The crystal structure of the small, mostly helical α domain of the AAA+module of the Escherichia coli ATP-dependent protease Lon has been solved by single isomorphous replacement combined with anomalous scattering and refined at 1.9Å resolution to a crystallographic R factor of 17.9%. This domain, comprising residues 491-584, was obtained by chymotrypsin digestion of the recombinant full-length protease. The α domain of Lon contains four α helices and two parallel strands and resembles similar domains found in a variety of ATPases and helicases, including the oligomeric proteases HslVU and ClpAP. The highly conserved "sensor-2" Arg residue is located at the beginning of the third helix. Detailed comparison with the structures of 11 similar domains established the putative location of the nucleotide-binding site in this first fragment of Lon for which a crystal structure has become available. © 2003 Elsevier Inc. All rights reserved.