Calcium-regulated non-lysosomal thiol-protease. Involved in membrane trafficking in the gastric surface mucus cells (pit cells) and may involve the membrane trafficking of mucus cells via interactions with coat protein. Proteolytically cleaves the beta-subunit of coatomer complex (By similarity). Broad endopeptidase specificity. Binds 2 calcium ions. Monomer and homooligomer. Interacts with COPS1/GPS1, COPB1, EYA2, NME2, NME4 and TOMM70 (By similarity). Stomach. The domain III mediates oligomerization. Undergoes autolytic cleavage between Ala-5 and Ala-6 which gives rise to fragments extending from Ala-6 to the C-terminus, Ala-6 to the EF-hand 2 domain and from Ala-6 to the beginning of domain III. Belongs to the peptidase C2 family.