This is a family of cysteine proteases, found in actinobacteria, protobacteria and firmicutes. Papain-like cysteine proteases play a crucial role in plant-pathogen/pest interactions. On entering the host they act on non-self substrates, thereby manipulating the host to evade proteolysis. AvrRpt2 from Pseudomonas syringae pv. tomato DC3000 triggers resistance to P. syringae-2-dependent defense responses, including hypersensitive cell death, by cleaving the Arabidopsis RIN4 protein which is monitored by the cognate resistance protein RPS2.