Cathepsin C (dipeptidyl peptidase I) is the physiological activator of a group of serine proteases. This domain corresponds to the exclusion domain whose structure excludes the approach of a polypeptide apart from its termini. It forms an enclosed beta barrel structure composed from 8 anti-parallel beta strands [1]. Based on a structural comparison and interaction data, it is suggested that the exclusion domain originates from a metallo-protease inhibitor [1].

1: Structure of human dipeptidyl peptidase I (cathepsin C): exclusion domain added to an endopeptidase framework creates the machine for activation of granular serine proteases. Turk D, Janjic V, Stern I, Podobnik M, Lamba D, Dahl SW, Lauritzen C, Pedersen J, Turk V, Turk B; EMBO J. 2001;20:6570-6582. PMID:11726493