Author: Ralf Koebnik
Internal reviewer:
Expert reviewer: WANTED!

Class: XopAI
Family: XopAI
Prototype: XAC3230 (Xanthomonas citri pv. citri; strain 306)
GenBank ID: AAM38074.1 (296 aa)
RefSeq ID: WP_011052119.1 (296 aa)
3D structure: 6K93, 6K94, 6KLY (Liu et al., 2019)

Based on the complete genome sequence, XopAI (XAC3230) was proposed as a Hrp regulon candidate (da Silva et al., 2002). Based on homology to effectors from Pseudomonas syringae and a strongly conserved 43‐amino‐acid N‐terminal domain that is also found in the N‐termini of effectors in class XopE and XopJ, it was proposed to be a T3E (White et al., 2009).

Unknown.

Unknown.

Unknown.

The XopAI N‐terminal domain contains a myristoylation motif, which was previously identified in several T3Es of P. syringae, indicating that effectors with this N‐terminal domain are targeted to host cellular membranes (White et al., 2009).

The C-terminal region of XopAI has similarity to predicted ADP-ribosyl transferase domains of the effector HopO1-1 of Pseudomonas syringae (Moreira et al., 2010).

XopAI was predicted to be a member of the arginine-specific mono-ADP-ribosyltransferase (mART) family. However, the crystal structure of XopAI revealed an altered active site that is unsuitable to bind the cofactor NAD+, but with the capability to capture an arginine-containing peptide from XopAI itself. Based on this finding, it was proposed that XopAI may not be a qualified mART, and it would exert different effects on host cells (Liu et al., 2019).

Structural homologs of XopAI are, among others, the HopU1 (Pseudomonas syringae T3SS-secreted effector HopU1, PDB code 3U0J), Tre1 (Serratia proteamaculans T6SS-secreted ADP-ribosyltransferase effector 1, PDB code 6DRH), ART2.2 (rat mART2.2, PDB code 1GXY), and ExoS (Pseudomonas aeruginosa exoenzyme S, PDB code 6GN8).

XopAI is conserved in the Citrus canker strains, including X. citri pv. citri and X. citri pv. aurantifolii (ex X. fuscans pv. aurantifolii) (Moreira et al., 2010). However, XopAI is absent from the Citrus bacterial spot pathogen, X. euvesicatoria pv. citrumelonis (ex X. axonopodis pv. citrumelo) strain F1 (Jalan et al., 2011).

XopAI homologs were also found in X. citri pv. bilvae and X. citri pv. glycines, as well as in the species X. arboricola, X. hortorum, X. vesicatoria (Moreira et al., 2010; Liu et al., 2019).

Yes (e.g., Acidovorax citrulli, Pseudomonas syringae, Ralstonia solanacearum) (Moreira et al., 2010)

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Gaurav I, Thakur A, Kumar G, Long Q, Zhang K, Sidu RK, Thakur S, Sarkar RK, Kumar A, Iyaswamy A, Yang Z (2023). Delivery of apoplastic extracellular vesicles encapsulating green-synthesized silver nanoparticles to treat citrus canker. Nanomaterials (Basel) 13: 1306. DOI: 10.3390/nano13081306

Jalan N, Aritua V, Kumar D, Yu F, Jones JB, Graham JH, Setubal JC, Wang N (2011). Comparative genomic analysis of Xanthomonas axonopodis pv. citrumelo F1, which causes citrus bacterial spot disease, and related strains provides insights into virulence and host specificity. J. Bacteriol. 193: 6342-6357. DOI: 10.1128/JB.05777-11

Liu JH, Yang JY, Hsu DW, Lai YH, Li YP, Tsai YR, Hou MH (2019). Crystal structure-based exploration of arginine-containing peptide binding in the ADP-ribosyltransferase domain of the type III effector XopAI protein. Int. J. Mol. Sci. 20: 5085. DOI: 10.3390/ijms20205085

Moreira LM, Almeida NF Jr, Potnis N, Digiampietri LA, Adi SS, Bortolossi JC, da Silva AC, da Silva AM, de Moraes FE, de Oliveira JC, de Souza RF, Facincani AP, Ferraz AL, Ferro MI, Furlan LR, Gimenez DF, Jones JB, Kitajima EW, Laia ML, Leite RP Jr, Nishiyama MY, Rodrigues Neto J, Nociti LA, Norman DJ, Ostroski EH, Pereira HA Jr, Staskawicz BJ, Tezza RI, Ferro JA, Vinatzer BA, Setubal JC (2010). Novel insights into the genomic basis of citrus canker based on the genome sequences of two strains of Xanthomonas fuscans subsp. aurantifolii. BMC Genomics 11: 238. DOI: 10.1186/1471-2164-11-238

White FF, Potnis N, Jones JB, Koebnik R (2009). The type III effectors of Xanthomonas. Mol. Plant Pathol. 10: 749-766. DOI: 10.1111/j.1364-3703.2009.00590.x