Figure 3.

S3D binding affinity but not cooperativity is linked to cation release. A, binding titrations of S3D (from left to right) in 10 (squares), 25 (circles), 50 (upright triangles), 75 (diamonds), or 100 mm KCl (rightward triangles). B, binding titrations of S3D (from left to right) in 0.5 (squares), 2 (circles), or 5 mm MgCl₂ (triangles). The smooth lines though data in A and B are fits to Equations 2 and 4. C and D, the KCl and MgCl₂ concentration dependence of the intrinsic S3D cofilin binding affinity (K_d) (C) and binding cooperativity (ω) (D) obtained from the fits to Equations 2 and 4 in A and B. KCl and MgCl₂ dependence of K_d in C yields slopes of −1.9 ± 0.6 and −1.4 ± 0.2, respectively, corresponding to dissociation of ∼2.3 K⁺ or ∼0.9 Mg²⁺ with S3D binding (see Ref. 27 for details on calculations). These values compare with the number of ions released with WT cofilin binding (Table 2) (27). In D, S3D cofilin binding cooperativity depends weakly on the solution salt concentration, similarly to WT cofilin (27).