Synergy between two active sites of human complement receptor type 1 (CD35) in complement regulation: implications for the structure of the classical pathway C3 convertase and generation of more potent inhibitors.
The extracellular domain of the complement receptor type 1 (CR1; CD35) consists entirely of 30 complement control protein repeats (CCPs). CR1 has two distinct functional sites, site 1 (CCPs 1-3) and two copies of site 2 (CCPs 8-10 and CCPs 15-17). In this report we further define the structural requirements for decay-accelerating activity (DAA) for the classical pathway (CP) C3 and C5 convertases and, using these results, generate more potent decay accelerators. Previously, we demonstrated that both sites 1 and 2, tandemly arranged, are required for efficient DAA for C5 convertases. We show that site 1 dissociates the CP C5 convertase, whereas the role of site 2 is to bind the C3b subunit. The intervening CCPs between two functional sites are required for optimal DAA, suggesting that a spatial orientation of the two sites is important. DAA for the CP C3 convertase is increased synergistically if two copies of site 1, particularly those carrying DAA-increasing mutations, are contained within one protein. DAA in such constructs may exceed that of long homologous repeat A (CCPs 1-7) by up to 58-fold. To explain this synergy, we propose a dimeric structure for the CP C3 convertase on cell surfaces. We also extended our previous studies of the amino acid requirements for DAA of site 1 and found that the CCP 1/CCP 2 junction is critical and that Phe82 may contact the C3 convertases. These observations increase our understanding of the mechanism of DAA. In addition, a more potent decay-accelerating form of CR1 was generated.
Krych-Goldberg
Malgorzata
M
Division of Rheumatology, Department of Internal Medicine, Washington University School of Medicine, St. Louis, MO 63110, USA.
Hauhart
Richard E
RE
Porzukowiak
Tina
T
Atkinson
John P
JP
D001665:Binding Sites; D018958:CD55 Antigens / Q000502:physiology; D050577:Complement C3-C5 Convertases / Q000037:antagonists & inhibitors / Q000737:chemistry* / Q000502:physiology*; D003179:Complement C3b / Q000378:metabolism; D003169:Complement Inactivator Proteins / Q000737:chemistry / Q000378:metabolism*; D003171:Complement Pathway, Classical / Q000235:genetics; D005998:Glycine / Q000235:genetics / Q000378:metabolism; D006801:Humans; D009154:Mutation; D010649:Phenylalanine / Q000235:genetics / Q000378:metabolism; D017434:Protein Structure, Tertiary; D017463:Receptors, Complement 3b / Q000235:genetics / Q000378:metabolism / Q000502:physiology*
D018958:CD55 Antigens; D003169:Complement Inactivator Proteins; D017463:Receptors, Complement 3b; D010649:Phenylalanine; D003179:Complement C3b; D050577:Complement C3-C5 Convertases; D005998:Glycine