Mutational analyses of the recombinant globular regions of human C1q A, B, and C chains suggest an essential role for arginine and histidine residues in the C1q-IgG interaction.
The first step in the activation of the classical complement pathway by immune complexes involves the binding of the globular domain (gC1q) of C1q to the Fc regions of aggregated IgG or IgM. Each gC1q domain is a heterotrimer of the C-terminal halves of one A (ghA), one B (ghB), and one C (ghC) chain. Our recent studies have suggested a modular organization of gC1q, consistent with the view that ghA, ghB, and ghC are functionally autonomous modules and have distinct and differential ligand-binding properties. Although C1q binding sites on IgG have been previously identified, the complementary interacting sites on the gC1q domain have not been precisely defined. The availability of the recombinant constructs expressing ghA, ghB, and ghC has allowed us, for the first time, to engineer single-residue substitution mutations and identify residues on the gC1q domain, which are involved in the interaction between C1q and IgG. Because C1q is a charge pattern recognition molecule, we have sequentially targeted arginine and histidine residues in each chain. Consistent with previous chemical modification studies and the recent crystal structure of gC1q, our results support a central role for arginine and histidine residues, especially Arg(114) and Arg(129) of the ghB module, in the C1q-IgG interaction.
Kojouharova
Mihaela S
MS
Department of Biochemistry, Sofia University, St. Kliment Ohridski, Sofia, Bulgaria.
Gadjeva
Mihaela G
MG
Tsacheva
Ivanka G
IG
Zlatarova
Aleksandra
A
Roumenina
Liubka T
LT
Tchorbadjieva
Magdalena I
MI
Atanasov
Boris P
BP
Waters
Patrick
P
Urban
Britta C
BC
Sim
Robert B
RB
Reid
Kenneth B M
KB
Kishore
Uday
U
D000409:Alanine / Q000235:genetics; D000818:Animals; D001120:Arginine / Q000235:genetics; D003174:Complement C1 Inactivator Proteins / Q000502:physiology; D015922:Complement C1q / Q000037:antagonists & inhibitors / Q000235:genetics* / Q000378:metabolism; D004252:DNA Mutational Analysis / Q000379:methods; D004912:Erythrocytes / Q000276:immunology; D004926:Escherichia coli / Q000235:genetics; D006461:Hemolysis / Q000276:immunology; D006639:Histidine / Q000235:genetics; D006801:Humans; D007074:Immunoglobulin G / Q000378:metabolism*; D016297:Mutagenesis, Site-Directed; D017354:Point Mutation; D021122:Protein Subunits / Q000235:genetics* / Q000502:physiology; D011994:Recombinant Proteins / Q000235:genetics*; D012756:Sheep
D003174:Complement C1 Inactivator Proteins; D007074:Immunoglobulin G; D021122:Protein Subunits; D011994:Recombinant Proteins; D006639:Histidine; D015922:Complement C1q; D001120:Arginine; D000409:Alanine