We report here the first crystal structure of the N-terminal domain of an A-type Lon protease. Lon proteases are ubiquitous, multidomain, ATP-dependent enzymes with both highly specific and nonspecific protein binding, unfolding, and degrading activities. We expressed and purified a stable, monomeric 119-amino acid N-terminal subdomain of the Escherichia coli A-type Lon protease and determined its crystal structure at 2.03 Å (Protein Data Bank [PDB] code 2ANE). The structure was solved in two crystal forms, yielding 14 independent views. The domain exhibits a unique fold consisting primarily of three twisted β-sheets and a single long α-helix. Analysis of recent PDB depositions identified a similar fold in BPP1347 (PDB code 1ZBO), a 203-amino acid protein of unknown function from Bordetella parapertussis, crystallized as part of a structural genomics effort. BPP1347 shares sequence homology with Lon N-domains and with a family of other independently expressed proteins of unknown functions. We postulate that, as is the case in Lon proteases, this structural domain represents a general protein and polypeptide interaction domain. Copyright © 2005 The Protein Society.