Anthrax lethal factor proteolysis and inactivation of MAPK kinase

Anthrax lethal toxin produced by the bacterium Bacillus anthracis is the major cause of death in animals infected with anthrax. One component of this toxin, lethal factor (LF), inactivates members of the mitogen-activated protein kinase kinase or MEK family through proteolysis of their NH2 termini. However, neither the substrate requirements for LF cleavage nor the mechanism by which proteolysis inactivates MEK have been demonstrated. By means of deletion mutant analysis and site-directed mutagenesis, we have identified an LFIR ((LF) under bar (i) under bar nteracting region) in the COOH-terminal kinase domain of MEK1 adjacent to the proline-rich region, which is essential for LF-mediated proteolysis of MEK. Point mutations in this region block proteolysis but do not alter the kinase activity of MEK. Similar mutations in MEK6 also prevent proteolysis, indicating that this region is functionally conserved among MEKs. In addition, NH2-terminal proteolysis of MEK1 by LF was found to reduce not only the affinity of MEK1 for its substrate mitogen-activated protein kinase but also its intrinsic kinase activity, indicating that the NH2-terminal end of MEK is important not only for substrate interaction but also for catalytic activity.