Table 1.
Examples of repeat-containing protein (RCP) effectors from plant-associated bacteria.
| RCP (aaa) | Plant-associated organism | Host plant (relationship with host) | Repeat featuresb | Localization in planta | Part of an RCP effector family? | References |
|---|---|---|---|---|---|---|
| AvrPtoB/HopAB2 (553) | Pseudomonas syringae pv. tomato (plant-pathogenic bacterium) | Tomato (causes speck disease) | Two amphipathic degenerate non-tandem repeats of 85 and 110 aa identified by structural analyses that each adopt a four-helix bundle fold | Host cell cytoplasm | No | Kim et al., 2002; Abramovitch et al., 2003, 2006; de Torres et al., 2006; He et al., 2006; Janjusevic et al., 2006; Mucyn et al., 2006; Rosebrock et al., 2007; Xiao et al., 2007; Göhre et al., 2008; Shan et al., 2008; Dong et al., 2009; Gimenez-Ibanez et al., 2009; Cheng et al., 2011; Zeng et al., 2012; Mathieu et al., 2014 |
| Biological function: AvrPtoB, a type III effector that suppresses host immunity, carries an amino (N)-terminal and central repeat unit (repeat units one and two, respectively), as well as a carboxyl (C)-terminal U-box-type E3 ubiquitin ligase domain. Repeat units one and two bind and inhibit the kinase domain of the plasma membrane (PM)-localized host lysin motif (LysM)-receptor-like kinase (RLK) and leucine-rich repeat (LRR)-RLK immune receptors, Bti9 and BAK1, respectively, to suppress immunity related signaling. Repeat units one and two also bind the kinase domain of the LysM-RLK CERK1 and LRR-RLK FLS2 immune receptors, respectively, which may promote their ubiquitination and subsequent proteasome-dependent degradation via the E3 ligase domain. In addition, repeat unit one interacts with the host receptor-like cytoplasmic kinase (RLCK), Pto, while repeat unit two interacts with Pto and a related host RLCK, Fen. Following interaction with AvrPtoB, Pto activates host immunity in conjunction with Prf, an immune receptor of tomato. Fen however, can only activate host immunity in the absence of the E3 ubiquitin ligase domain. Interaction of Pto or Fen with repeat unit two results in the proteasome-dependent degradation of these proteins as above. Pto however, is able to resist degradation to activate Prf-dependent immunity upon interaction with repeat unit one, as this repeat unit is further away from the E3 ubiquitin ligase domain | ||||||
| HopI1 (432) | Pseudomonas syringae pv. maculicola (plant-pathogenic bacterium) | Brassicaceae (causes leaf spot disease) | Four hydrophilic imperfect intrinsically disordered tandem proline and glutamine (P/Q)-rich repeats of 27, 37, or 38 aa | Host cell chloroplast | No | Guttman et al., 2002; Jelenska et al., 2007, 2010 |
| Biological function: HopI1 is a type III effector that carries an N-terminal region of unknown function, a central repeat domain, and a C-terminal J-domain. HopI1 suppresses salicylic acid (SA) accumulation and related plant defenses. HopI1 also induces the remodeling of thylakoid stacks within chloroplasts. The J-domain of HopI1 directly binds to different plant Hsp70 isoforms and stimulates Hsp70 ATP hydrolysis activity in vitro. In association with Hsp70, HopI1 forms large complexes in planta, and recruits cytosolic Hsp70 to chloroplasts, a requirement for its virulence function. It has been suggested that Hsp70 may affect the folding/complex assembly of chloroplast factors related to plant immunity, including those required for SA biosynthesis and transport. The HopI1 repeat domain is not required for the interaction with Hsp70 or the association of this effector with chloroplasts. However, it is required for HopI1 virulence function. Thus, the HopI1 repeat domain may for example, interfere with these processes by actively affecting Hsp70 activity and/or substrate specificity | ||||||
| HsvG (671) | Pantoea agglomerans pv. gypsophilae (plant-pathogenic bacterium) | Gypsophila (root and crown gall disease) | Two amphipathic imperfect tandem repeats of 75 and 71 aa | Host cell nucleus | Yes | Valinsky et al., 1998; Nissan et al., 2006, 2012 |
| Biological function: HsvG is a type III effector that carries a central DNA-binding region and repeat domain (transcription activation domain; TAD). HsvG functions as a transcription factor that binds and activates the HSVGT gene promoter from Gypsophila paniculata. HSVGT encodes a predicted protein of the DnaJ family that has features typical of eukaryotic transcription factors, but lacks a J-domain. In addition to transcriptional activation, the HsvG repeat domain is required for host specificity (P. agglomerans pv. gypsophilae pathogenicity on gypsophila). HsvG requires two repeat units for pathogenicity on gypsophila (one is not sufficient) | ||||||
| PthXo1 (1373) | Xanthomonas oryzae pv. oryzae (plant-pathogenic bacterium) | Rice (causes blight disease) | Four amphipathic degenerate tandem repeats of 25 or 34 aa, followed by 23.5 amphipathic imperfect tandem repeats of 33 or 34 aa. All repeat units adopt a two-helix bundle fold | Host cell nucleus | Yes | Yang and White, 2004; Chu et al., 2006; Yang et al., 2006; Yuan et al., 2009; Chen et al., 2010; Gao et al., 2012; Mak et al., 2012 |
| Biological function: PthXo1 is a type III transcription activator-like (TAL) effector that binds and transcriptionally activates the promoter of OsSWEET11, a susceptibility gene from rice that encodes a SWEET sugar transporter, to promote colonization. It is thought that OsSWEET11 expression results in an excess of sucrose transport to the site of infection by X. oryzae pv. oryzae, which in turn provides the pathogen with a source of carbon. PthXo1 carries a central repeat domain, which forms a left-handed superhelix (α-solenoid) that physically wraps around the effector-binding element (EBE) of OsSWEET11, as well as a C-terminal eukaryotic activation domain (AD), which induces OsSWEET11 transcription. The first two degenerate repeat units of PthXo1 mediate non-base-specific interactions with the EBE, while the two subsequent degenerate repeat units mediate pairing with the EBE's initial 5′ thymine base. The remaining imperfect repeat units mediate base-specific interactions with the EBE, with specificity determined by the repeat-variable di-residues (RVDs) at positions 12 and 13 of each repeat unit. The recessive OsSWEET11 allele, xa13, confers resistance to X. oryzae pv. oryzae, and is based on a naturally mutated EBE that can neither be bound nor transcriptionally activated by PthXo1 | ||||||
| RipG7/GALA7 (647) | Ralstonia solanacearum (plant-pathogenic bacterium) | Broad host range (causes wilt disease) | Fifteen amphipathic degenerate mostly tandem LRRs of ~21–25 aa | Intracellular (host) | Yes | Cunnac et al., 2004; Angot et al., 2006; Remigi et al., 2011; Wang et al., 2015a |
| Biological function: RipG7 is a type III effector that carries an N-terminal F-box domain followed by a LRR domain. RipG7 interacts with several Arabidopsis thaliana SKP1-like (ASK) proteins. Together with six of its paralogs (RipG1–RipG6), RipG7 is essential for pathogenicity on A. thaliana, although functionally redundant with RipG2, 3 and 6, and required for full virulence on tomato. RipG7 is a virulence factor required for host-specific colonization of Medicago truncatula, with the F-box being essential for virulence, suggesting that RipG7 may mimic host F-box proteins and be recruited to SCF-type E3 ubiquitin ligase complexes to interfere with host ubiquitination and proteasome processing. The LRR domain is expected to recruit specific plant proteins to a SCFRipG7 E3 ubiquitin ligase for subsequent ubiquitination and possible degradation. Ten of 11 amino acid residue sites identified as being under strong positive selection across RipG7 from phylogenetically diverse strains of R. solanacearum are located within, or in loops between, predicted LRRs. This suggests an evolutionary arms race between R. solanacearum and its hosts that occurs at the interaction interface between RipG7 and its putative host targets | ||||||
| RipL (1390) | R. solanacearum | Five amphipathic degenerate tandem pentatricopeptide repeats (PPRs) of 35 aac | Intracellular (host) | No | Cunnac et al., 2004 | |
| Biological function: RipL is a type III effector with unknown function. PRRs possibly mediate the binding of RNA | ||||||
| RipS4/SKWP4 (2574) | R. solanacearum | At least 18 amphipathic imperfect/degenerate tandem HEAT/armadillo repeats of 40–42 aa | Intracellular (host) | Yes | Mukaihara and Tamura, 2009; Macho et al., 2010 | |
| Biological function: RipS4 is a type III effector required for full virulence on eggplant, although its specific function is unknown | ||||||
| RipTAL1 (1245) | R. solanacearum | Two amphipathic degenerate tandem repeats of 34 or 35 aa, followed by 16 amphipathic imperfect tandem repeats of 35 aa, and two amphipathic degenerate tandem repeats of 34 aa | Host cell nucleus | No | Macho et al., 2010; de Lange et al., 2013; Li et al., 2013 | |
| Biological function: RipTAL1 is a type III TAL effector required for full virulence of R. solanacearum on eggplant, and probably promotes virulence through the transcriptional activation of a host susceptibility gene. RipTAL1 carries a central repeat domain, which mediates interaction with the EBE of a target host gene promoter, and a C-terminal eukaryotic acidic AD, which induces transcription of the target host gene. The N-terminal degenerate repeat units of RipTAL1 mediate pairing with EBEs containing an initial 5′ guanine base. The imperfect repeat units mediate base-specific interactions with the EBE, with specificity mainly determined by RVDs at positions 12 and 13 of each repeat unit, although certain non-RVD residues also have a significant impact on DNA recognition | ||||||
| RipY (912) | R. solanacearum | At least six amphipathic degenerate mostly tandem ankyrin repeats of 31 aac | Intracellular (host) | No | Cunnac et al., 2004; Macho et al., 2010 | |
| Biological function: RipY is a type III effector required for full virulence on eggplant, although its specific function is unknown | ||||||
| XopAC/AvrAC (536) | Xanthomonas campestris pv. campestris | Brassicaceae (causes black rot disease) | Six amphipathic degenerate tandem LRRs of 23–24 aa | Intracellular (host PM) | No | Xu et al., 2008; Feng et al., 2012; Guy et al., 2013; Wang et al., 2015b |
| Biological function: XopAC is a type III effector that enhances the virulence of X. campestris pv. campestris and suppresses plant immunity. It has an N-terminal region, followed by a LRR domain, and a C-terminal FIC (filamentation induced by cyclic AMP) domain, with the latter possessing uridine 5′-monophosphate (UMP) transferase enzymatic activity. In susceptible A. thaliana plants, the FIC domain transfers UMP to phosphorylation sites in the activation loop of several immunity-related RLCKs, including BIK1, PBL1, and RIPK. This prevents their phosphorylation, thereby reducing their kinase activities and inhibiting their downstream immune signaling. In A. thaliana ecotype Col-0 vascular tissues, XopAC is recognized as an avirulence protein, with the LRRs, as well as the FIC domain and its associated uridylylation activity required to trigger the avirulent phenotype. Such immunity is dependent upon the RLCK, PBL2. Although PBL2 is a paralog of BIK1, it is solely required for immunity, indicating that PBS2 is a decoy of BIK1 that enables XopAC recognition by the host. Notably, the N-terminal region and LRR domain of XopAC are required for the interaction with RLCKs, with localization of XopAC to the host PM also dependent upon the LRRs | ||||||
| XopD (760) | Xanthomonas euvesicatoria (plant-pathogenic bacterium) | Tomato/pepper (causes leaf spot disease) | Two amphipathic tandem ERF-associated repression (EAR) motifs of 6 aa | Host cell nucleus (subnuclear foci) | No | Hotson et al., 2003; Chosed et al., 2007; Kim et al., 2008, 2013 |
| Biological function: XopD is a type III effector that promotes pathogen growth by suppressing activation of host immunity via plant SUMO protease mimicry. It has an N-terminal DNA-binding domain (DBD), two EAR motifs (typically found in plant repressors that regulate stress-induced transcription) in the central domain and a C-terminal SUMO peptidase domain. XopD possesses both plant-specific peptidase activity, resulting in cleavage of SUMO isoforms, and isopeptidase activity, resulting in cleavage of SUMO from SUMO conjugates. All three domains are collectively required to desumoylate the transcription factor SIERF4 to suppress ethylene production and signaling. The mechanism by which the DBD and EAR motifs modulate the protease activity is not known, however they may mediate critical interactions with DNA or proteins within plant transcription factor complexes to influence effector specificity | ||||||
| XopL (660) | X. euvesicatoria | Nine amphipathic degenerate tandem LRRs of 23–33 aa | Intracellular (host) | No | Singer et al., 2013 | |
| Biological function: XopL is a type III effector that has E3 ubiquitin ligase activity responsible for initiating cell death in the non-host Nicotiana benthamiana, but not in the hosts tomato and pepper. The N-terminal LRR domain is solely required for host immunity-related gene expression when assayed in A. thaliana. XopL recruits plant E2 enzymes, mimicking components of the host ubiquitination machinery, with the LRRs possibly acting as protein–protein interaction modules for ubiquitination target recognition | ||||||
| XopN (733) | X. euvesicatoria | Seven amphipathic degenerate tandem HEAT/armadillo-like repeats | Host cytoplasm and PM | No | Roden et al., 2004; Kim et al., 2009; Taylor et al., 2012 | |
| Biological function: XopN is a type III effector that suppresses host immune responses. It interacts with the atypical LRR-RLK, TARK1 (via the non-repetitive N-terminal region), and the tomato 14-3-3 isoform TFT1 (via the C-terminal HEAT/armadillo-like repeats), both of which are positive regulators of host immunity in tomato. XopN is expected to promote and/or stabilize TARK1/TFT1 complex formation by functioning as a protein bridge or molecular scaffold, since these proteins only interact in the presence of XopN. It remains unclear how these interactions repress the host immune response, although XopN may interfere with TARK1 protein–protein interactions, stability and/or signal transduction, and TFT1 client interactions. Another possibility is that the action of XopN leads to the sequestration of inactive immune complexes, preventing downstream immune signaling | ||||||
a
Protein length in amino acids (aa).
b
Repeat hydropathy profiles were determined using the Expasy ProtScale server (http://web.expasy.org/protscale/), with default server settings.
c
PPR and ankyrin repeats were predicted using TPRpred (http://toolkit.tuebingen.mpg.de/tprpred) and InterProScan 5 (http://www.ebi.ac.uk/Tools/pfa/iprscan5/), respectively.