(D) Fluorescence polarization competition assay measuring the ability of unlabeled COP1 PSL, COP1 PSL 4A, and TRIB1 tail peptides to compete with FITC-TRIB1 tail for binding to the COP1 WD40 domain (376-731). The COP1 WD40 domain was preincubated with FITC-TRIB1 tail before addition of the indicated concentrations of unlabeled peptide. The change in polarization is plotted as a function of unlabeled peptide concentration. Curves were fit to a sigmoidal dose-response curve. Data are presented as mean values ± S.E.M. for three independent experiments where each measurement was made in duplicate.. 