FIGURE 4.

NMR titration analysis reveals that SCR1 and SCR2 of CR2 are both involved in the ligation with C3d and C3d-inhibitory peptide C3dp1. A and B, five superimposed ¹H-¹⁵N TROSY-HSQC spectra of ¹⁵N-labeled CR2 SCR1-2 (0.3 mm in 1/3× PBS) collected during titration with increasing amounts of C3d. Black, no C3d; blue, with 50 μm C3d; purple, with 100 μm C3d; green, with 300 μm C3d; red, with 600 μm C3d. Inset on B, the binding curve for the CR2-C3d weak interaction. C and D, five superimposed ¹H-¹⁵N TROSY-HSQC spectra of ¹⁵N-labeled CR2 SCR1-2 (0.3 mm in 1/3× PBS) collected during titration with increasing amounts of C3dp1. Black, no C3dp1; blue, with 250 μm C3dp1; purple, with 500 μm C3dp1; green, with 750 μm C3dp1; red, with 1000 μm C3dp1. Inset in D, binding curve for the CR2-C3dp1 weak interaction. Residues monitored were Arg¹³ (A and C) and Arg⁸³ (B and D). The numbering system used here within for CR2 is based on the reported amino acid sequence for the mature protein (5).