Updated FTP location. Purified GST-fused Nab2, Foxk1, and HURP were incubated with ATP in the presence or absence of His-tagged active ERK in vitro, separated by Phos-tag SDS-PAGE, and visualized by CBB staining. The most shifted regions of protein bands on the Phos-tag-containing gels were excised, digested with trypsin, and analyzed by LC-MS/MS.