Plays a role in lysosomal amyloid precursor protein (APP) processing by cleaving and activating CTSD/cathepsin D which leads to APP degradation (PubMed:27333034). Binds 1 zinc ion per subunit. Interacts with CTSD; this leads to activation of CTSD. Expressed in brain neurons (at protein level) (PubMed:27333034). Expressed in testis (PubMed:10512762). The conserved cysteine present in the cysteine-switch motif binds the catalytic zinc ion, thus inhibiting the enzyme. The dissociation of the cysteine from the zinc ion upon the activation-peptide release activates the enzyme.