Thiol protease which is believed to participate in intracellular degradation and turnover of proteins (By similarity). Cleaves matrix extracellular phosphoglycoprotein MEPE (By similarity). Involved in the solubilization of cross-linked TG/thyroglobulin in the thyroid follicle lumen (By similarity). Has also been implicated in tumor invasion and metastasis (By similarity). Hydrolysis of proteins with broad specificity for peptide bonds. Preferentially cleaves -Arg-Arg-|-Xaa bonds in small molecule substrates (thus differing from cathepsin L). In addition to being an endopeptidase, shows peptidyl-dipeptidase activity, liberating C-terminal dipeptides. Dimer of a heavy chain and a light chain cross-linked by a disulfide bond. Interacts with SRPX2. Directly interacts with SHKBP1. Localizes to the lumen of thyroid follicles and to the apical membrane of thyroid epithelial cells. Expressed in heart (at protein level). Belongs to the peptidase C1 family.