Crystal structure of the N‐terminal domain of E. coli Lon protease

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 non‐specific 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.