Abstract
Exposure of mammalian cells to stressful stimuli results in activation of the c-Jun NH2-terminal kinase (JNK)/stress-activated protein kinases (SAPKs), a family of protein kinases related to mitogen-activated protein (MAP) kinase. JNK/SAPKs are activated by specific MAP kinase kinases (MKKs), one of which, MKK4/SEK1, has been characterized extensively. In Drosophila, the JNK/SAPK Basket (Bsk) and the MKK Hemipterous (Hep), are important for embryonic development. Loss of function of either gene inhibits dorsal closure, a morphogenetic movement in which the edges of the embryonic ectoderm move together over the amnioserosa. There is evidence that the Rho GTPases Rac and Cdc42 are also required for dorsal closure, suggesting that Rac or Cdc42 may regulate Hep and Bsk. We have identified MKK7, a murine homolog of Hep. MKK7 functionally rescues hep mutant flies. In fibroblasts, MKK7 is activated by stress and by the GTPase Rac1. MKK7 directly phosphorylates and activates JNK/SAPK. Thus, MKK7 is a homolog of hep and functions in a conserved signaling pathway involving JNK/SAPK and the GTPase Rac1.
