Members of this family adopt a secondary structure consisting of a helical bundle of eight alpha helices and three beta strands, the last alpha helix connecting to the first strand of the catalytic domain. The first strand of the N-terminus also forms a small parallel beta sheet with strand 5' of catalytic domain. The domain mediates dimerisation of the protein, with two proline residues present in the domain being critical for interaction [1].

1: The structural basis for catalysis and specificity of the X-prolyl dipeptidyl aminopeptidase from Lactococcus lactis. Rigolet P, Mechin I, Delage MM, Chich JF; Structure. 2002;10:1383-1394. PMID:12377124