Bacillus subtilis DppA is a binuclear zinc-dependent, D-specific aminopeptidase. The X-ray structure of the enzyme has been determined at 2.4 A resolution by a three-wavelength MAD experiment. The structure reveals that DppA is a new example of a 'self-compartmentalizing protease', a family of proteolytic complexes. Proteasomes are the most extensively studied representatives of this family. The DppA enzyme is composed of identical 30 kDa subunits organized in a decamer with 52 point-group symmetry. A 20 A wide channel runs through the complex, giving access to a central chamber holding the active sites. The structure shows DppA to be a prototype of a new family of metalloaminopeptidases characterized by the SXDXEG key sequence.