E Comparison of the signal-averaged sedimentation coefficients of wild-type (red) and monomeric mutant FICDL258D (blue), as measured by analytical ultracentrifugation. A fit for monomer-dimer association (solid red line), constrained using the average value for the monomeric protein (dashed line, 2.82 S, Sw,20 = 3.02 S), yielded a Kd of 1.2 nM with a 95% confidence interval between 1.1 to 1.4 nM and a value of 4.08 S for the dimer (Sw,20 = 4.36 S). The fitted data points are from three independent experiments. 