Fig. 1

A model of cathepsin (CTS) structure and maturation. A: Each CTS has a signaling peptide, a pre-region, a light chain, and a heavy chain. B: Normal and alternative translations. The general maturation processing of a CTS is as follows. After normal translation, the pro-cathepsin is targeted to the endoplasmic reticulum (ER) for the cleaving and folding of the signaling peptide; to the trans-Golgi network (TGN) for the phosphorylation of mannose-6 residue (M6p); to the early endosome for acidification; to the endosome for the removal of the pre-region; to the lysosome for further activation (light and heavy chains); and then is finally subjected to Ca2+-dependent organelle blending and secretion into extracellular spaces. Some of the CTSs are released by an exocytosis process without proceeding to the M6p form. In the alternative translation, alternative splicing and skipping-related translation generates a CTS without a signal peptide, which moves to the cytoplasm, nucleus, and mitochondria, leading to apoptosis and proliferation