Thiol protease important for the overall degradation of proteins in lysosomes (By similarity). Plays a critical for normal cellular functions such as general protein turnover, antigen processing and bone remodeling. Involved in the solubilization of cross-linked TG/thyroglobulin and in the subsequent release of thyroid hormone thyroxine (T4) by limited proteolysis of TG/thyroglobulin in the thyroid follicle lumen (By similarity). In neuroendocrine chromaffin cells secretory vesicles, catalyzes the prohormone proenkephalin processing to the active enkephalin peptide neurotransmitter (By similarity). In thymus, regulates CD4(+) T cell positive selection by generating the major histocompatibility complex class II (MHCII) bound peptide ligands presented by cortical thymic epithelial cells. Also mediates invariant chain processing in cortical thymic epithelial cells. Major elastin-degrading enzyme at neutral pH. Accumulates as a mature and active enzyme in the extracellular space of antigen presenting cells (APCs) to regulate degradation of the extracellular matrix in the course of inflammation (By similarity). Secreted form generates endostatin from COL18A1 (By similarity). Critical for cardiac morphology and function. Plays an important role in hair follicle morphogenesis and cycling, as well as epidermal differentiation (By similarity). Required for maximal stimulation of steroidogenesis by TIMP1 (PubMed:7777858). Specificity close to that of papain. As compared to cathepsin B, cathepsin L exhibits higher activity toward protein substrates, but has little activity on Z-Arg-Arg-NHMec, and no peptidyl-dipeptidase activity. Inhibited by the propeptide produced by autocleavage (By similarity). Long isoform of CD74/Ii chain stabilizes the conformation of mature CTSL by binding to its active site and serving as a chaperone to help maintain a pool of mature enzyme in endocytic compartments and extracellular space of APCs. IFNG enhances the conversion into the CTSL mature and active form (By similarity). Inhibited by CST6. Inhibited by the glycopeptide antibiotic teicoplanin. Inhibited by amantadine (By similarity). Dimer of a heavy and a light chain linked by disulfide bonds. Interacts with Long isoform of CD74/Ii chain; the interaction stabilizes the conformation of mature CTSL. Localizes to the apical membrane of thyroid epithelial cells. Released at extracellular space by activated dendritic cells and macrophages. Both mature cathepsin L1 and procathepsin L are found in the upper epidermis. The lower epidermis predominantly contains procathepsin L. In seminiferous tubules expression is greater at stages VI-VII than at stages IX-XII. Expression in Sertoli cells is repressed by germ cells. During export along the endocytic pathway, pro-CTSL undergoes several proteolytic cleavages to generate the CTSL single-chain and two-chain mature forms, composed of a heavy chain linked to a light chain by disulfide bonds (By similarity). Autocleavage; produces the single-chain CTSL after cleavage of the propeptide. The cleavage can be intermolecular (By similarity). Belongs to the peptidase C1 family.