SummaryThe 155-kDa glycoprotein, complement factor H (CFH), is a regulator of complement activation that is abundant in human plasma. Threedimensional structures of over half the 20 complement control protein (CCP) modules in CFH have been solved in the context of single-, double-and triple-module segments. Proven binding sites for C3b occupy the N and C termini of this elongated molecule and may be brought together by a bend in CFH mediated by its central CCP modules. The C-terminal CCP 20 is key to the ability of the molecule to adhere to polyanionic markers on self-surfaces where CFH acts to regulate amplification of the alternative pathway of complement. The surface patch on CCP 20 that binds to model glycosaminoglycans has been mapped using nuclear magnetic resonance (NMR), as has a second glycosaminoglycan-binding patch on CCP 7. These patches include many of the residue positions at which sequence variations have been linked to three complement-mediated disorders: dense deposit disease, age-related macular degeneration and atypical haemolytic uraemic syndrome. In one plausible model, CCP 20 anchors CFH to self-surfaces via a C3b/polyanion composite binding site, CCP 7 acts as a 'proof-reader' to help discriminate selffrom non-self patterns of sulphation, and CCPs 1-4 disrupt C3/C5 convertase formation and stability.

The plasma protein factor H primarily controls the activation of the alternative pathway of complement. The C-terminal of factor H is known to be involved in protection of host cells from complement attack. In the present study, we show that domains 19–20 alone are capable of discriminating between host-like and complement-activating cells. Furthermore, although factor H possesses three binding sites for C3b, binding to cell-bound C3b can be almost completely inhibited by the single site located in domains 19–20. All of the regulatory activities of factor H are expressed by the N-terminal four domains, but these activities toward cell-bound C3b are inhibited by isolated recombinant domains 19–20 (rH 19–20). Direct competition with the N-terminal site is unlikely to explain this because regulation of fluid phase C3b is unaffected by domains 19–20. Finally, we show that addition of isolated rH 19–20 to normal human serum leads to aggressive complement-mediated lysis of normally nonactivating sheep erythrocytes and moderate lysis of human erythrocytes, which possess membrane-bound regulators of complement. Taken together, the results highlight the importance of the cell surface protective functions exhibited by factor H compared with other complement regulatory proteins. The results may also explain why atypical hemolytic uremic syndrome patients with mutations affecting domains 19–20 can maintain complement homeostasis in plasma while their complement system attacks erythrocytes, platelets, endothelial cells, and kidney tissue.

Factor H is a regulatory glycoprotein of the complement system. We expressed the three N-terminal complement control protein modules of human factor H (FH1-3) and confirmed FH1-3 to be the minimal unit with cofactor activity for C3b proteolysis by factor I. We reconstructed FH1-3 from NMR-derived structures of FH1-2 and FH2-3 revealing an ϳ105-Å -long rodlike arrangement of the modules. In structural comparisons with other C3b-engaging proteins, factor H module 3 most closely resembles factor B module 3, consistent with factor H competing with factor B for binding C3b. Factor H modules 1, 2, and 3 each has a similar backbone structure to first, second, and third modules, respectively, of functional sites in decay accelerating factor and complement receptor type 1; the equivalent intermodular tilt and twist angles are also broadly similar. Resemblance between molecular surfaces is closest for first modules but absent in the case of second modules. Substitution of buried Val-62 with Ile (a factor H single nucleotide polymorphism potentially protective for age-related macular degeneration and dense deposit disease) causes rearrangements within the module 1 core and increases thermal stability but does not disturb the interface with module 2. Replacement of partially exposed (in module 1) Arg-53 by His (an atypical hemolytic uremic syndrome-linked mutation) did not impair structural integrity at 37°C, but this FH1-2 mutant was less stable at higher temperatures; furthermore, chemical shift differences indicated potential for small structural changes at the module 1-2 interface.

Characterisation of the structure and dynamics of large biomolecules and biomolecular complexes by NMR spectroscopy is hampered by increasing overlap and severe broadening of NMR signals. As a consequence, the number of available NMR spectroscopy data is often sparse and new approaches to provide complementary NMR spectroscopy data are needed. Paramagnetic relaxation enhancements (PREs) obtained from inert and soluble paramagnetic probes (solvent PREs) provide detailed quantitative information about the solvent accessibility of NMR-active nuclei. Solvent PREs can be easily measured without modification of the biomolecule; are sensitive to molecular structure and dynamics; and are therefore becoming increasingly powerful for the study of biomolecules, such as proteins, nucleic acids, ligands and their complexes in solution. In this Minireview, we give an overview of the available solvent PRE probes and discuss their applications for structural and dynamic characterisation of biomolecules and biomolecular complexes.

BACKGROUND AND PURPOSE The µ‐conopeptide family is defined by its ability to block voltage‐gated sodium channels (VGSCs), a property that can be used for the development of myorelaxants and analgesics. We characterized the pharmacology of a new µ‐conopeptide (µ‐CnIIIC) on a range of preparations and molecular targets to assess its potential as a myorelaxant. EXPERIMENTAL APPROACH µ‐CnIIIC was sequenced, synthesized and characterized by its direct block of elicited twitch tension in mouse skeletal muscle and action potentials in mouse sciatic and pike olfactory nerves. µ‐CnIIIC was also studied on HEK‐293 cells expressing various rodent VGSCs and also on voltage‐gated potassium channels and nicotinic acetylcholine receptors (nAChRs) to assess cross‐interactions. Nuclear magnetic resonance (NMR) experiments were carried out for structural data. KEY RESULTS Synthetic µ‐CnIIIC decreased twitch tension in mouse hemidiaphragms (IC50= 150 nM), and displayed a higher blocking effect in mouse extensor digitorum longus muscles (IC = 46 nM), compared with µ‐SIIIA, µ‐SmIIIA and µ‐PIIIA. µ‐CnIIIC blocked NaV1.4 (IC50= 1.3 nM) and NaV1.2 channels in a long‐lasting manner. Cardiac NaV1.5 and DRG‐specific NaV1.8 channels were not blocked at 1 µM. µ‐CnIIIC also blocked the α3β2 nAChR subtype (IC50= 450 nM) and, to a lesser extent, on the α7 and α4β2 subtypes. Structure determination of µ‐CnIIIC revealed some similarities to α‐conotoxins acting on nAChRs. CONCLUSION AND IMPLICATIONS µ‐CnIIIC potently blocked VGSCs in skeletal muscle and nerve, and hence is applicable to myorelaxation. Its atypical pharmacological profile suggests some common structural features between VGSCs and nAChR channels.