The C-terminus of complement regulator Factor H mediates target recognition: evidence for a compact conformation of the native protein

The complement inhibitor Factor H has three distinct binding sites for C3b and for heparin, but in solution uses specifically the most C-terminal domain, i.e. short consensus repeats (SCR) 20 for ligand interaction. Two novel monoclonal antibodies (mABs C14 and C18) that bind to the most C-terminal domain SCR 20 completely blocked interaction of Factor H with the ligands C3b, C3d, heparin and binding to endothelial cells. In contrast, several mAbs that bind to the N-terminus and to the middle regions of the molecule showed no or minor inhibitory effects when assayed by enzyme-linked immunosorbent assay (ELISA) and ligand interaction assays. This paradox between a single functional binding site identified for native Factor H versus multiple interaction sites reported for deletion constructs is explained by a compact conformation of the fluid phase protein with one accessible binding site. On zymosan particles mAbs C14 and C18 blocked alternative pathway activation completely. Thus demonstrating that native Factor H makes the first and initial contact with the C terminus, which is followed by N terminally mediated complement regulation. These results are explained by a conformational hypothetical model: the native Factor H protein has a compact structure and only one binding site accessible. Upon the first contact the protein unfolds and exposes the additional binding sites. This model does explain how Factor H mediates recognition functions during complement control and the clustering of disease associated mutations in patients with haemolytic uraemic syndrome that have been reported in the C-terminal recognition domain of Factor H

Complement in age-related macular degeneration: a focus on function

Age-related macular degeneration (AMD) is an inflammatory disease, which causes visual impairment and blindness in older people. The proteins of the complement system are central to the development of this disease. Local and systemic inflammation in AMD are mediated by the deregulated action of the alternative pathway of the complement system. Variants in complement system genes alter an individual's risk of developing AMD. Recent studies have shown how some risk-associated genetic variants alter the function of the complement system. In this review, we describe the evolution of the complement system and bring together recent research to form a picture of how changes in complement system genes and proteins affect the function of the complement cascade, and how this affects the development of AMD. We discuss the application of this knowledge to prevention and possible future treatments of AMD

How novel structures inform understanding of complement function

33 p.-3 fig.During the last decade, the complement field has experienced outstanding advancements in the mechanistic understanding of how complement activators are recognized, what C3 activation means, how protein complexes like the C3 convertases and the membrane attack complex are assembled, and how positive and negative complement regulators perform their function. All of this has been made possible mostly because of the contributions of structural biology to the study of the complement components. The wealth of novel structural data has frequently provided support to previously held knowledge, but often has added alternative and unexpected insights into complement function. Here we will review some of these findings focusing in the alternative and terminal complement pathways.SRdeC is supported by the Spanish “Ministerio de Economía y Competitividad-FEDER” (SAF2015-66287R), the Seventh Framework Programme European Union Project EURenOmics (305608) and the Autonomous Region of Madrid (S2010/BMD- 2316). SRdeC is member of the “CIB intramural Program “Molecular Machines for Better Life (MACBET)”. EGdeJ is supported by the Spanish “Ministerio de Economía y Competitividad-FEDER” (RYC-2013-13395 and SAF2014-52339P). OL is supported by the Spanish Ministry of Economy, Industry and Competitiveness (SAF2014-52301-R).AT and MS are supported by the Spanish “Ministerio de Economía y Competitividad-FEDER” (IJCI-2015-25222 and IJCI-2015-24388, respectively).Peer reviewe