Polycomb-repressive complexes (PRCs) play key roles in development by repressing a large number of genes involved in various functions. Much, however, remains to be discovered about PRC-silencing mechanisms as well as their targeting to specific genomic regions. Besides other mechanisms, GAGA-binding factors in animals can guide PRC members in a sequencespecific manner to Polycomb-responsive DNA elements. Here, we show that the Arabidopsis (Arabidopsis thaliana) GAGA-motif binding factor protein BASIC PENTACYSTEINE6 (BPC6) interacts with LIKE HETEROCHROMATIN PROTEIN1 (LHP1), a PRC1 component, and associates with VERNALIZATION2 (VRN2), a PRC2 component, in vivo. By using a modified DNAprotein interaction enzyme-linked immunosorbant assay, we could show that BPC6 was required and sufficient to recruit LHP1 to GAGA motif-containing DNA probes in vitro. We also found that LHP1 interacts with VRN2 and, therefore, can function as a possible scaffold between BPC6 and VRN2. The lhp1-4 bpc4 bpc6 triple mutant displayed a pleiotropic phenotype, extreme dwarfism and early flowering, which disclosed synergistic functions of LHP1 and group II plant BPC members. Transcriptome analyses supported this synergy and suggested a possible function in the concerted repression of homeotic genes, probably through histone H3 lysine-27 trimethylation. Hence, our findings suggest striking similarities between animal and plant GAGAbinding factors in the recruitment of PRC1 and PRC2 components to Polycomb-responsive DNA element-like GAGA motifs, which must have evolved through convergent evolution.

Fluorescence-based protein-protein interaction techniques are vital tools for understanding in vivo cellular functions on a mechanistic level. However, only under the condition of highly efficient (co)transformation and accumulation can techniques such as Förster resonance energy transfer (FRET) realize their potential for providing highly accurate and quantitative interaction data. FRET as a fluorescence-based method unifies several advantages, such as measuring in an in vivo environment, real-time context, and the ability to include transient interactions as well as detecting the mere proximity of proteins. Here, we introduce a novel vector set that incorporates the benefit of the recombination-based 2in1 cloning system with the latest state-of-the-art fluorescent proteins for optimal coaccumulation and FRET output studies. We demonstrate its utility across a range of methods. Merging the 2in1 cloning system with new-generation FRET fluorophore pairs allows for enhanced detection, speeds up the preparation of clones, and enables colocalization studies and the identification of meaningful protein-protein interactions in vivo.Two technological advances allowed the field of modern cell biology to emerge: the development of high-resolution microscopes and the groundbreaking work in the development of fluorescent proteins (FPs) that were originally isolated from jellyfish (for review, see Day and Davidson, 2009;Zimmer, 2009). Genetically encoded FPs can be fused directly to proteins of choice, offering insights into their functional properties by allowing researchers to monitor subcellular localization, chemical environment, interaction, movement, and/or turnover rate.Research on the FPs themselves has exploded in the past two decades, and a range of technological improvements to and variations of the FPs were generated. These improvements include novel spectral properties from all areas of the color palette and from a diverse range of organisms. They also extend to a multitude of parameters, such as brightness, folding efficiency, chromophore oxidation rate, quantum yield, pH, and photostability (Day and Davidson, 2009). Taken together, these optimizations further the possibilities of available methods or prompt the development of new techniques.In 1946, the German scientist Theodor Förster laid the theoretical foundation for resonance energy transfer, now known as Förster resonance energy transfer (FRET;Forster, 1946). According to this theory, a donor chromophore in an excited state can transfer the excitation energy to a nearby acceptor chromophore via dipole-dipole coupling without the emission of a photon (for review, see Clegg, 2009;Ishikawa-Ankerhold et al., 2012). FRET depends on the spectral overlap between the emission spectrum of the donor and the absorbance spectrum of the acceptor chromophores. It also depends on the fluorescence quantum yield and the excited state lifetime of the donor in the absence of an acceptor as well as the relative orientation of the two chromophores. Most importantly, FRET ef...

GAGA-motif binding proteins control transcriptional activation or repression of homeotic genes. Interestingly, there are no sequence similarities between animal and plant proteins. Plant BBR/BPC-proteins can be classified into two distinct groups: Previous studies have elaborated on group I members only and so little is known about group II proteins. Here, we focused on the initial characterization of AtBPC6, a group II protein from Arabidopsis thaliana. Comparison of orthologous BBR/BPC sequences disclosed two conserved signatures besides the DNA binding domain. A first peptide signature is essential and sufficient to target AtBPC6-GFP to the nucleus and nucleolus. A second domain is predicted to form a zipper-like coiled-coil structure. This novel type of domain is similar to Leucine zippers, but contains invariant alanine residues with a heptad spacing of 7 amino acids. By yeast-2-hybrid and BiFC-assays we could show that this Alanine zipper domain is essential for homotypic dimerization of group II proteins in vivo. Interhelical salt bridges and charge-stabilized hydrogen bonds between acidic and basic residues of the two monomers are predicted to form an interaction domain, which does not follow the classical knobs-into-holes zipper model. FRET-FLIM analysis of GFP/RFP-hybrid fusion proteins validates the formation of parallel dimers in planta. Sequence comparison uncovered that this type of domain is not restricted to BBR/BPC proteins, but is found in all kingdoms.

Cytokinin plays diverse roles in plant growth and development, generally acting by modulating gene transcription in target tissues. The type-B Arabidopsis response regulators (ARR) transcription factors have emerged as primary targets of cytokinin signaling and are required for essentially all cytokinin-mediated changes in gene expression. The diversity of cytokinin function is likely imparted by the activity of various transcription factors working with the type-B ARRs to alter specific sets of target genes. One potential set of co-regulators modulating the cytokinin response are the BARLEY B-RECOMBINANT/BASIC PENTACYSTEINE (BBR/BPC) family of plant-specific transcription factors. Here, we show that disruption of multiple BPCs results in reduced sensitivity to cytokinin. Further, the BPCs are necessary for the induction of a subset of genes in response to cytokinin. We identified direct in vivo targets of BPC6 using ChIP-Seq and found an enrichment of promoters of genes differentially expressed in response to cytokinin. Further, a significant number of BPC6 regulated genes are also direct targets of the type-B ARRs. Potential cis-binding elements for a number of other transcription factors linked to cytokinin action are enriched in the BPC binding fragments, including those for the cytokinin response factors (CRFs). In addition, several BPCs interact with a subset of type-A ARRs. Consistent with these results, a significant number of genes whose expression is altered in bpc mutant roots are also mis-expressed in crf1,3,5,6 and type-A arr3,4,5,6,7,8,9,15 mutant roots. These results suggest that the BPCs are part of a complex network of transcription factors that are involved in the response to cytokinin.

Clathrin ensures mitotic spindle stability and efficient chromosome alignment, independently of its vesicle trafficking function. Although clathrin localizes to the mitotic spindle and kinetochore fiber microtubule bundles, the mechanisms by which clathrin stabilizes microtubules are unclear. We show that clathrin adaptor interaction sites on clathrin heavy chain (CHC) are repurposed during mitosis to directly recruit the microtubule-stabilizing protein GTSE1 to the spindle. Structural analyses reveal that these sites interact directly with clathrin-box motifs on GTSE1. Disruption of this interaction releases GTSE1 from spindles, causing defects in chromosome alignment. Surprisingly, this disruption destabilizes astral microtubules, but not kinetochore-microtubule attachments, and chromosome alignment defects are due to a failure of chromosome congression independent of kinetochore–microtubule attachment stability. GTSE1 recruited to the spindle by clathrin stabilizes microtubules by inhibiting the microtubule depolymerase MCAK. This work uncovers a novel role of clathrin adaptor-type interactions to stabilize nonkinetochore fiber microtubules to support chromosome congression, defining for the first time a repurposing of this endocytic interaction mechanism during mitosis.