TY - JOUR T1 - Regulation of MTK1/MEKK4 Kinase Activity by Its N-Terminal Autoinhibitory Domain and GADD45 Binding JF - Molecular and Cellular Biology JO - Mol. Cell. Biol. SP - 4544 LP - 4555 DO - 10.1128/MCB.22.13.4544-4555.2002 VL - 22 IS - 13 AU - Mita, Hiroaki AU - Tsutsui, Junichiro AU - Takekawa, Mutsuhiro AU - Witten, Elizabeth A. AU - Saito, Haruo Y1 - 2002/07/01 UR - http://mcb.asm.org/content/22/13/4544.abstract N2 - A variety of cellular stresses activate the stress-responsive mitogen-activated protein (MAP) kinases p38 and JNK. In this study, we studied the activation mechanism of a human MAP kinase kinase kinase, MTK1 (also known as MEKK4), which mediates activation of both p38 and JNK. MTK1 has an extensive N-terminal noncatalytic domain composed of ∼1,300 amino acids. Full-length or near full-length MTK1 is catalytically inactive when expressed in Saccharomyces cerevisiae cells, as it is in mammalian cells. Deletion of a segment including positions 253 to 553 activates kinase, indicating that this segment contains the autoinhibitory domain. In the autoinhibited conformation, the MTK1 kinase domain cannot interact with its substrate, MKK6. By a functional complementation screening with yeast cells, GADD45 proteins (GADD45α, β, and γ) were identified as MTK1 activators. GADD45 proteins bind a site in MTK1 near the inhibitory domain and relieve autoinhibition. Mutants of full-length MTK1 were isolated that can interact with MKK6 in the absence of the activator GADD45 proteins. These MTK1 mutants are constitutively active, in both yeast and mammalian cells. A model of MTK1 autoinhibition by the N-terminal inhibitory domain and activation by GADD45 binding is presented. ER -