The PA (Protease associated) domain is found as an insert domain in diverse proteases. The PA domain is also found in a plant vacuolar sorting receptor Swiss:O22925 and members of the RZF family Swiss:O43567. It has been suggested that this domain forms a lid-like structure that covers the active site in active proteases, and is involved in protein recognition in vacuolar sorting receptors [1].

1: The protease-associated domain: a homology domain associated with multiple classes of proteases. Luo X, Hofmann K; Trends Biochem Sci 2001;26:147-148. PMID:11246007