Dimerization of the bacterial effector protein AvrBs3 in the plant cell cytoplasm prior to nuclear import

The effector protein AvrBs3 from the bacterial phytopathogen Xanthomonas campestris pv. vesicatoria is translocated into the plant cell where it specifically induces hypertrophy symptoms or the hypersensitive reaction. Activity of AvrBs3 depends on nuclear localization signals (NLSs) and an acidic activation domain, suggesting a role in regulation of plant transcription. Here, we show that AvrBs3 dimerizes in the plant cell prior to its nuclear import. AvrBs3 deletion derivatives were tested in the yeast two-hybrid system revealing that the repeat region, which confers specific recognition in resistant plants and is crucial for virulence function, is also essential for the self-interaction. GST pull-down assays showed that the AvrBs3-AvrBs3 interaction occurs independent of plant proteins. Coexpression of two different inactive mutant AvrBs3 derivatives in Bs3-resistant pepper plants resulted in 'trans-complementation', i.e., the induction of a hypersensitive reaction. This clearly indicates that AvrBs3-dimerization occurs in planta. Interestingly, 'trans-complementation' was not observed in susceptible plants suggesting that wild-type homodimers are needed for the AvrBs3 virulence function in plants. Furthermore, a green fluorescent protein (GFP) fusion of AvrBs3 deleted in the NLSs (AvrBs3DeltaNLS-GFP), normally localized in the cytoplasm, was imported into the nucleus upon coexpression with wild-type AvrBs3 in Nicotiana benthamiana. Thus, AvrBs3 dimerization takes place in the cytoplasm of the plant cell prior to nuclear import. Given the fact that dimerization is a common feature of transcriptional regulators, our data are consistent with the idea that AvrBs3 manipulates expression of plant genes involved in the establishment of compatible and incompatible interactions.