Lysosomal serine protease with tripeptidyl-peptidase I activity. May act as a non-specific lysosomal peptidase which generates tripeptides from the breakdown products produced by lysosomal proteinases. Requires substrates with an unsubstituted N-terminus (By similarity). Release of an N-terminal tripeptide from a polypeptide, but also has endopeptidase activity. Binds 1 Ca(2+) ion per subunit. Monomer. Interacts with CLN5. Interacts with CLN3 (By similarity). Activated by autocatalytic proteolytical processing upon acidification. N-glycosylation is required for processing and activity (By similarity).