Anthrax lethal toxin (LeTx) stands for the major virulence factor of the anthrax disease. It comprises a 90 kDa highly specific metalloprotease, the anthrax lethal factor (LF). LF possesses a catalytic Zn2+ binding site and is highly specific against MAPK kinases, thus representing the most potent native biomolecule to alter and inactivate MKK [MAPK (mitogen-activated protein kinase) kinases] signalling pathways. Given the importance of the interaction between LF and substrate for the development of anti-anthrax agents as well as the potential treatment of nascent tumours, the analysis of the structure and dynamic properties of the LF catalytic site are essential to elucidate its enzymatic properties. Here we report the recombinant expression and purification of a C-terminal part of LF (LF672-776) that harbours the enzyme's core protease domain. The biophysical characterization and backbone assignments (H-1, C-13, N-15) of the polypeptide revealed a stable, well folded structure even in the absence of Zn2+, suitable for high resolution structural analysis by NMR. (C) 2010 Elsevier Inc. All rights reserved.