Structure-based mapping of DAF active site residues that accelerate the decay of C3 convertases.
Focused complement activation on foreign targets depends on regulatory proteins that decay the bimolecular C3 convertases. Although this process is central to complement control, how the convertases engage and disassemble is not established. The second and third complement control protein (CCP) modules of the cell surface regulator, decay-accelerating factor (DAF, CD55), comprise the simplest structure mediating this activity. Positioning the functional effects of 31 substitution mutants of DAF CCP2 to -4 on partial structures was previously reported. In light of the high resolution crystal structure of the DAF four-CCP functional region, we now reexamine the effects of these and 40 additional mutations. Moreover, we map six monoclonal antibody epitopes and overlap their effects with those of the amino acid substitutions. The data indicate that the interaction of DAF with the convertases is mediated predominantly by two patches approximately 13 A apart, one centered around Arg69 and Arg96 on CCP2 and the other around Phe148 and Leu171 on CCP3. These patches on the same face of the adjacent modules bracket an intermodular linker of critical length (16 A.) Although the key DAF residues in these patches are present or there are conservative substitutions in all other C3 convertase regulators that mediate decay acceleration and/or provide factor I-cofactor activity, the linker region is highly conserved only in the former. Intra-CCP regions also differ. Linker region comparisons suggest that the active CCPs of the decay accelerators are extended, whereas those of the cofactors are tilted. Intra-CCP comparisons suggest that the two classes of regulators bind different regions on their respective ligands.
Kuttner-Kondo
Lisa
L
Institute of Pathology, Case Western Reserve University, Cleveland, Ohio 44106.
Hourcade
Dennis E
DE
Department of Medicine, Washington University School of Medicine, St. Louis, Missouri 63110.
Anderson
Vernon E
VE
Department of Biochemistry, Case Western Reserve University, Cleveland, Ohio 44106.
Muqim
Nasima
N
Institute of Pathology, Case Western Reserve University, Cleveland, Ohio 44106.
Mitchell
Lynne
L
Department of Medicine, Washington University School of Medicine, St. Louis, Missouri 63110.
Soares
Dinesh C
DC
Institute of Structural and Molecular Biology and School of Chemistry, University of Edinburgh, Edinburgh EH9 3JJ, Scotland, United Kingdom.
Barlow
Paul N
PN
Institute of Structural and Molecular Biology and School of Chemistry, University of Edinburgh, Edinburgh EH9 3JJ, Scotland, United Kingdom.
Medof
M Edward
ME
Institute of Pathology, Case Western Reserve University, Cleveland, Ohio 44106. Electronic address: mxm16@po.cwru.edu.
D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D018958:CD55 Antigens / Q000737:chemistry* / Q000378:metabolism; D050577:Complement C3-C5 Convertases / Q000737:chemistry* / Q000378:metabolism; D018360:Crystallography, X-Ray; D000939:Epitopes / Q000737:chemistry; D006801:Humans; D007930:Leucine / Q000737:chemistry; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009154:Mutation; D010649:Phenylalanine / Q000737:chemistry; D011485:Protein Binding; D017386:Sequence Homology, Amino Acid
D018958:CD55 Antigens; D000939:Epitopes; D010649:Phenylalanine; D050577:Complement C3-C5 Convertases; D007930:Leucine