Ready, set, cleave: proteases in action.

Activity-based probes are small molecules that can be used to monitor enzyme activity by covalently binding to specific residues in the active site. In this issue of Chemistry & Biology, Lu and colleagues developed a specific fluorescent activity-based probe that targets the papain-like cysteine bacterial type III effector protease AvrPphB and used it to demonstrate the regulation of the protease secretion and pathogenesis.