Plasmid construction, mutagenesis, protein expression, purification, and the AlphaScreen peptide-binding assay were as previously described with minor modifications to the AlphaScreen assay (Hill and Pioszak, 2013; Moad and Pioszak, 2013). Synthetic peptides were from RS Synthesis, Bachem, or were synthesized in-house. Details are in Supplemental Experimental Procedures.