| MEKK 4 is an effector of the embryonic TRAF 4 for JNK activation . ^^^ Here , we show that endogenous TRAF 4 and MEKK 4 associate in both human K 562 cells and mouse E10 . 5 embryos . ^^^ TRAF 4 interacts with the kinase domain of MEKK 4 . ^^^ The interaction of MEKK 4 and TRAF 4 are further demonstrated by the colocalization of TRAF 4 and MEKK 4 in cells . ^^^ Importantly , although TRAF 4 has little or no ability to activate JNK independently , coexpression of TRAF 4 and MEKK 4 results in synergistic activation of JNK that is inhibited by a kinase inactive mutant of MEKK 4 , MEKK4K1361R . ^^^ |