The Xanthomonas campestris type III effector XopJ proteolytically degrades proteasome subunit RPT6.
Many animal and plant pathogenic bacteria inject type III effector (T3E) proteins into their eukaryotic host cells to suppress immunity. The YopJ-family of T3Es is a widely distributed family of effector proteins found in both, animal and plant pathogens and its members are highly diversified in virulence functions. Some members have been shown to possess acetyltransferase activity; however, whether this is a general feature of YopJ-family T3Es is currently unknown. The T3E XopJ, a YopJ-family effector from the plant pathogen Xanthomonas campestris pv. vesicatoria, interacts with the proteasomal subunit RPT6 in planta to suppress proteasome activity resulting in the inhibition of salicylic acid (SA)-related immune responses. Here we show that XopJ has protease activity to specifically degrade RPT6, leading to reduced proteasome activity in the cytoplasm as well as in the nucleus. Proteolytic degradation of RPT6 was dependent on localization of XopJ to the plasma membrane as well as on its catalytic triad. Mutation of the Walker-B motif of RPT6 prevented XopJ-mediated degradation of the protein but not XopJ interaction. This indicates that interaction of RPT6 with XopJ is dependent on ATP-binding activity of RPT6 but proteolytic cleavage additionally requires its ATPase activity. 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. Our results show that YopJ-family T3Es are not only highly diversified in virulence function, but also appear to possess different biochemical activities.
Üstün, S.; Börnke, F. 2015. The Xanthomonas campestris type III effector XopJ proteolytically degrades proteasome subunit RPT6. Plant Physiology 116, 107-119.