MRK, a mixed lineage kinase-related molecule that plays a role in γ-radiation-induced cell cycle arrest

Abstract

Mitogen-activated protein (MAP) kinase pathways are three-kinase modules that mediate diverse cellular processes and have been highly conserved among eukaryotes. By using a functional complementation screen in yeast, we have identified a human MAP kinase kinase kinase (MAPKKK) that shares homology with members of the mixed lineage kinase (MLK) family and therefore was called MRK (MLK-related kinase). We report the structure of the MRK gene, from which are generated two splice forms of MRK, MRK-α and MRK-β, encoding for proteins of 800 and 456 amino acids, respectively. By using a combination of solid phase protein kinase assays, transient transfections in cells, and analysis of endogenous proteins in stably transfected Madin-Darby canine kidney cells, we found that MRK-β preferentially activates ERK6/p38γ via MKK3/MKK6 and JNK through MKK4/MKK7. We also show that expression of wild type MRK increases the cell population in the G2/M phase of the cell cycle, whereas dominant negative MRK attenuates the G2 arrest caused by γ-radiation. In addition, exposure of cells to γ-radiation induces MRK activity. These data suggest that MRK may mediate γ-radiation signaling leading to cell cycle arrest and that MRK activity is necessary for the cell cycle checkpoint regulation in cells.