MST MLK2, A MEMBER OF THE MIXED LINEAGE KINASE FAMILY, DIRECTLY PHOSPHORYLATES AND ACTIVATES SEK1, AN ACTIVATOR OF C-JUN N-TERMINAL KINASE/STRESS-ACTIVATED PROTEIN-KINASE/

c-Jun N-terminal kinases/stress-activated protein kinases (JNKs/SAPKs) are mitogen-activated protein kinase (MAPK)-related protein kinases t hat are involved in several cellular events, including growth, differe ntiation, and apoptosis. Mixed lineage kinases (MLKs) form a family of protein kinases sharing two leucine zipper-like motifs and a kinase d omain whose primary structure is similar to both the tyrosine-specific and the serine/threonine-specific kinase classes. We have reported th at a member of the MLK family, MUK/DLK/ZPK, can activate JNK/SAPK in v ivo, and here we show that another member of the MLK family, MST/MLK2, activates JNK/SAPK. Both MUK/DLK/ZPK and MST/MLK2 cause a slight acti vation of p38/Mpk2 when overexpressed in COS-1 cells, whereas MST/MLK2 , but not MCK/DLK/ ZPK, activates extracellular response kinase (ERK) to a certain degree. The activity of SEK1/MKK4/JNKK, a MAPK kinase cla ss protein kinase designated as a direct activator of JNK/SAPK, is als o induced by MUK/DLK/ ZPK or MST/MLK2 overexpression. Furthermore, rec ombinant MST/MLK2 produced in bacteria directly phosphorylates and act ivates SEK1/MKK4/JNKK in vitro, showing that MST/MLK2 acts like a MAPK kinase kinase. Taken together, these results suggest that MLK family members are MAPK kinase kinases preferentially acting on the JNK/SAPK pathway.