Mixed lineage kinase LZK forms a functional signaling complex with JIP-1, a scaffold protein of the c-Jun NH2-terminal kinase pathway

Leucine zipper-bearing kinase (LZK) is a novel member of the mixed lineage kinase (MLK) protein family, the cDNA of which was first cloned from a huma n brain cDNA library [Sakuma, H., Ikeda, A., Oka, S., Kozutsumi, Y., Zanett a, J.-P., and Kawasaki, T. (1997) J. Mot Chein. 272,28622-28629]. Several M LK family proteins have been proposed to function as MAP kinase kinase kina ses in the c-Jun NH2 terminal kinase (JNK)/stress-activated protein kinase (SA-PK) pathway. In the present study, we demonstrated that, like other MLK s, LZK activated the JNK/SAPK pathway but not the ERK pathway. LZK directly phosphorylated and activated MKK7, one of the two MAPKKs in the JNK/SAPK p athway, to a comparable extent to a constitutive active form of MEKK1 (MEKK 1 DeltaN), suggesting a biological role of LZK as a MAPKKK in the JNK/SAPK pathway. Recent studies have revealed the essential roles of scaffold prote ins in intracellular signaling pathways including MAP kinase pathways. JIP- 1, one of the scaffold proteins, has been shown to be associated with MLKs, MKK7, and JNK [Whitmarsh, AA, Cavanagh, J., Tournier, C., Yasuda, J., and Davis, R.J. (1998) Science 281, 1671-1674], suggesting the presence of a se lective signaling pathway including LZK, MKK7, and JNK. Consistent with thi s hypothesis, we provided evidence that LZK is associated with the C-termin al region of JIP-1 through its kinase catalytic domain. In addition, LZK-in duced JNK activation was markedly enhanced when LZK and JNK were co-express ed with JIP-1. These results constituted important clues for understanding the molecular mechanisms regulating the signaling specificities of various JNK activators under different cellular conditions.