Author: Joël F. Pothier & Trainees from the 2^nd EuroXanth Training School (Amandine Cunty, Filip Gazdik, Leonor Martins, Cinzia Van Malderghem, Esther Van Veen)
Internal reviewer: Alexandre B. de Menezes
Expert reviewer: Rebecca Bart

Class: XopK
Family: XopK
Prototype: XOO1669 (Xanthomonas oryzae pv. oryzae; strain T7174)
GenBank ID: BAE68424.1 (845 aa)
RefSeq ID: WP_027703763.1 (843 aa)
3D structure: Unknown

XopK was discovered as a putative T3E based on the presence of a plant-inducible promoter box-like sequence and a -10 box-like sequence (Furutani et al., 2006).

Mutation of a putative ubiquitin-conjugation enzyme (E2) binding site abolished XopK-induced degradation of rice somatic receptor kinase 2 (OsSERK2) and compromised XopK-dependent virulence (Qin et al., 2018). Expression of XopK is HrpX-dependent (Furutani et al., 2006) and was observed to translocate using a Cya reporter system (Furutani et al., 2009).

Preceded by both a PIP box and a -10 box-like motif (Schulze et al., 2012; Furutani et al., 2006).

qRT-PCR revealed that transcript levels of 15 out of 18 tested non-TAL effector genes (as well as the regulatory genes hrpG and hrpX), including xopK, were significantly reduced in the Xanthomonas oryzae pv. oryzae ΔxrvC mutant compared with those in the wild-type strain PXO99^A (Liu et al., 2016).

• Deletion of XopK has been shown not to affect the virulence of X. oryzae pv. oryzae PXO99A in rice IR24 plants; these differential results could be attributed to different genotypes of the rice cultivar or field conditions for plant growth (Song & Yang, 2010).
• A ∆xopK mutant strain of Xanthomonas phaseoli pv. manihotis (aka Xanthomonas axonopodis pv. manihotis) exhibited enhanced induction of disease symptoms in cassava at the site of inoculation but reduced spread through the vasculature (Mutka et al., 2016).
• XopK inhibits pathogen-associated molecular pattern-triggered immunity upstream of mitogen-activated protein kinase cascades (Qin et al., 2018)

The XopK sequence contains 54% hydrophobic residues and several predicted transmembrane domains. Thus, it is possible this protein is associated with host cell membranes following secretion (Mutka et al., 2016)

The protein has E3 ubiquinol ligase activity. The putative E2-binding site is highly conserved in the majority of members from different Xanthomonas strains except for X. oryzae pv. oryzicola strains (W420C).

XopK interacted with and directly ubiquitinated rice somatic embryogenic receptor kinase 2 (OsSERK2), resulting in its degradation (Qin et al., 2018)

Yes (based on EDGAR; e.g., X. oryzae pvs. oryzae and oryzicola, X. citri pvs. citri, malvacearum, fuscans and glycines, X. euvesicatoria, X. perforans, X. campestris, X. cynarae pv. gardneri, X. fragariae, X. translucens, X. vesicatoria). In addition to the taxa above, BLAST against the GenBank nt database also shows matches to X. arboricola, X. hortorum, X. hyacinthi.

Yes (e.g., Acidovorax spp. with 42% AAI). BLAST against GenBank nt DB suggests that Acidovorax spp. is the only group in which this gene may be present outside Xanthomonadaceae.

Furutani A, Nakayama T, Ochiai H, Kaku H, Kubo Y, Tsuge S (2006). Identification of novel HrpXo regulons preceded by two cis-acting elements, a plant-inducible promoter box and a -10 box-like sequence, from the genome database of Xanthomonas oryzae pv. oryzae. FEMS Microbiol. Lett. 259: 133-141. DOI: 10.1111/j.1574-6968.2006.00265.x

Furutani A, Takaoka M, Sanada H, Noguchi Y, Oku T, Tsuno K, Ochiai H, Tsuge S (2009). Identification of novel type III secretion effectors in Xanthomonas oryzae pv. oryzae. Mol. Plant Microbe Interact. 22: 96-106. DOI: 10.1094/MPMI-22-1-0096

Liu Y, Long J, Shen D, Song C (2016). Xanthomonas oryzae pv. oryzae requires H-NS-family protein XrvC to regulate virulence during rice infection. FEMS Microbiol. Lett. 363: fnw067. DOI: 10.1093/femsle/fnw067

Mutka AM, Fentress SJ, Sher SW, Berry JC, Pretz C, Nusinow DA, Bart R (2016). Quantitative, image-based phenotyping methods provide insight into spatial and temporal dimensions of plant disease. Plant Physiol. 172: 650-660. DOI: 10.1104/pp.16.00984

Qin J, Zhou X, Sun L, Wang K, Yang F, Liao H, Rong W, Yin J, Chen H, Chen X, Zhang J (2018). The Xanthomonas effector XopK harbours E3 ubiquitin-ligase activity that is required for virulence. New Phytol. 220: 219-231. DOI: 10.1111/nph.15287

Schulze S, Kay S, Büttner D, Egler M, Eschen-Lippold L, Hause G, Krüger A, Lee J, Müller O, Scheel D, Szczesny R, Thieme F, Bonas U (2012). Analysis of new type III effectors from Xanthomonas uncovers XopB and XopS as suppressors of plant immunity. New Phytol. 195: 894-911. DOI: 10.1111/j.1469-8137.2012.04210.x

Song C, Yang B (2010). Mutagenesis of 18 type III effectors reveals virulence function of XopZ_PXO99 in Xanthomonas oryzae pv. oryzae. Mol. Plant Microbe Interact. 23: 893-902. DOI: 10.1094/MPMI-23-7-0893

This fact sheet is based upon work from COST Action CA16107 EuroXanth, supported by COST (European Cooperation in Science and Technology).