Bacillus subtilis DppA is a binuclear zinc-dependent, D-specific aminopeptidase. The structure reveals that DppA is a new example of a 'self-compartmentalising 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. The only known substrates are D-ala-D-ala and D-ala-gly-gly.