This domain is normally found at the C-terminus of secreted bacterial peptidases. They are not present in the active peptidase. It is possible that they fulfill a similar role to the PKD (Pfam:PF00801) domain, which also are found in this context. Visual analysis suggests that PKD and PPC are distantly related (personal obs:Bateman A, Yeats C).

1: New Knowledge from Old: In silico discovery of novel protein domains in Streptomyces coelicolor. Yeats C, Bentley S, Bateman A; BMC Microbiol 2003;3:3-3. PMID:12625841