Calcium-regulated non-lysosomal thiol-protease (By similarity). 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. Broad endopeptidase specificity. Binds 2 calcium ions. The concentration of calcium for half-maximal activity is 0.3 mM. Inhibited by calpastatin and calpeptin. Optimum pH is 6. Optimum temperature is 20 degrees Celsius. Monomer and homooligomer. Interacts with COPS1/GPS1, COPB1, EYA2, NME2, NME4 and TOMM70. Predominantly expressed in the stomach. Localizes strictly to the surface mucus cells in the gastric epithelium and the mucus-secreting goblet cells in the duodenum. 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.