May have a role in immune function. Probably involved in the processing of antigenic peptides during MHC class II-mediated antigen presentation (By similarity). Similar to cathepsin D, but slightly broader specificity. Homodimer; disulfide-linked. The proenzyme is localized to the endoplasmic reticulum and Golgi apparatus, while the mature enzyme is localized to the endosome. Expressed predominantly in the anterior and posterior adult stomach and at much lower levels in the larval foregut. Expression levels are low in surface mucous cells and manicotto gland cells of the foregut epithelium of pro-metamorphic tadpoles. During metamorphosis, expression levels rise markedly in proliferating adult epithelial primordia. In the adult stomach, expression was strongest in oxynticopeptic cells, but was also detected at a lower level in surface mucose cells. Glycosylated. Contains high mannose-type oligosaccharide (By similarity). Belongs to the peptidase A1 family.