NopD of Bradyrhizobium sp. XS1150 Possesses SUMO Protease Activity

Effectors secreted by the type III protein secretion system (T3SS) of rhizobia are host-specific determinants of the nodule symbiosis. Here, we have characterized NopD, a putative type III effector of sp. XS1150. NopD was found to possess a functional N-terminal secretion signal sequence that could replace that of the NopL effector secreted by sp. NGR234. Recombinant NopD and the C-terminal domain of NopD alone can process small ubiquitin-related modifier (SUMO) proteins and cleave SUMO-conjugated proteins. Activity was abolished in a NopD variant with a cysteine-to-alanine substitution in the catalytic core (NopD-C A). NopD recognizes specific plant SUMO proteins (AtSUMO1 and AtSUMO2 of ; GmSUMO of ; PvSUMO of ). Subcellular localization analysis with protoplasts showed that NopD accumulates in nuclear bodies. NopD, but not NopD-C A, induces cell death when expressed in . Likewise, inoculation tests with constructed mutant strains of XS1150 indicated that nodulation of is negatively affected by the protease activity of NopD. In conclusion, our findings show that NopD is a symbiosis-related protein that can process specific SUMO proteins and desumoylate SUMO-conjugated proteins.