A wild-type human procathepsin B was expressed, crystallized in two crystal forms and its crystal structure determined at 3.2 and 3.3 Å resolution. The structure reveals that the propeptide folds on the cathepsin B surface, shielding the enzyme active site from exposure to solvent. The structure of the enzymatically active domains is virtually identical to that of the native enzyme [Musil et al. (1991) EMBO J. 10, 2321–2330]: the main difference is that the occluding loop residues are lifted above the body of the mature enzyme, supporting the propeptide structure.
