MEKK3 directly regulates MEK5 activity as part of the big mitogen-activated protein kinase 1 (BMK1) signaling pathway

Big mitogen-activated protein (MAP) kinase (BMK1), also known as ERK5, is a member of the MAP kinase family whose cellular activity is elevated in res ponse to growth factors, oxidative stress, and hyperosmolar conditions. Pre vious studies have identified MEK5 as a cellular kinase directly regulating BMK1 activity; however, signaling molecules that directly regulate MEK5 ac tivity have not yet been defined. Through utilization of a yeast two-hybrid screen, we have identified MEKK3 as a molecule that physically interacts w ith MEK5, This interaction appears to take place in mammalian cells as evid enced by the fact that cellular MEK5 and MEKK3 co-immunoprecipitate. In add ition, we show that a dominant active form of MEKK3 stimulates BMK1 activit y through MEK5. Moreover, we demonstrate that MEKK3 activity is required fo r growth factor mediated cellular activation of endogenous BMK1. Taken toge ther, these results identify MEKK3 as a kinase that regulates the activity of MEK5 and BMK1 during growth factor-induced cellular stimulation.