Commercial recombinant protein kinase fusion proteins were purchased from Medical Research Council Protein Phosphorylation and Ubiquitlyation Unit (PPUU) reagents (University of Dundee) and were purified from Sf21 cells or E. coli. Full details are provided in table S3. GST and His6-tagged kinases were purified using standard procedures, before storage in 1 mM DTT (except where indicated) at −80°C. All kinases were assayed using standard enzyme preparations (128). Bacterially expressed GST-MAPKAP-K2 and GST-MAPKAP-K3 were activated in vitro by incubation with catalytically active p38α, which was removed by repurification before assay. General biochemicals and all redox reagents, including GRX, were purchased from Sigma-Aldrich.