THE MAP KINASE KINASE KINASE MLK2 COLOCALIZES WITH ACTIVATED JNK ALONG MICROTUBULES AND ASSOCIATES WITH KINESIN SUPERFAMILY MOTOR KIF3

The MLK (mixed lineage) ser/thr kinases are most closely related to th e MAP kinase kinase kinase family, In addition to a kinase domain, MLK 1, MLK2 and MLK3 each contain an SH3 domain, a leucine zipper domain a nd a potential Rac/Cdc42 GTPase-binding (CRIB) motif, The C-terminal r egions of the proteins are essentially unrelated, Using yeast two-hybr id analysis and in vitro dot-blots, we show that MLK2 and MLK3 interac t with the activated (GTP-bound) forms of Rac and Cdc42, with a slight preference for Rac, Transfection of MLK2: into COS cells leads to str ong and constitutive activation of the JNK (c-Jun N-terminal kinase) M AP kinase cascade, but also to activation of ERK (extracellular signal -regulated kinase) and p38, When expressed in fibroblasts, MLK2 co-loc alizes with active, dually phosphorylated JNK1/2 to punctate structure s along microtubules. In an attempt to identify proteins that affect t he activity and localization of MLK2, we have screened a yeast two-hyb rid cDNA library. MLK2 and MLK3 interact with members of the KIF3 fami ly of kinesin superfamily motor proteins and with KAP3A, the putative targeting component of KIF3 motor complexes, suggesting a potential li nk between stress activation and motor protein function.