Activation of a phytopathogenic bacterial effector protein by a eukaryotic cyclophilin.

Innate immunity in higher plants invokes a sophisticated surveillance system capable of recognizing bacterial effector proteins. In Arabidopsis, resistance to infection by strains of Pseudomonas syringae expressing the effector AvrRpt2 requires the plant resistance protein RPS2. AvrRpt2 was identified as a putative cysteine protease that results in the elimination of the Arabidopsis protein RIN4. RIN4 cleavage serves as a signal to activate RPS2-mediated resistance. AvrRpt2 is delivered into the plant cell, where it is activated by a eukaryotic factor that we identify as cyclophilin. This activation of AvrRpt2 is necessary for protease activity. Active AvrRpt2 can then directly cleave RIN4.