MEF2 participates in IL-6–Erk5 signal transduction. (A) Expression of MEF2 isoforms in MM1S and MM144 cells, analyzed by RT-PCR using primers specific for each of the MEF2 isoforms. (B) Expression of a dominant-negative form of Erk5 in MM1S cells. Cells were infected with retroviruses that included an empty or HA-Erk5^AEF–containing vector, and the expression of the infected protein was analyzed by immunoprecipitation with the anti-Erk5 C-terminal antibody followed by blotting with anti-HA (top) or anti-Erk5 (bottom) antibodies. (C) Effect of the dominant-negative form of Erk5 on an MEF2 luciferase reporter gene. MM1S infected with either the empty retroviral vector or with the retroviral vector containing HA-Erk5^AEF were electroporated with the indicated plasmid reporter gene and 24 hours later treated for 3 hours with IL-6 (5 nM; ▪) or not (□) as indicated. Cell extracts were prepared and analyzed for the relative luciferase activity as described in “Materials and methods.” Data are the mean ± SD of duplicates of an experiment repeated twice.