Functional analysis of bacterial type III effector proteins in plants

One of the most diverse and widely distributed families of T3Es is the YopJ family of cysteine proteases/acetyltransferases. Members of this large family of T3Es are found among both, plant and animal pathogens as well as plant symbionts and a characteristic feature of these proteins is their catalytic triad consisting of the amino acids histidine, glutamic/aspartic acid, and a cysteine and which is required for their biological activity. Although acetyltransferase activity has been demonstrated for some YopJ-family members, it is currently unclear whether other members possess other biochemical activities. In the course of our experiments, we could show that XopJ, a T3E from Xanthomonas campestris pv. vesicatoria, inhibits the host cell proteasome to interfere with salicylic acid mediated defense responses. XopJ acts as a protease to specifically degrade the proteasomal subunit RPT6 and thereby interferes with the assembly of a functional 26S proteasome. Inhibition of the proteasome impairs the proteasomal turnover of NPR1, the master regulator of SA responses, leading to the accumulation of ubiquitinated NPR1 which likely interferes with full induction of NPR1 target genes. Future experiments will have to clarify whether other members of the YopJ-effector family have similar functions and whether T3Es from other bacteria will also target the host proteasome.