Telomere repeat binding proteins (TRBs) belong to a family of proteins possessing a Myb-like domain which binds to telomeric repeats. Three members of this family (TRB1, TRB2, TRB3) from Arabidopsis thaliana have already been described as associated with terminal telomeric repeats (telomeres) or short interstitial telomeric repeats in gene promoters (telo-boxes). They are also known to interact with several protein complexes: telomerase, Polycomb repressive complex 2 (PRC2) E(z) subunits and the PEAT complex (PWOs-EPCRs-ARIDs-TRBs). Here we characterize two novel members of the TRB family (TRB4 and TRB5). Our wide phylogenetic analyses have shown that TRB proteins evolved in the plant kingdom after the transition to a terrestrial habitat in Streptophyta, and consequently TRBs diversified in seed plants. TRB4-5 share common TRB motifs while differing in several others and seem to have an earlier phylogenetic origin than TRB1-3. Their common Myb-like domains bind long arrays of telomeric repeats in vitro, and we have determined the minimal recognition motif of all TRBs as one telo-box. Our data indicate that despite the distinct localization patterns of TRB1-3 and TRB4-5 in situ, all members of TRB family mutually interact and also bind to telomerase/PRC2/PEAT complexes. Additionally, we have detected novel interactions between TRB4-5 and EMF2 and VRN2, which are Su(z)12 subunits of PRC2.

• Keywords
• PEAT, PRC2, TERT, TRB, Telomere repeat binding, Telomeric,
• MeSH
• Arabidopsis * genetics   metabolism   MeSH
• Phylogeny MeSH
• Arabidopsis Proteins * genetics   metabolism   MeSH
• Telomere-Binding Proteins genetics   metabolism   MeSH
• Soil MeSH
• Telomerase * genetics   metabolism   MeSH
• Telomere genetics   metabolism   MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Arabidopsis Proteins * MeSH
• Telomere-Binding Proteins MeSH
• Soil MeSH
• Telomerase * MeSH

Telomeres are basic structures of eukaryote genomes. They distinguish natural chromosome ends from double-stranded breaks in DNA and protect chromosome ends from degradation or end-to-end fusion with other chromosomes. Telomere sequences are usually tandemly arranged minisatellites, typically following the formula (TxAyGz)n. Although they are well conserved across large groups of organisms, recent findings in plants imply that their diversity has been underestimated. Changes in telomeres are of enormous evolutionary importance as they can affect whole-genome stability. Even a small change in the telomere motif of each repeat unit represents an important interference in the system of sequence-specific telomere binding proteins. Here, we provide an overview of telomere sequences, considering the latest phylogenomic evolutionary framework of plants in the broad sense (Archaeplastida), in which new telomeric sequences have recently been found in diverse and economically important families such as Solanaceae and Amaryllidaceae. In the family Lentibulariaceae and in many groups of green algae, deviations from the typical plant telomeric sequence have also been detected recently. Ancestry and possible homoplasy in telomeric motifs, as well as extant gaps in knowledge are discussed. With the increasing availability of genomic approaches, it is likely that more telomeric diversity will be uncovered in the future. We also discuss basic methods used for telomere identification and we explain the implications of the recent discovery of plant telomerase RNA on further research about the role of telomerase in eukaryogenesis or on the molecular causes and consequences of telomere variability.

• Keywords
• Allium, Cestrum, Genlisea, circular chromosomes, green algae, linear chromosomes, telomerase, telomeres,
• Publication type
• Journal Article MeSH
• Review MeSH

Telomeric repeat binding factor 2 (TRF2) folds human telomeres into loops to prevent unwanted DNA repair and chromosome end-joining. The N-terminal basic domain of TRF2 (B-domain) protects the telomeric displacement loop (D-loop) from cleavage by endonucleases. Repressor activator protein 1 (Rap1) binds TRF2 and improves telomeric DNA recognition. We found that the B-domain of TRF2 stabilized the D-loop and thus reduced unwinding by BLM and RPA, whereas the formation of the Rap1-TRF2 complex restored DNA unwinding. To understand how the B-domain of TRF2 affects DNA binding and D-loop processing, we analyzed DNA binding of full-length TRF2 and a truncated TRF2 construct lacking the B-domain. We quantified how the B-domain improves TRF2's interaction with DNA via enhanced long-range electrostatic interactions. We developed a structural envelope model of the B-domain bound on DNA. The model revealed that the B-domain is flexible in solution but becomes rigid upon binding to telomeric DNA. We proposed a mechanism for how the B-domain stabilizes the D-loop.

• MeSH
• DNA chemistry   metabolism   MeSH
• Humans MeSH
• Telomeric Repeat Binding Protein 2 chemistry   metabolism   MeSH
• Protein Domains MeSH
• Telomere-Binding Proteins metabolism   MeSH
• Shelterin Complex MeSH
• Static Electricity MeSH
• Telomere chemistry   metabolism   MeSH
• Protein Binding MeSH
• Check Tag
• Humans MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• DNA MeSH
• Telomeric Repeat Binding Protein 2 MeSH
• Telomere-Binding Proteins MeSH
• Shelterin Complex MeSH
• TERF2 protein, human MeSH Browser
• TERF2IP protein, human MeSH Browser

Telomeres, as physical ends of linear chromosomes, are targets of a number of specific proteins, including primarily telomerase reverse transcriptase. Access of proteins to the telomere may be affected by a number of diverse factors, e.g., protein interaction partners, local DNA or chromatin structures, subcellular localization/trafficking, or simply protein modification. Knowledge of composition of the functional nucleoprotein complex of plant telomeres is only fragmentary. Moreover, the plant telomeric repeat binding proteins that were characterized recently appear to also be involved in non-telomeric processes, e.g., ribosome biogenesis. This interesting finding was not totally unexpected since non-telomeric functions of yeast or animal telomeric proteins, as well as of telomerase subunits, have been reported for almost a decade. Here we summarize known facts about the architecture of plant telomeres and compare them with the well-described composition of telomeres in other organisms.

• Keywords
• plant, shelterin, telomerase, telomere, telomeric proteins, telomeric repeat binding (TRB),
• Publication type
• Journal Article MeSH
• Review MeSH

Recently we characterised TRB1, a protein from a single-myb-histone family, as a structural and functional component of telomeres in Arabidopsis thaliana. TRB proteins, besides their ability to bind specifically to telomeric DNA using their N-terminally positioned myb-like domain of the same type as in human shelterin proteins TRF1 or TRF2, also possess a histone-like domain which is involved in protein-protein interactions e.g., with POT1b. Here we set out to investigate the genome-wide localization pattern of TRB1 to reveal its preferential sites of binding to chromatin in vivo and its potential functional roles in the genome-wide context. Our results demonstrate that TRB1 is preferentially associated with promoter regions of genes involved in ribosome biogenesis, in addition to its roles at telomeres. This preference coincides with the frequent occurrence of telobox motifs in the upstream regions of genes in this category, but it is not restricted to the presence of a telobox. We conclude that TRB1 shows a specific genome-wide distribution pattern which suggests its role in regulation of genes involved in biogenesis of the translational machinery, in addition to its preferential telomeric localization.

• Keywords
• Arabidopsis thaliana, ChIP-seq, Ribosome, Telomere repeat binding (TRB), Translation machinery, snoRNA,
• MeSH
• Arabidopsis genetics   metabolism   MeSH
• Gene Library MeSH
• Histones metabolism   MeSH
• Molecular Sequence Data MeSH
• Nucleotide Motifs MeSH
• Promoter Regions, Genetic genetics   MeSH
• Arabidopsis Proteins genetics   metabolism   MeSH
• Telomere-Binding Proteins genetics   metabolism   MeSH
• Protein Biosynthesis MeSH
• Ribosomes genetics   MeSH
• Base Sequence MeSH
• Sequence Analysis, DNA MeSH
• Oligonucleotide Array Sequence Analysis MeSH
• Telomere metabolism   MeSH
• Protein Binding MeSH
• Computational Biology MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Histones MeSH
• Arabidopsis Proteins MeSH
• Telomere-Binding Proteins MeSH
• TRB1 protein, Arabidopsis MeSH Browser

More than two decades of genetic research have identified and assigned main biological functions of shelterin proteins that safeguard telomeres. However, a molecular mechanism of how each protein subunit contributes to the protecting function of the whole shelterin complex remains elusive. Human Repressor activator protein 1 (Rap1) forms a multifunctional complex with Telomeric Repeat binding Factor 2 (TRF2). Rap1-TRF2 complex is a critical part of shelterin as it suppresses homology-directed repair in Ku 70/80 heterodimer absence. To understand how Rap1 affects key functions of TRF2, we investigated full-length Rap1 binding to TRF2 and Rap1-TRF2 complex interactions with double-stranded DNA by quantitative biochemical approaches. We observed that Rap1 reduces the overall DNA duplex binding affinity of TRF2 but increases the selectivity of TRF2 to telomeric DNA. Additionally, we observed that Rap1 induces a partial release of TRF2 from DNA duplex. The improved TRF2 selectivity to telomeric DNA is caused by less pronounced electrostatic attractions between TRF2 and DNA in Rap1 presence. Thus, Rap1 prompts more accurate and selective TRF2 recognition of telomeric DNA and TRF2 localization on single/double-strand DNA junctions. These quantitative functional studies contribute to the understanding of the selective recognition of telomeric DNA by the whole shelterin complex.

• MeSH
• Sodium Chloride pharmacology   MeSH
• DNA chemistry   genetics   metabolism   MeSH
• Fluorescence Polarization MeSH
• Spectrometry, Fluorescence MeSH
• Kinetics MeSH
• Binding, Competitive drug effects   MeSH
• Humans MeSH
• Surface Plasmon Resonance MeSH
• Telomeric Repeat Binding Protein 2 chemistry   genetics   metabolism   MeSH
• Telomere-Binding Proteins chemistry   genetics   metabolism   MeSH
• Shelterin Complex MeSH
• Static Electricity MeSH
• Telomere genetics   metabolism   MeSH
• Protein Binding drug effects   MeSH
• Check Tag
• Humans MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Sodium Chloride MeSH
• DNA MeSH
• Telomeric Repeat Binding Protein 2 MeSH
• Telomere-Binding Proteins MeSH
• Shelterin Complex MeSH
• TERF2 protein, human MeSH Browser
• TERF2IP protein, human MeSH Browser

Proteins that bind to telomeric DNA form the key structural and functional constituents of telomeres. While telomere binding proteins have been described in the majority of organisms, their identity in plants remains unknown. Several protein families containing a telomere binding motif known as the telobox have been previously described in Arabidopsis thaliana. Nonetheless, functional evidence for their involvement at telomeres has not been obtained, likely due to functional redundancy. Here we performed genetic analysis on the TRF-like family consisting of six proteins (TRB1, TRP1, TRFL1, TRFL2, TRFL4, and TRF9) which have previously shown to bind telomeric DNA in vitro. We used haploid genetics to create multiple knock-out plants deficient for all six proteins of this gene family. These plants did not exhibit changes in telomere length, or phenotypes associated with telomere dysfunction. This data demonstrates that this telobox protein family is not involved in telomere maintenance in Arabidopsis. Phylogenetic analysis in major plant lineages revealed early diversification of telobox proteins families indicating that telomere function may be associated with other telobox proteins.

• Keywords
• centromere, haploid, protein family, telobox, telomeres,
• Publication type
• Journal Article MeSH

Although telomere-binding proteins constitute an essential part of telomeres, in vivo data indicating the existence of a structure similar to mammalian shelterin complex in plants are limited. Partial characterization of a number of candidate proteins has not identified true components of plant shelterin or elucidated their functional mechanisms. Telomere repeat binding (TRB) proteins from Arabidopsis thaliana bind plant telomeric repeats through a Myb domain of the telobox type in vitro, and have been shown to interact with POT1b (Protection of telomeres 1). Here we demonstrate co-localization of TRB1 protein with telomeres in situ using fluorescence microscopy, as well as in vivo interaction using chromatin immunoprecipitation. Classification of the TRB1 protein as a component of plant telomeres is further confirmed by the observation of shortening of telomeres in knockout mutants of the trb1 gene. Moreover, TRB proteins physically interact with plant telomerase catalytic subunits. These findings integrate TRB proteins into the telomeric interactome of A. thaliana.

• Keywords
• Arabidopsis thaliana, plant shelterin, telomerase, telomere, telomere protein interaction, telomere repeat binding (TRB),
• MeSH
• Arabidopsis enzymology   genetics   MeSH
• Arabidopsis Proteins metabolism   MeSH
• Telomere-Binding Proteins metabolism   MeSH
• Telomerase metabolism   MeSH
• Telomere metabolism   MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Arabidopsis Proteins MeSH
• Telomere-Binding Proteins MeSH
• Telomerase MeSH