MAPK/ERK kinase kinase 1 (MEKK1) is a mitogen-activated protein kinase kinase kinase (MAP3K) of the
stress-induced JNK pathway. Once activated, MEKK1 phosphorylates
the MAP2K MKK4, which in turn, phosphorylates JNK.
MEKK1 also has the capacity to activate IKK, the central protein kinase of the NF-kappaB pathway.
The molecular determinants responsible for the ability of MEKK1 to recognize
specific substrates are poorly understood. We report here that select point
mutations in subdomain VIII of the protein kinase domain of MEKK1 (MEKK1delta) differentially affect
its ability to activate MKK4 and IKK, and consequently AP1 and NF-kappaB reporter genes. Moreover, binding of MKK4 to
MEKK1delta protects the latter from cleavage at an engineered protease target
site in subdomain VIII. Collectively these results
provide evidence that subdomain VIII of MEKK1 is
involved not only in binding to, but also in discrimination of, protein