Furin-processing compartments of the trans-Golgi network (TGN)/endosomal system. At steady-state, furin (represented by scissors) is localized principally to the TGN, where it cycles between this sorting compartment, the cell surface and the early endosomes. In the TGN/biosynthetic pathway, furin cleaves many substrates including pro-β-nerve growth factor (pro-β-NGF), pro-bone morphogenetic protein-4 (pro-BMP-4), the insulin pro-receptor and Ebola Zaire pro-glycoprotein (pro-GP). At the cell surface, furin cleaves substrates such as anthrax protective antigen (PA), proaerolysin and Clostridium septicum α-toxin. In mildly acidic early endosomes (endocytic pathway), furin cleaves substrates including diphtheria toxins, shiga toxin and shiga-like toxin-1, and Pseudomonas exotoxin A. See text for more details.