Figure 3.

C1r p.(D290G) variant strongly inhibits binding of the tetramer to C1q whereas C1r p.W435R induces a new cleavage site. (A) Size exclusion chromatography of the tetramers (containing C1r p.S654A with/without p.(D290G)) in the presence of CaCl₂ (plain lines) or EDTA (dotted lines). WT and variant C1r yielded similar major peaks in the presence of CaCl₂. In the presence of EDTA, the tetramer was dissociated into two later eluting peaks, corresponding to the C1r dimer and C1s monomer. (B) SPR analysis of the binding of the tetramers to immobilized C1q (14,300 RU). Ninety microliter of each tetramer (5 nM) were injected at a flow rate of 30 μl/min followed by 300 s dissociation. C1r p.(D290G) allows formation of the C1s-C1r-C1r-C1s tetramer but strongly inhibits binding of the tetramer to C1q. (C) N-terminal sequencing of C1r p.W435R identified two cleavage sites: IQYY (new cleavage) and IIGGQ (activation cleavage) indicated by red arrows. The preceding Arg is highlighted in green. The 3D structure shows proximity between p.W435R and the new cleavage site. The two cleavage sites correspond to C1r specificity (cleavage between Arg and Ile). In the SP domain disulfide bond maintaining integrity after activation cleavage (orange) and active serine (red) are marked (PDB ID 1GPZ).