Figure 7.

| Unconventional targeting of an emerging tumour target. (a) Schematic illustration of dysregulated CatB secretion from cancer and stromal cells (i.e., tumour-associated macrophages, TAMs) in the tumour microenvironment. In healthy tissues, CatB is contained intracellularly in endo-lysosomal vesicles. (b) Structural conservation (determined by ConSurf (http://consurf.tau.ac.il/)) within the cathepsin family in human CatB. Cathepsins have highly conserved active site clefts, making truly selective targeting using activity-based approaches difficult, whereas other regions with high structural variability provide uncharted territory for probe design with affinity probes such as DARPin 8h6. Engineering a unique cysteine residue at the C terminus of DARPins enables site-specific labelling with various chemical tags for the purpose of imaging, pull-down or drug delivery. The image was produced in PyMOL (www.pymol.org).