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

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

Overlapping expression patterns among the genes encoding Arabidopsis chromosomal high mobility group (HMG) proteins

AbstractHigh mobility group (HMG) proteins are usually considered ubiquitous components of the eukaryotic chromatin. Using HMG gene promoter-GUS reporter gene fusions we have examined the expression of the reporter gene in transgenic Arabidopsis plants. These experiments have revealed that the different HMGA and HMGB promoters display overlapping patterns of activity, but they also show tissue- and developmental stage-specific differences. Moreover, leader introns that are present in some of the HMGB genes can modulate reporter gene expression. The differential HMG gene expression supports the view that the various HMG proteins serve partially different architectural functions in plant chromatin

Arabidopsis chromatin-associated HMGA and HMGB use different nuclear targeting signals and display highly dynamic localization within the nucleus

Launholt D, Merkle T, Houben A, Schulz A, Grasser KD. Arabidopsis chromatin-associated HMGA and HMGB use different nuclear targeting signals and display highly dynamic localization within the nucleus. The Plant Cell. 2006;18(11):2904-2918.In plants, the chromatin-associated high mobility group ( HMG) proteins occur in two subfamilies termed HMGA and HMGB. The HMGA proteins are characterized by the presence of four AT-hook DNA binding motifs, and the HMGB proteins contain an HMG box DNA binding domain. As architectural factors, the HMG proteins appear to be involved in the regulation of transcription and other DNA-dependent processes. We have examined the subcellular localization of Arabidopsis thaliana HMGA, HMGB1, and HMGB5, revealing that they localize to the cell nucleus. They display a speckled distribution pattern throughout the chromatin of interphase nuclei, whereas none of the proteins associate with condensed mitotic chromosomes. HMGA is targeted to the nucleus by a monopartite nuclear localization signal, while efficient nuclear accumulation of HMGB1/5 requires large portions of the basic N-terminal part of the proteins. The acidic C-terminal domain interferes with nucleolar targeting of HMGB1. Fluorescence recovery after photobleaching experiments revealed that HMGA and HMGB proteins are extremely dynamic in the nucleus, indicating that they bind chromatin only transiently before moving on to the next site, thereby continuously scanning the genome for targets. By contrast, the majority of histone H2B is basically immobile within the nucleus, while linker histone H1.2 is relatively mobile