The TAL Effector PthA4 Interacts with Nuclear Factors Involved in RNA-Dependent Processes Including a HMG Protein That Selectively Binds Poly(U) RNA

Abstract

Plant pathogenic bacteria utilize an array of effector proteins to cause disease. Among them, transcriptional activator-like (TAL) effectors are unusual in the sense that they modulate transcription in the host. Although target genes and DNA specificity of TAL effectors have been elucidated, how TAL proteins control host transcription is poorly understood. Previously, we showed that the Xanthomonas citri TAL effectors, PthAs 2 and 3, preferentially targeted a citrus protein complex associated with transcription control and DNA repair. To extend our knowledge on the mode of action of PthAs, we have identified new protein targets of the PthA4 variant, required to elicit canker on citrus. Here we show that all the PthA4-interacting proteins are DNA and/or RNA-binding factors implicated in chromatin remodeling and repair, gene regulation and mRNA stabilization/modification. The majority of these proteins, including a structural maintenance of chromosomes protein (CsSMC), a translin-associated factor X (CsTRAX), a VirE2-interacting protein (CsVIP2), a high mobility group (CsHMG) and two poly(A)-binding proteins (CsPABP1 and 2), interacted with each other, suggesting that they assemble into a multiprotein complex. CsHMG was shown to bind DNA and to interact with the invariable leucine-rich repeat region of PthAs. Surprisingly, both CsHMG and PthA4 interacted with PABP1 and 2 and showed selective binding to poly(U) RNA, a property that is novel among HMGs and TAL effectors. Given that homologs of CsHMG, CsPABP1, CsPABP2, CsSMC and CsTRAX in other organisms assemble into protein complexes to regulate mRNA stability and translation, we suggest a novel role of TAL effectors in mRNA processing and translational control.

Effector Biology of Plant-Associated Organisms: Concepts and Perspectives

“…Although it appears that the majority of Pseudomonas syringae T3SS effectors function as immunosuppressors, many other distinct functions have been assigned to pathogen effectors. The TAL effectors of xanthomonads activate host genes involved in processes as diverse as cell size, sugar transport, and epigenetics (Duan et al 1999;Chen et al 2010;Domingues et al 2010;de Souza et al 2012). Aster yellows phytoplasmas secrete SAP proteins that directly perturb plant development to increase the mass of host green tissue and the likelihood of transmission by their insect vector Sugio et al 2011).…”

“…Another group of effectors have evolved to bind nucleic acids acting as modulators of gene expression. Xanthomonas TAL effectors directly bind elements in plant gene promoters to activate gene expression of host genes that benefit the pathogen (Duan et al 1999;Boch et al 2009;Domingues et al 2010;de Souza et al 2012). Among the host susceptibility genes that are induced by TAL effectors are the SWEET sugar transporters that are thought to release sugar to contribute to the nutrition of the invading bacteria .…”

Citrus MAF1, a Repressor of RNA Polymerase III, Binds the Xanthomonas citri Canker Elicitor PthA4 and Suppresses Citrus Canker Development

“…Moreover, we found that some of the PthA interactors form protein complexes and that distinct PthA proteins, although highly homologous to each other, have differential binding specificity or affinity to certain host targets de Souza et al, 2012). We showed that, while PthA2 and PthA3 preferentially interacted with the sweet orange protein complex comprising the cyclophilin CsCYP, thioredoxin CsTDX, and the CsUEV/ UBC13 heterodimer, PthA4 selectively bound to CsVIP2, the citrus homolog of the Arabidopsis (Arabidopsis thaliana) VirE-INTERACTING PROTEIN2 de Souza et al, 2012). Importantly, we found that not only CsCYP and CsTDX, but all PthA variants, interact with the C-terminal domain of the citrus RNA polymerase II (CTD), a regulatory domain that plays critical roles in all phases of the transcription cycle (Sobennikova et al, 2007;Domingues et al, 2012;Campos et al, 2013).…”

“…Accordingly, we identified numerous sweet orange (Citrus sinensis) proteins implicated in nuclear import, transcriptional regulation, DNA repair, and mRNA stabilization as possible targets of the X. citri PthA proteins de Souza et al, 2012). Moreover, we found that some of the PthA interactors form protein complexes and that distinct PthA proteins, although highly homologous to each other, have differential binding specificity or affinity to certain host targets de Souza et al, 2012).…”

“…Accordingly, we identified numerous sweet orange (Citrus sinensis) proteins implicated in nuclear import, transcriptional regulation, DNA repair, and mRNA stabilization as possible targets of the X. citri PthA proteins de Souza et al, 2012). Moreover, we found that some of the PthA interactors form protein complexes and that distinct PthA proteins, although highly homologous to each other, have differential binding specificity or affinity to certain host targets de Souza et al, 2012). We showed that, while PthA2 and PthA3 preferentially interacted with the sweet orange protein complex comprising the cyclophilin CsCYP, thioredoxin CsTDX, and the CsUEV/ UBC13 heterodimer, PthA4 selectively bound to CsVIP2, the citrus homolog of the Arabidopsis (Arabidopsis thaliana) VirE-INTERACTING PROTEIN2 de Souza et al, 2012).…”