Fluorescence anisotropy measurements of P C-terminal peptide binding to N(13-391), effect of KCl, and competition by full-length P. (A) Affinity of unphosphorylated and doubly phosphorylated P-derived C-terminal peptides BP1 and BP5, respectively, for N(13-391). Data points and error bars represent the means and standard deviations, respectively, of 3 replicates. The error bars are mostly hidden by the data points. The Kds for BP1 and BP5 were 760 ± 20 nM and 78 ± 5 nM, respectively (best-fit value ± standard error of fit). (B) Effect of KCl concentration on affinity of BP5 for N(13-391). Kds were measured with N(13-391) concentrations ranging from 1.95 to 2000 nM in triplicate. The data were fit as above. Data points and error bars represent the best-fit Kds and the standard errors of the fits, respectively. (C) Inhibition of BP5 binding to N(13-391) by full-length P protein. The IC50 was 180 ± 10 nM (best-fit value ± standard error of fit).