JNKK1 ORGANIZES A MAP KINASE MODULE THROUGH SPECIFIC AND SEQUENTIAL INTERACTIONS WITH UPSTREAM AND DOWNSTREAM COMPONENTS MEDIATED BY ITS AMINO-TERMINAL EXTENSION

MAP kinase (MAPK) cascades are composed of a MAPK, MAPK kinase (MAPKK) , and a MAPKK kinase (MAPKKK). Despite the existence of numerous compo nents and ample opportunities for crosstalk, most MAPKs are specifical ly and distinctly activated. We investigated the basis for specific ac tivation of the JNK subgroup of MAPKs. The specificity of JNK activati on is determined by the MAPKK JNKK1, which interacts with the MAPKKK M EKK1 and INK through its amino-terminal extension. Inactive JNKK1 muta nts can disrupt INK activation by MEKK1 or tumor necrosis factor (TNF) in intact cells only if they contain an intact amino-terminal extensi on. Mutations in this region interfere with the ability of JNKK1 to re spond to TNF but do not affect its activation by physical stressors. A s INK and MEKK1 compete for binding to JNKK1 and activation of JNKK1 p revents its binding to MEKK1, activation of this module is likely to o ccur through sequential MEKK1:JNKK1 and JNKK1:JNK interactions. These results underscore a role for the amino-terminal extension of MAPKKs i n determination of response specificity.