The Ser(186) phospho-acceptor site within ERK4 is essential for its ability to interact with and activate PRAK/MK5.

*Department of Pharmacology, Institute of Medical Biology, University of Tromso, N-9037 Tromso, Norway, †Department of Biochemistry, Institute of Medical Biology, University ofTromso, N-9037 Tromso, Norway, and ‡Cancer Research UK Stress Response Laboratory, Biomedical Research Centre, Level 5, Ninewells Hospital and Medical School,Dundee DD1 9SY, U.K.