Structural Basis for the Function of Complement Component C4 within the Classical and Lectin Pathways of Complement.
Complement component C4 is a central protein in the classical and lectin pathways within the complement system. During activation of complement, its major fragment C4b becomes covalently attached to the surface of pathogens and altered self-tissue, where it acts as an opsonin marking the surface for removal. Moreover, C4b provides a platform for assembly of the proteolytically active convertases that mediate downstream complement activation by cleavage of C3 and C5. In this article, we present the crystal and solution structures of the 195-kDa C4b. Our results provide the molecular details of the rearrangement accompanying C4 cleavage and suggest intramolecular flexibility of C4b. The conformations of C4b and its paralogue C3b are shown to be remarkably conserved, suggesting that the convertases from the classical and alternative pathways are likely to share their overall architecture and mode of substrate recognition. We propose an overall molecular model for the classical pathway C5 convertase in complex with C5, suggesting that C3b increases the affinity for the substrate by inducing conformational changes in C4b rather than a direct interaction with C5. C4b-specific features revealed by our structural studies are probably involved in the assembly of the classical pathway C3/C5 convertases and C4b binding to regulators.
Department of Molecular Biology and Genetics, Aarhus University, DK-8000 Aarhus, Denmark;
Sofia
S
Mortensen
Department of Molecular Biology and Genetics, Aarhus University, DK-8000 Aarhus, Denmark;
Rune T
RT
Kidmose
Department of Biomedicine, Aarhus University, DK-8000 Aarhus, Denmark; and.
Steen V
SV
Petersen
3rd Department of Internal Medicine, Semmelweis University, Budapest 1125, Hungary.
Ágnes
Á
Szilágyi
3rd Department of Internal Medicine, Semmelweis University, Budapest 1125, Hungary.
Zoltan
Z
Prohászka
Department of Molecular Biology and Genetics, Aarhus University, DK-8000 Aarhus, Denmark; gra@mbg.au.dk.
Gregers R
http://orcid.org/0000-0001-6292-3319
GR
Andersen
D003167:Complement Activation / Q000276:immunology*; D050577:Complement C3-C5 Convertases / Q000378:metabolism; D003179:Complement C3b / Q000235:genetics / Q000276:immunology; D015935:Complement C4b / Q000737:chemistry* / Q000276:immunology; D003182:Complement C5 / Q000235:genetics / Q000276:immunology; D003171:Complement Pathway, Classical / Q000276:immunology*; D037582:Complement Pathway, Mannose-Binding Lectin / Q000276:immunology*; D018360:Crystallography, X-Ray; D006801:Humans; D009895:Opsonin Proteins / Q000276:immunology; D011485:Protein Binding / Q000276:immunology; D011499:Protein Processing, Post-Translational; D017434:Protein Structure, Tertiary
D003182:Complement C5; D009895:Opsonin Proteins; D003179:Complement C3b; D015935:Complement C4b; D050577:Complement C3-C5 Convertases