ERG is phosphorylated at S215 by ERK via an FXFP motif. (A) Mass spectrum of ERK2 phosphorylated ERG peptide fragment (ALQNS*PR) identifying phosphorylation of S215. (B) Schematic representation of ERG and ERGi10 protein isoforms showing possible ERK binding motifs (D and DEF domains, indicated as boxes) and potential phosphorylation sites (TP and SP sequences, indicated with lines). Serine and threonine residues interrogated in (C) are annotated. Dashed lines define the region coded by exon 9. (C) In vitro kinase assays, as in Figure 1, show ERK2 phosphorylation of the indicated proteins. Note that bovine serum albumin (BSA) was included in the kinase buffer in the left panels, but had no effect on the specificity of the assay (compare to center panel), and was therefore not used in other kinase assays.