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).

Co-regulation of xopAI with other HrpG-controlled genes was also proposed by Noël et al. (2006) based on the observation that 200 bp of the 5' sequence including promoter and coding regions of xopE2 (avrXacE3) and xopAI (XAC3230) from X. citri pv. citri are more than 85% identical to the corresponding region of xopJ, which is a member of the HrpG regulon from X. euvesicatoria pv. euvesicatoria (ex X. campestris pv. vesicatoria).

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, XopAI was proposed to be a T3E (Stavrinides et al., 2006; White et al., 2009).

Unknown.

Using microarrays, seven T3E genes from X. citri pv. citri were found to be upregulated in planta, five of which in all the three times investigated, i.e. 24 hpi, 72 hpi and 120 hpi (xopE1, xopN, xopK, xopE3, xopAI), and the two remaining at the later times of the infectious process (xopE2 and xopV) (de Laia et al., 2019).

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) (Liu et al., 2019).

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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de Laia ML, Moreira LM, Gonçalves JF, Ferro MI, Pinto Rodrigues AC, dos Santos JN, Felestrino ÉB, Ferro JA (2019). Gene expression analysis identifies hypothetical genes that may be critical during the infection process of Xanthomonas citri subsp. citri.
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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

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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

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