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. Found in the larval foregut and adult stomach. Glycosylated. Contains high mannose-type oligosaccharide. Belongs to the peptidase A1 family.