The MAP kinase kinase kinase MLK2 co‐localizes with activated JNK along microtubules and associates with kinesin superfamily motor KIF3

The MLK (mixed lineage) ser/thr kinases are most closely related to the MAP kinase kinase kinase family. In addition to a kinase domain, MLK1, MLK2 and MLK3 each contain an SH3 domain, a leucine zipper domain and a potential Rac/Cdc42 GTPase‐binding (CRIB) motif. The C‐terminal regions of the proteins are essentially unrelated. Using yeast two‐hybrid analysis and in vitro dot‐blots, we show that MLK2 and MLK3 interact with the activated (GTP‐bound) forms of Rac and Cdc42, with a slight preference for Rac. Transfection of MLK2 into COS cells leads to strong and constitutive activation of the JNK (c‐Jun N–terminal kinase) MAP kinase cascade, but also to activation of ERK (extracellular signal‐regulated kinase) and p38. When expressed in fibroblasts, MLK2 co‐localizes with active, dually phosphorylated JNK1/2 to punctate structures along microtubules. In an attempt to identify proteins that affect the activity and localization of MLK2, we have screened a yeast two‐hybrid cDNA library. MLK2 and MLK3 interact with members of the KIF3 family of kinesin superfamily motor proteins and with KAP3A, the putative targeting component of KIF3 motor complexes, suggesting a potential link between stress activation and motor protein function.

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