the joint ultracentrifugation-scattering approach provides a means to obtain an overall macromolecular structure.
    Methods 01/2011; 54(1):181-99. · 3.64 Impact Factor
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    ABSTRACT: Component C3 is the central protein of the complement system. During complement activation, the thioester group in C3 is slowly hydrolysed to form C3u, then the presence of C3u enables the rapid conversion of C3 into functionally active C3b. C3u shows functional similarities to C3b. To clarify this mechanism, the self-association properties and solution structures of C3 and C3u were determined using analytical ultracentrifugation and X-ray scattering. Sedimentation coefficients identified two different dimerization events in both proteins. A fast dimerization was observed in 50 mM NaCl but not in 137 mM NaCl. Low amounts of a slow dimerization was observed for C3u and C3 in both buffers. The X-ray radius of gyration RG values were unchanged for both C3 and C3u in 137 mM NaCl, but depend on concentration in 50 mM NaCl. The C3 crystal structure gave good X-ray fits for C3 in 137 mM NaCl. By randomization of the TED (thioester-containing domain)/CUB (for complement protein subcomponents C1r/C1s, urchin embryonic growth factor and bone morphogenetic protein 1) domains in the C3b crystal structure, X-ray fits showed that the TED/CUB domains in C3u are extended and differ from the more compact arrangement of C3b. This TED/CUB conformation is intermediate between those of C3 and C3b. The greater exposure of the TED domain in C3u (which possesses the hydrolysed reactive thioester) accounts for the greater self-association of C3u in low-salt conditions. This conformational variability of the TED/CUB domains would facilitate their interactions with a broad range of antigenic surfaces. The second dimerization of C3 and C3u may correspond to a dimer observed in one of the crystal structures of C3b.
    Biochemical Journal 10/2010; 431(1):63-72. · 4.65 Impact Factor
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    ABSTRACT: Factor H (FH) is the major regulator of the central complement protein C3b in the alternative pathway of complement activation, and is comprised of 20 SCR domains. A FH Tyr402His polymorphism in SCR-7 is associated with age-related macular degeneration (AMD) and leads to deposition of complement in drusen. The unravelling of how FH interacts with five major physiological and patho-physiological ligands is complicated by the weak nature of these interactions, coupled with the multivalency of FH. Using multiple biophysical methods, we summarise our recent results for these five FH ligands: (1) FH by itself shows a folded-back SCR domain structure in solution, and self-associates in a manner dependent on electrostatic forces. (2) FH activity is inhibited by zinc, which causes FH to aggregate. The onset of FH-zinc aggregation for zinc concentrations above 20 muM appears to be enhanced with the His402 allotype, and may be relevant to AMD. (3) The FH and C-reactive protein (CRP) interaction has been controversial; however our new work resolves earlier discrepancies. The FH-CRP interaction is only observed when native CRP is at high acute-phase concentration levels, and CRP binds weakly to the His402 FH allotype to suggest a molecular mechanism that leads to AMD. (4) Heparin is an analogue of the polyanionic host cell surface, and FH forms higher oligomers with larger heparin fragments, suggesting a mechanism for more effective FH regulation. (5) The interaction of C3b with FH also depends on buffer, and FH forms multimers with the C3d fragment of C3b. This FH-C3d interaction at high FH concentration may also facilitate complement regulation. Overall, our results to date suggest that the FH interactions involving zinc and native CRP have the closest relevance for explaining the onset of AMD.
    Advances in experimental medicine and biology 01/2010; 703:25-47. · 1.83 Impact Factor
  • Molecular Immunology - MOL IMMUNOL. 01/2010; 47(13):2262-2263.
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    ABSTRACT: X-ray and neutron-scattering techniques characterize proteins in solution and complement high-resolution structural studies. They are useful when either a large protein cannot be crystallized, in which case scattering yields a solution structure, or a crystal structure has been determined and requires validation in solution. These solution structures are determined by the application of constrained modelling methods based on known subunit structures. First, an appropriate starting model is generated. Next, its conformation is randomized to generate thousands of models for trial-and-error fits. Comparison with the experimental data identifies a small family of best-fit models. Finally, their significance for biological function is assessed. We illustrate this in application to structure determinations for secretory immunoglobulin A, the most prevalent antibody in the human body and a first line of defence in mucosal immunity. We also discuss the applications to the large multi-domain proteins of the complement system, most notably its major regulator factor H, which is important in age-related macular degeneration and renal diseases. We discuss the importance of complementary data from analytical ultracentrifugation, and structural studies of protein-protein complexes. We conclude that constrained scattering modelling makes useful contributions to our understanding of antibody and complement structure and function.
    Journal of The Royal Society Interface 08/2009; 6 Suppl 5:S679-96. · 4.91 Impact Factor
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    ABSTRACT: Activation of C3 to C3b signals the start of the alternative complement pathway. The C-terminal short complement regulator (SCR)-20 domain of factor H (FH), the major serum regulator of C3b, possesses a binding site for C3d, a 35-kDa physiological fragment of C3b. Size distribution analyses of mixtures of SCR-16/20 or FH with C3d by analytical ultracentrifugation in 50 and 137 mM NaCl buffer revealed a range of discrete peaks, showing that multimeric complexes had formed at physiologically relevant concentrations. Surface plasmon resonance studies showed that native FH binds C3d in two stages. An equilibrium dissociation constant K(D)(1) of 2.6 microM in physiological buffer was determined for the first stage. Overlay experiments indicated that C3d formed multimeric complexes with FH. X-ray scattering showed that the maximum dimension of the C3d complexes with SCR-16/20 at 29 nm was not much longer than that of the unbound SCR-16/20 dimer. Molecular modelling suggested that the ultracentrifugation and scattering data are most simply explained in terms of associating dimers of each of SCR-16/20 and C3d. We conclude that the physiological interaction between FH and C3d is not a simple 1:1 binding stoichiometry between the two proteins that is often assumed. Because the multimers involve the C-terminus of FH, which is bound to host cell surfaces, our results provide new insight on FH regulation during excessive complement activation, both in the fluid phase and at host cell surfaces decorated by C3d.
    Journal of Molecular Biology 07/2009; 391(1):119-35. · 3.91 Impact Factor
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    ABSTRACT: Complement receptor type 2 (CR2, CD21) is a cell surface protein that links the innate and adaptive immune response during the activation of B-cells through its binding to C3d, a cleavage fragment of the major complement component C3. The extracellular portion of CR2 comprises 15 or 16 short complement regulator (SCR) domains in a partially folded-back but flexible structure. Here, the effect of C3d binding to CR2 was determined by analytical ultracentrifugation and X-ray scattering. The sedimentation coefficient of unbound CR2 is 4.03 S in 50 mM NaCl. Because this agrees well with a value of 3.93 S in 137 mM NaCl, the overall CR2 structure is unaffected by change in ionic strength. Unbound C3d exists in monomer-dimer and monomer-trimer equilibria in 50 mM NaCl, but as a monomer only in 137 mM NaCl. In c(s) size-distribution analyses, an equimolar mixture of the CR2–C3d complex in 50 mM NaCl revealed a single peak shifted to 4.52 S when compared to unbound CR2 at 4.03 S to show that the complex had formed. The CR2–C3d complex in 137 mM NaCl showed two peaks at 2.52 S and 4.07 S to show that this had dissociated. Solution structural models for the CR2 SCR-1/2 complex with C3d and CR2 SCR-1/15 were superimposed. These gave an average sedimentation coefficient of 4.57 S for the complex, in good agreement with the observed value of 4.52 S. It is concluded that CR2 does not detectably change conformation when C3d is bound to it. Consistent with previous analyses, its C3d complex is not formed in physiological salt conditions. The implications of these solution results for its immune role are discussed. To our knowledge, this is the first solution structural study of a large multidomain SCR protein CR2 bound to its physiological ligand C3d.
    Journal of Molecular Biology 10/2008; · 3.91 Impact Factor