SUMOylation regulates telomere length by targeting the shelterin subunit Tpz1(Tpp1) to modulate shelterin-Stn1 interaction in fission yeast

Pot1 promotes telomere DNA replication via the Stn1-Ten1 complex in fission yeast

Telomeres are nucleoprotein complexes that protect the chromosome-ends from eliciting DNA repair while ensuring their complete duplication. Pot1 is a subunit of telomere capping complex that binds to the G-rich overhang and inhibits the activation of DNA damage checkpoints. In this study, we explore new functions of fission yeast Pot1 by using a pot1-1 temperature sensitive mutant. We show that pot1 inactivation impairs telomere DNA replication resulting in the accumulation of ssDNA leading to the complete loss of telomeric DNA. Recruitment of Stn1 to telomeres, an auxiliary factor of DNA lagging strand synthesis, is reduced in pot1-1 mutants and overexpression of Stn1 rescues loss of telomeres and cell viability at restrictive temperature. We propose that Pot1 plays a crucial function in telomere DNA replication by recruiting Stn1-Ten1 and Polα-primase complex to telomeres via Tpz1, thus promoting lagging-strand DNA synthesis at stalled replication forks.

Aspergillus SUMOylation mutants exhibit chromosome segregation defects including chromatin bridges

Functions of protein SUMOylation remain incompletely understood in different cell types. Via forward genetics, here we identified ubaBQ247*, a loss-of-function mutation in a SUMO activation enzyme UbaB in the filamentous fungus Aspergillus nidulans. The ubaBQ247*, ΔubaB, and ΔsumO mutants all produce abnormal chromatin bridges, indicating the importance of SUMOylation in the completion of chromosome segregation. The bridges are enclosed by nuclear membrane containing peripheral nuclear pore complex proteins that normally get dispersed during mitosis, and the bridges are also surrounded by cytoplasmic microtubules typical of interphase cells. Time-lapse sequences further indicate that most bridges persist through interphase prior to the next mitosis, and anaphase chromosome segregation can produce new bridges that persist into the next interphase. When the first mitosis happens at a higher temperature of 42°C, SUMOylation deficiency produces not only chromatin bridges but also many abnormally shaped single nuclei that fail to divide. UbaB-GFP localizes to interphase nuclei just like the previously studied SumO-GFP, but the nuclear signals disappear during mitosis when the nuclear pores are partially open, and the signals reappear after mitosis. The nuclear localization is consistent with many SUMO targets being nuclear proteins. Finally, although the budding yeast SUMOylation machinery interacts with LIS1, a protein critical for dynein activation, loss of SUMOylation does not cause any obvious defect in dynein-mediated transport of nuclei and early endosomes, indicating that SUMOylation is unnecessary for dynein activation in A. nidulans.

Control of telomere length in yeast by SUMOylated PCNA and the Elg1 PCNA unloader

Telomeres cap and protect the linear eukaryotic chromosomes. Telomere length is determined by an equilibrium between positive and negative regulators of telomerase activity. A systematic screen for yeast mutants that affect telomere length maintenance in the yeast Saccharomyces cerevisiae revealed that mutations in any of ~500 genes affects telomere length. One of the genes that, when mutated, causes telomere elongation is ELG1, which encodes an unloader of PCNA, the processivity factor for replicative DNA polymerases. PCNA can undergo SUMOylation on two conserved residues, K164 and K127, or ubiquitination at lysine 164. These modifications have already been implicated in genome stability processes. We report that SUMOylated PCNA acts as a signal that positively regulates telomerase activity. We also uncovered physical interactions between Elg1 and the CST (Cdc13-Stn1-Ten) complex and addressed the mechanism by which Elg1 and Stn1 negatively regulates telomere elongation, coordinated by SUMO. We discuss these results with respect to how chromosomal replication and telomere elongation are coordinated.

Stn1-Ten1 and Taz1 independently promote replication of subtelomeric fragile sequences in fission yeast

Efficient replication of terminal DNA is crucial to maintain telomere stability. In fission yeast, Taz1 and the Stn1-Ten1 (ST) complex play prominent roles in DNA-ends replication. However, their function remains elusive. Here, we have analyzed genome-wide replication and show that ST does not affect genome-wide replication but is crucial for the efficient replication of a subtelomeric region called STE3-2. We further show that, when ST function is compromised, a homologous recombination (HR)-based fork restart mechanism becomes necessary for STE3-2 stability. While both Taz1 and Stn1 bind to STE3-2, we find that the STE3-2 replication function of ST is independent of Taz1 but relies on its association with the shelterin proteins Pot1-Tpz1-Poz1. Finally, we demonstrate that the firing of an origin normally inhibited by Rif1 can circumvent the replication defect of subtelomeres when ST function is compromised. Our results help illuminate why fission yeast telomeres are terminal fragile sites.

The Cajal body marker protein coilin is SUMOylated and possesses SUMO E3 ligase-like activity

Cajal bodies (CBs) are subnuclear domains that contribute to the biogenesis of several different classes of ribonucleoproteins (RNPs) including small nuclear RNPs. Only some cell types contain abundant CBs, such as neuronal cells and skeletal muscle, but CBs are invariant features of transformed cells. In contrast, coilin, the CB marker protein, is a ubiquitously expressed nuclear protein but the function of coilin in cell types that lack CBs is not well understood. We have previously shown that coilin promotes microRNA biogenesis by promoting phosphorylation of DGCR8, a component of the Microprocessor. Here we identify 7 additional residues of DGCR8 with decreased phosphorylation upon coilin knockdown. In addition to phosphorylation, the addition of a small ubiquitin-like modifier (SUMO) to DGCR8 also increases its stability. Because of coilin's role in the promotion of DGCR8 phosphorylation, we investigated whether coilin is involved in DGCR8 SUMOylation. We show that coilin knockdown results in global decrease of protein SUMOylation, including decreased DGCR8 and Sp100 (a PML body client protein) SUMOylation and decreased SMN expression. Alternatively, we found that coilin expression rescued Sp100 SUMOylation and increased DGCR8 and SMN levels in a coilin knockout cell line. Furthermore, we found that coilin facilitates RanGAP1 SUMOylation, interacts directly with components of the SUMOylation machinery (Ubc9 and SUMO2), and itself is SUMOylated in vitro and in vivo. In summary, we have identified coilin as a regulator of DGCR8 phosphorylation and a promotor of protein SUMOylation with SUMO E3 ligase-like activity.

Rice SUMOs and unification of their names

Posttranslational modifications (PTMs) to proteins are regulatory mechanisms that play a critical role in regulating growth and development. The SUMO system is a rapid and dynamic PTM system employed by eukaryotic cells. Plant SUMOs are involved in many physiological processes, such as stress responses, regulation of flowering time and defense reactions to pathogen attack. In Arabidopsis thaliana and rice (Oryza sativa), eight and seven SUMO genes, respectively, were predicted by sequence analysis. Phylogenetic tree analysis of these SUMOs shows that they are divided into two groups. One consists of SUMOs that contain no SUMO acceptor site and are involved in monoSUMOylation of their target proteins. Rice OsSUMO1 and OsSUMO2 are in this group, and are structurally similar to each other and to Arabidopsis AtSUMO1. The other group is composed of SUMOs in which an acceptor site (ΨKXE/D) occurs inside the SUMO molecule, suggesting their involvement in polySUMOylation. Several studies on the rice SUMOs have been performed independently and reported. Individual names of rice SUMOs are confusing, because a unified nomenclature has not been proposed. This review clarifies the attribution of seven rice SUMOs and unifies the individual SUMO names.

A nuclear pore complex-associated regulation of SUMOylation in meiosis

The nuclear pore complex (NPC) provides a permeable barrier between the nucleoplasm and cytoplasm. In a subset of NPC constituents that regulate meiosis in the fission yeast Schizosaccharomyces pombe, we found that nucleoporin Nup132 (homolog of human Nup133) deficiency resulted in transient leakage of nuclear proteins during meiosis I, as observed in the nup132 gene-deleted mutant. The nuclear protein leakage accompanied the liberation of the small ubiquitin-like modifier (SUMO)-specific ubiquitin-like protease 1 (Ulp1) from the NPC. Ulp1 retention at the nuclear pore prevented nuclear protein leakage and restored normal meiosis in a mutant lacking Nup132. Furthermore, using mass spectrometry analysis, we identified DNA topoisomerase 2 (Top2) and RCC1-related protein (Pim1) as the target proteins for SUMOylation. SUMOylation levels of Top2 and Pim1 were altered in meiotic cells lacking Nup132. HyperSUMOylated Top2 increased the binding affinity at the centromeres of nup132 gene-deleted meiotic cells. The Top2-12KR sumoylation mutant was less localized to the centromeric regions. Our results suggest that SUMOylation of chromatin-binding proteins is regulated by the NPC-bound SUMO-specific protease and is important for the progression of meiosis.

A proto-telomere is elongated by telomerase in a shelterin-dependent manner in quiescent fission yeast cells

Telomere elongation is coupled with genome replication, raising the question of the repair of short telomeres in post-mitotic cells. We investigated the fate of a telomere-repeat capped end that mimics a single short telomere in quiescent fission yeast cells. We show that telomerase is able to elongate this single short telomere during quiescence despite the binding of Ku to the proto-telomere. While Taz1 and Rap1 repress telomerase in vegetative cells, both shelterin proteins are required for efficient telomere extension in quiescent cells, underscoring a distinct mode of telomerase control. We further show that Rad3ATR and Tel1ATM are redundantly required for telomere elongation in quiescence through the phosphorylation of Ccq1 and that Rif1 and its associated-PP1 phosphatases negatively regulate telomerase activity by opposing Ccq1 phosphorylation. The distinct mode of telomerase regulation in quiescent fission yeast cells may be relevant to that in human stem and progenitor cells.

Structural insights into Pot1-ssDNA, Pot1-Tpz1 and Tpz1-Ccq1 Interactions within fission yeast shelterin complex

The conserved shelterin complex caps chromosome ends to protect telomeres and regulate telomere replication. In fission yeast Schizosaccharomyces pombe, shelterin consists of telomeric single- and double-stranded DNA-binding modules Pot1-Tpz1 and Taz1-Rap1 connected by Poz1, and a specific component Ccq1. While individual structures of the two DNA-binding OB folds of Pot1 (Pot1_OB1-GGTTAC and Pot1_OB2-GGTTACGGT) are available, structural insight into recognition of telomeric repeats with spacers by the complete DNA-binding domain (Pot1_DBD) remains an open question. Moreover, structural information about the Tpz1-Ccq1 interaction requires to be revealed for understanding how the specific component Ccq1 of S. pombe shelterin is recruited to telomeres to function as an interacting hub. Here, we report the crystal structures of Pot1_DBD-single-stranded-DNA, Pot1_372-555-Tpz1_185-212 and Tpz1_425-470-Ccq1_123-439 complexes and propose an integrated model depicting the assembly mechanism of the shelterin complex at telomeres. The structure of Pot1_DBD-DNA unveils how Pot1 recognizes S. pombe degenerate telomeric sequences. Our analyses of Tpz1-Ccq1 reveal structural basis for the essential role of the Tpz1-Ccq1 interaction in telomere recruitment of Ccq1 that is required for telomere maintenance and telomeric heterochromatin formation. Overall, our findings provide valuable structural information regarding interactions within fission yeast shelterin complex at 3’ ss telomeric overhang.

Telomeres, composed of repetitive DNA sequences and specialized proteins, are protective structures at the ends of linear chromosomes. The telomere structure is essential for the maintenance of genome integrity and stability, and telomere dysfunction has been linked to human development, aging, cancer and a variety of degenerative diseases. An evolutionarily conserved multiple-protein complex called shelterin plays versatile roles in telomere homeostasis regulation, end protection and heterochromatin establishment. However, the highly flexible nature of shelterin complex has greatly impeded our structural and functional understanding for this important complex. In fission yeast, structures of the shelterin dsDNA-binding protein subcomplex Taz1-Rap1 and the bridge subcomplex Tpz1-Poz1-Rap1 are available. Although individual OB-fold subdomains structures have been characterized, structural information about the complete Pot1_DBD bound to telomeric repeats with spacers remains to be revealed. Here, by determining the crystal structures of the telomeric overhang binding Pot1_DBD-ssDNA, Pot1_372-555-Tpz1_185-212 and Tpz1_425-470-Ccq1_123-439 subcomplexes, we provide structural basis not only for the recognition of S. pombe degenerate telomeric sequences by Pot1, but also for the essential function of the Tpz1-Ccq1 interaction in Ccq1 recruitment to telomeres for telomere maintenance and telomeric heterochromatin formation. These findings provide an integrated model depicting the assembly mechanism of the shelterin complex at telomeres and its multiple roles in telomere biology.

Structural insights into telomere protection and homeostasis regulation by yeast CST complex

Budding yeast Cdc13-Stn1-Ten1 (CST) complex plays an essential role in telomere protection and maintenance. Despite extensive studies, only structural information of individual domains of CST is available; the architecture of CST still remains unclear. Here, we report crystal structures of Kluyveromyces lactis Cdc13-telomeric-DNA, Cdc13-Stn1 and Stn1-Ten1 complexes and propose an integrated model depicting how CST assembles and plays its roles at telomeres. Surprisingly, two oligonucleotide/oligosaccharide-binding (OB) folds of Cdc13 (OB2 and OB4), previously believed to mediate Cdc13 homodimerization, actually form a stable intramolecular interaction. This OB2-OB4 module of Cdc13 is required for the Cdc13-Stn1 interaction that assembles CST into an architecture with a central ring-like core and multiple peripheral modules in a 2:2:2 stoichiometry. Functional analyses indicate that this unique CST architecture is essential for both telomere capping and homeostasis regulation. Overall, our results provide fundamentally valuable structural information regarding the CST complex and its roles in telomere biology.

The Role of Ubiquitination and SUMOylation in Telomere Biology

Telomeres are a unique structure of DNA repeats covered by proteins at the ends of the chromosomes that protect the coding regions of the genome and function as a biological clock. They require a tight regulation of the factors covering and protecting their structure, as they are shortened with each cell division to limit the ability of cells to replicate uncontrollably. Additionally, they protect the chromosome ends from DNA damage responses and thereby, prevent genomic instability. Telomere dysfunction can lead to chromosomal abnormalities and cancer. Therefore, dysregulation of any of the factors that regulate the integrity of the telomeres will have implications to chromosomal stability, replicative lifespan and may lead to cell transformation. This review will cover the main factors participating in the normal function of the telomeres and how these are regulated by the ubiquitin and SUMO systems. Accumulating evidence indicate that the ubiquitin and SUMO pathways are significant regulators of the shelterin complex and other chromatin modifiers, which are important for telomere structure integrity. Furthermore, the crosstalk between these two pathways has been reported in telomeric DNA repair. A better understanding of the factors contributing to telomere biology, and how they are regulated, is important for the design of new strategies for cancer therapies and regenerative medicine.

Tpz1TPP1 prevents telomerase activation and protects telomeres by modulating the Stn1-Ten1 complex in fission yeast

In both mammalian and fission yeast cells, conserved shelterin and CST (CTC1-STN1-TEN1) complexes play critical roles in protection of telomeres and regulation of telomerase, an enzyme required to overcome the end replication problem. However, molecular details that govern proper coordination among shelterin, CST, and telomerase have not yet been fully understood. Here, we establish a conserved SWSSS motif, located adjacent to the Lys242 SUMOylation site in the fission yeast shelterin subunit Tpz1, as a new functional regulatory element for telomere protection and telomere length homeostasis. The SWSSS motif works redundantly with Lys242 SUMOylation to promote binding of Stn1-Ten1 at telomere and sub-telomere regions to protect against single-strand annealing (SSA)-dependent telomere fusions, and to prevent telomerase accumulation at telomeres. In addition, we provide evidence that the SWSSS motif defines an unanticipated role of Tpz1 in limiting telomerase activation at telomeres to prevent uncontrolled telomere elongation.

Fission yeast Ccq1 is a modulator of telomerase activity

Shelterin, the telomeric protein complex, plays a crucial role in telomere homeostasis. In fission yeast, telomerase is recruited to chromosome ends by the shelterin component Tpz1 and its binding partner Ccq1, where telomerase binds to the 3' overhang to add telomeric repeats. Recruitment is initiated by the interaction of Ccq1 with the telomerase subunit Est1. However, how telomerase is released following elongation remains to be established. Here, we show that Ccq1 also has a role in the suppression of telomere elongation, when coupled with the Clr4 histone H3 methyl-transferase complex and the Clr3 histone deacetylase and nucleosome remodelling complex, SHREC. We have dissected the functions of Ccq1 by establishing a Ccq1-Est1 fusion system, which bypasses the telomerase recruitment step. We demonstrate that Ccq1 forms two distinct complexes for positive and negative telomerase regulation, with Est1 and Clr3 respectively. The negative form of Ccq1 promotes dissociation of Ccq1-telomerase from Tpz1, thereby restricting local telomerase activity. The Clr4 complex also has a negative regulation activity with Ccq1, independently of SHREC. Thus, we propose a model in which Ccq1-Est1 recruits telomerase to mediate telomere extension, whilst elongated telomeric DNA recruits Ccq1 with the chromatin-remodelling complexes, which in turn releases telomerase from the telomere.

Ssu72 phosphatase is a conserved telomere replication terminator

Telomeres, the protective ends of eukaryotic chromosomes, are replicated through concerted actions of conventional DNA polymerases and elongated by telomerase, but the regulation of this process is not fully understood. Telomere replication requires (Ctc1/Cdc13)‐Stn1‐Ten1, a telomeric ssDNA‐binding complex homologous to RPA. Here, we show that the evolutionarily conserved phosphatase Ssu72 is responsible for terminating the cycle of telomere replication in fission yeast. Ssu72 controls the recruitment of Stn1 to telomeres by regulating Stn1 phosphorylation at Ser74, a residue located within its conserved OB‐fold domain. Consequently, ssu72∆ mutants are defective in telomere replication and exhibit long 3′‐ssDNA overhangs, indicative of defective lagging‐strand DNA synthesis. We also show that hSSU72 regulates telomerase activation in human cells by controlling recruitment of hSTN1 to telomeres. These results reveal a previously unknown yet conserved role for the phosphatase SSU72, whereby this enzyme controls telomere homeostasis by activating lagging‐strand DNA synthesis, thus terminating the cycle of telomere replication.

Telomere association of Oryza sativa telomere repeat-binding factor like 1 and its roles in telomere maintenance and development in rice, Oryza sativa L

Telomeres are specialized nucleoprotein complexes that function to protect eukaryotic chromosomes from recombination and erosion. Several telomere binding proteins (TBPs) have been characterized in higher plants, but their detailed in vivo functions at the plant level are largely unknown. In this study, we identified and characterized OsTRFL1 (Oryza sativa Telomere Repeat-binding Factor Like 1) in rice, a monocot model crop. Although OsTRFL1 did not directly bind to telomere repeats (TTTAGGG)₄in vitro, it was associated with telomeric sequences in planta. OsTRFL1 interacted with rice TBPs, such as OsTRBF1 and RTBP1, in yeast and plant cells as well as in vitro. Thus, it seems likely that the association of OsTRFL1 with other TBPs enables OsTRFL1 to bind to telomeres indirectly. T-DNA inserted OsTRFL1 knock-out mutant rice plants displayed significantly longer telomeres (6–25 kb) than those (5–12 kb) in wild-type plants, indicating that OsTRFL1 is a negative factor for telomere lengthening. The reduced levels of OsTRFL1 caused serious developmental defects in both vegetative and reproductive organs of rice plants. These results suggest that OsTRFL1 is an essential factor for the proper maintenance of telomeres and normal development of rice.

CTC1-STN1 terminates telomerase while STN1-TEN1 enables C-strand synthesis during telomere replication in colon cancer cells

Telomerase elongates the telomeric G-strand to prevent telomere shortening through conventional DNA replication. However, synthesis of the complementary C-strand by DNA polymerase α is also required to maintain telomere length. Polymerase α cannot perform this role without the ssDNA binding complex CST (CTC1-STN1-TEN1). Here we describe the roles of individual CST subunits in telomerase regulation and G-overhang maturation in human colon cancer cells. We show that CTC1-STN1 limits telomerase action to prevent G-overhang overextension. CTC1^-/- cells exhibit telomeric DNA damage and growth arrest due to overhang elongation whereas TEN1^-/- cells do not. However, TEN1 is essential for C-strand synthesis and TEN1^-/- cells exhibit progressive telomere shortening. DNA binding analysis indicates that CTC1-STN1 retains affinity for ssDNA but TEN1 stabilizes binding. We propose CTC1-STN1 binding is sufficient to terminate telomerase action but altered DNA binding dynamics renders CTC1-STN1 unable to properly engage polymerase α on the overhang for C-strand synthesis.

The fission yeast Stn1-Ten1 complex limits telomerase activity via its SUMO-interacting motif and promotes telomeres replication

Mammalian CST (CTC1-STN1-TEN1) complex fulfills numerous functions including rescue of the stalled replication forks and termination of telomerase action. In fission yeast lacking the CTC1 ortholog, the Stn1-Ten1 complex restricts telomerase action via its sumoylation-mediated interaction with Tpz1^TPP1. We identify a small ubiquitin-like modifier (SUMO)-interacting motif (SIM) in the carboxyl-terminal part of Stn1 and show that this domain is crucial for SUMO and Tpz1-SUMO interactions. Point mutations in the SIM (Stn1-226) lead to telomere elongation, impair Stn1-Ten1 recruitment to telomeres, and enhance telomerase binding, revealing that Stn1 SIM domain contributes to the inhibition of telomerase activity at chromosome ends. Our results suggest that Stn1-Ten1 promotes DNA synthesis at telomeres to limit single-strand DNA accumulation. We further demonstrate that Stn1 functions in the replication of telomeric and subtelomeric regions in a Taz1-independent manner. Genetic analysis reveals that misregulation of origin firing and/or telomerase inhibition circumvents the replication defects of the stn1-226 mutant. Together, our results show that the Stn1-Ten1 complex has a dual function at telomeres by limiting telomerase action and promoting chromosome end replication.

Emerging roles of CST in maintaining genome stability and human disease

The human CTC1-STN1-TEN1 (CST) complex is a single-stranded DNA binding protein that shares homology with RPA and interacts with DNA polymerase alpha/primase. CST complexes are conserved from yeasts to humans and function in telomere maintenance. A common role of CST across species is in the regulation of telomere extension by telomerase and C-strand fill-in synthesis. However, recent studies also indicate that CST promotes telomere duplex replication as well the rescue of stalled DNA replication at non-telomeric sites. Furthermore, CST dysfunction and mutation is associated with several genetic diseases and cancers. In this review, we will summarize what is known about CST with a particular focus on the emerging roles of CST in DNA replication and human disease.

Fission yeast Stn1 is crucial for semi-conservative replication at telomeres and subtelomeres

The CST complex is a phylogenetically conserved protein complex consisting of CTC1/Cdc13, Stn1 and Ten1 that protects telomeres on linear chromosomes. Deletion of the fission yeast homologs stn1 and ten1 results in complete telomere loss; however, the precise function of Stn1 is still largely unknown. Here, we have isolated a high-temperature sensitive stn1 allele (termed stn1-1). stn1-1 cells abruptly lost telomeric sequence almost completely at the restrictive temperature. The loss of chromosomal DNA happened without gradual telomere shortening, and extended to 30 kb from the ends of chromosomes. We found transient and modest single-stranded G-strand exposure, but did not find any evidence of checkpoint activation in stn1-1 at the restrictive temperature. When we probed neutral-neutral 2D gels for subtelomere regions, we found no Y-arc-shaped replication intermediates in cycling cells. We conclude that the loss of telomere and subtelomere DNAs in stn1-1 cells at the restrictive temperature is caused by very frequent replication fork collapses specifically in subtelomere regions. Furthermore, we identified two independent suppressor mutants of the high-temperature sensitivity of stn1-1: a multi-copy form of pmt3 and a deletion of rif1. Collectively, we propose that fission yeast Stn1 primarily safeguards the semi-conservative DNA replication at telomeres and subtelomeres.

Functions of SUMO in the Maintenance of Genome Stability

Like in most other areas of cellular metabolism, the functions of the ubiquitin-like modifier SUMO in the maintenance of genome stability are manifold and varied. Perturbations of global sumoylation causes a wide spectrum of phenotypes associated with defects in DNA maintenance, such as hypersensitivity to DNA-damaging agents, gross chromosomal rearrangements and loss of entire chromosomes. Consistent with these observations, many key factors involved in various DNA repair pathways have been identified as SUMO substrates. However, establishing a functional connection between a given SUMO target, the cognate SUMO ligase and a relevant phenotype has remained a challenge, mainly because of the difficulties involved in identifying important modification sites and downstream effectors that specifically recognize the target in its sumoylated state. This review will give an overview over the major pathways of DNA repair and genome maintenance influenced by the SUMO system and discuss selected examples of SUMO's actions in these pathways where the biological consequences of the modification have been elucidated.

CTC1-mediated C-strand fill-in is an essential step in telomere length maintenance

To prevent progressive telomere shortening as a result of conventional DNA replication, new telomeric DNA must be added onto the chromosome end. The de novo DNA synthesis involves elongation of the G-rich strand of the telomere by telomerase. In human cells, the CST complex (CTC1-STN1-TEN1) also functions in telomere replication. CST first aids in duplication of the telomeric dsDNA. Then after telomerase has extended the G-rich strand, CST facilitates fill-in synthesis of the complementary C-strand. Here, we analyze telomere structure after disruption of human CTC1 and demonstrate that functional CST is essential for telomere length maintenance due to its role in mediating C-strand fill-in. Removal of CTC1 results in elongation of the 3΄ overhang on the G-rich strand. This leads to accumulation of RPA and telomeric DNA damage signaling. G-overhang length increases with time after CTC1 disruption and at early times net G-strand growth is apparent, indicating telomerase-mediated G-strand extension. In contrast, C-strand length decreases continuously, indicating a deficiency in C-strand fill-in synthesis. The lack of C-strand maintenance leads to gradual shortening of the telomeric dsDNA, similar to that observed in cells lacking telomerase. Thus, telomerase-mediated G-strand extension and CST-mediated C-strand fill-in are equally important for telomere length maintenance.

SUMO and Chromatin Remodeling

Many of the known SUMO substrates are nuclear proteins, which regulate gene expression and chromatin dynamics. Sumoylation, in general, appears to correlate with decreased transcriptional activity, and in many cases modulation of the chromatin template is implicated. Sumoylation of the core histones is associated with transcriptional silencing, and transcription factor sumoylation can decrease gene expression by promoting recruitment of chromatin modifying enzymes. Additionally, sumoylation of transcriptional corepressors and chromatin remodeling enzymes can influence interactions with other transcriptional regulators, and alter their enzymatic activity. In some cases, proteins that are components of transcriptional corepressor complexes have been shown to be SUMO E3 ligases, further emphasizing the integration of sumoylation with the regulation of chromatin remodeling. Despite the evidence suggesting that sumoylation is primarily repressive for access to chromatin, recent analyses suggest that protein sumoylation on the chromatin template may play important roles at highly expressed genes. Elucidating the dynamic interplay of sumoylation with other post-translational modifications of histones and chromatin associated proteins will be key to fully understanding the regulation of access to the chromatin template.

Ubiquitination and SUMOylation in Telomere Maintenance and Dysfunction

Telomeres are essential nucleoprotein structures at linear chromosomes that maintain genome integrity by protecting chromosome ends from being recognized and processed as damaged DNA. In addition, they limit the cell’s proliferative capacity, as progressive loss of telomeric DNA during successive rounds of cell division eventually causes a state of telomere dysfunction that prevents further cell division. When telomeres become critically short, the cell elicits a DNA damage response resulting in senescence, apoptosis or genomic instability, thereby impacting on aging and tumorigenesis. Over the past years substantial progress has been made in understanding the role of post-translational modifications in telomere-related processes, including telomere maintenance, replication and dysfunction. This review will focus on recent findings that establish an essential role for ubiquitination and SUMOylation at telomeres.

SUMO-targeted ubiquitin ligase activity can either suppress or promote genome instability, depending on the nature of the DNA lesion

The posttranslational modifiers SUMO and ubiquitin critically regulate the DNA damage response (DDR). Important crosstalk between these modifiers at DNA lesions is mediated by the SUMO-targeted ubiquitin ligase (STUbL), which ubiquitinates SUMO chains to generate SUMO-ubiquitin hybrids. These SUMO-ubiquitin hybrids attract DDR proteins able to bind both modifiers, and/or are degraded at the proteasome. Despite these insights, specific roles for SUMO chains and STUbL in the DDR remain poorly defined. Notably, fission yeast defective in SUMO chain formation exhibit near wild-type resistance to genotoxins and moreover, have a greatly reduced dependency on STUbL activity for DNA repair. Based on these and other data, we propose that a critical role of STUbL is to antagonize DDR-inhibitory SUMO chain formation at DNA lesions. In this regard, we identify a SUMO-binding Swi2/Snf2 translocase called Rrp2 (ScUls1) as a mediator of the DDR defects in STUbL mutant cells. Therefore, in support of our proposal, SUMO chains attract activities that can antagonize STUbL and other DNA repair factors. Finally, we find that Taz1^TRF1/TRF2-deficiency triggers extensive telomeric poly-SUMOylation. In this setting STUbL, together with its cofactor Cdc48^p97_, actually promotes genomic instability caused by the aberrant processing of taz1Δ telomeres by DNA repair factors. In summary, depending on the nature of the initiating DNA lesion, STUbL activity can either be beneficial or harmful.

Since its discovery in 2007, SUMO-targeted ubiquitin ligase (STUbL) activity has been identified as a key regulator of diverse cellular processes such as DNA repair, mitosis and DNA replication. In each of these processes, STUbL has been shown to promote the chromatin extraction and/or degradation of SUMO chain modified proteins. However, it remains unclear whether STUbL acts as part of a "programmed" cascade to remove specific proteins, or antagonizes localized SUMO chain formation that otherwise impedes each process. Here we determine that SUMO chains, the major recruitment signal for STUbL, are largely dispensable for genotoxin resistance in fission yeast. Moreover, when SUMO chain formation is compromised, the need for STUbL activity in DNA repair is strongly reduced. These results indicate a primary role for STUbL in antagonizing localized SUMO chain formation. Interestingly, we also find that STUbL activity can be toxic at certain genomic lesions that induce extensive local SUMOylation. For example, STUbL promotes the chromosome instability and cell death caused by deprotected telomeres following Taz1^TRF1/2 deletion. Together, our data suggest that STUbL limits DNA repair-inhibitory SUMO chain formation, and depending on the nature of the genomic lesion, can either suppress or cause genome instability.

Telomerase Mechanism of Telomere Synthesis

Telomerase is the essential reverse transcriptase required for linear chromosome maintenance in most eukaryotes. Telomerase supplements the tandem array of simple-sequence repeats at chromosome ends to compensate for the DNA erosion inherent in genome replication. The template for telomerase reverse transcriptase is within the RNA subunit of the ribonucleoprotein complex, which in cells contains additional telomerase holoenzyme proteins that assemble the active ribonucleoprotein and promote its function at telomeres. Telomerase is distinct among polymerases in its reiterative reuse of an internal template. The template is precisely defined, processively copied, and regenerated by release of single-stranded product DNA. New specificities of nucleic acid handling that underlie the catalytic cycle of repeat synthesis derive from both active site specialization and new motif elaborations in protein and RNA subunits. Studies of telomerase provide unique insights into cellular requirements for genome stability, tissue renewal, and tumorigenesis as well as new perspectives on dynamic ribonucleoprotein machines.

The role of SUMOylation in ageing and senescent decline

Posttranslational protein modifications are playing crucial roles in essential cellular mechanisms. SUMOylation is a reversible posttranslational modification of specific target proteins by the attachment of a small ubiquitin-like protein. Although the mechanism of conjugation of SUMO to proteins is analogous to ubiquitination, it requires its own, specific set of enzymes. The consequences of SUMOylation are widely variable, depending on the physiological state of the cell and the attached SUMO isoform. Accumulating recent findings have revealed a prominent role of SUMOylation in molecular pathways that govern senescence and ageing. Here, we review the link between SUMO attachment events and cellular processes that influence senescence and ageing, including promyelocytic leukaemia (PML) nuclear body and telomere function, autophagy, reactive oxygen species (ROS) homeostasis and growth factor signalling.

Telomere attrition and restoration in the normal teleost Oryzias latipes are linked to growth rate and telomerase activity at each life stage

Telomere shortening occurs when cells divide, both in vitro and in vivo. On the other hand, telomerase is able to maintain telomere length in cells by adding TTAGGG repeats to the ends of telomeres. However, the interrelationships existing among telomere length, telomerase activity and growth in vertebrates remain to be clarified. In the present study we measured telomere length (terminal restriction fragment length), telomerase activity and body growth of Oryzias latipes from the embryo stage until senescence. During the rapid growth stage (age 0–7 months), telomeres shortened in parallel with decreasing telomerase activity. Then, during adolescence (age 7 months – 1 year), telomeres lengthened quickly as growth slowed and telomerase activity increased. In the adult stage (age 1–4 years) characterized by little growth, telomerase activity decreased gradually and telomeres shortened. Our data indicate that telomere attrition and restoration are linked to growth and telomerase activity, and suggest that critical loss of telomere homeostasis is associated with mortality in this animal.

Sequential and counter-selectable cassettes for fission yeast

Background

Fission yeast is one of the most commonly used model organisms for studying genetics. For selection of desirable genotypes, antibiotic resistance cassettes are widely integrated into the genome near genes of interest. In yeasts, this is achieved by PCR amplification of the cassette flanked by short homology sequences, which can be incorporated by homology directed repair. However, the currently available cassettes all share the same tef promoter and terminator sequences. It can therefore be challenging to perform multiple genetic modifications by PCR-based targeting, as existing resistance cassettes in strains can be favored for recombination due to shared homology between the cassettes.

Results

Here we have generated new selection cassettes that do not recombine with those traditionally used. We achieved this by swapping the tef promoter and terminator sequences in the established antibiotic resistance MX6 cassette series for alternative promoters and/or terminators. The newly created selection cassettes did not recombine with the tef-containing MX6 cassettes already present in the genome, allowing for sequential gene targeting using the PCR-based method. In addition, we have generated a series of plasmids to facilitate the C-terminal tagging of genes with desired epitopes. We also utilized the anti-selection gene HSV-TK, which results in cell death in strains grown on the drug 5-Fluoro-2’-deoxyuridine (FdU, Floxuridin or FUDR). By fusing an antibiotic resistance gene to HSV-TK, we were able to select on the relevant antibiotic as well as counter-select on FdU media to confirm the desired genomic modification had been made. We noted that the efficiency of the counter selection by FdU was enhanced by treatment with hydroxyurea. However, a number of DNA replication checkpoint and homologous recombination mutants, including rad3∆, cds1∆, rad54∆ and rad55∆, exhibited sensitivity to FdU even though those strains did not carry the HSV-TK gene. To remove counter-selectable markers, we introduced the Cre-loxP irreversible recombination method. Finally, utilizing the negative selectable markers, we showed efficient induction of point mutations in an endogenous gene by a two-step transformation method.

Conclusions

The plasmid constructs and techniques described here are invaluable tools for sequential gene targeting and will simplify construction of fission yeast strains required for study.

STN1 OB Fold Mutation Alters DNA Binding and Affects Selective Aspects of CST Function

Mammalian CST (CTC1-STN1-TEN1) participates in multiple aspects of telomere replication and genome-wide recovery from replication stress. CST resembles Replication Protein A (RPA) in that it binds ssDNA and STN1 and TEN1 are structurally similar to RPA2 and RPA3. Conservation between CTC1 and RPA1 is less apparent. Currently the mechanism underlying CST action is largely unknown. Here we address CST mechanism by using a DNA-binding mutant, (STN1 OB-fold mutant, STN1-OBM) to examine the relationship between DNA binding and CST function. In vivo, STN1-OBM affects resolution of endogenous replication stress and telomere duplex replication but telomeric C-strand fill-in and new origin firing after exogenous replication stress are unaffected. These selective effects indicate mechanistic differences in CST action during resolution of different replication problems. In vitro binding studies show that STN1 directly engages both short and long ssDNA oligonucleotides, however STN1-OBM preferentially destabilizes binding to short substrates. The finding that STN1-OBM affects binding to only certain substrates starts to explain the in vivo separation of function observed in STN1-OBM expressing cells. CST is expected to engage DNA substrates of varied length and structure as it acts to resolve different replication problems. Since STN1-OBM will alter CST binding to only some of these substrates, the mutant should affect resolution of only a subset of replication problems, as was observed in the STN1-OBM cells. The in vitro studies also provide insight into CST binding mechanism. Like RPA, CST likely contacts DNA via multiple OB folds. However, the importance of STN1 for binding short substrates indicates differences in the architecture of CST and RPA DNA-protein complexes. Based on our results, we propose a dynamic DNA binding model that provides a general mechanism for CST action at diverse forms of replication stress.

Mammalian CST (CTC1/STN1/TEN1) is a three protein complex that aids in several steps during telomere replication and has genome-wide roles during recovery from replication fork stalling. Loss of CST leads to abnormalities in telomere structure, genomic instability and defects in chromosome segregation. Currently, we do not understand how CST acts to ensure the resolution of very diverse types of replication problem. We set out to address this question by studying a mutant form of CST that was predicted to alter DNA binding. The mutations are in the STN1 subunit. In vivo, the STN1 mutant (STN1-OBM) affects some aspects of CST function while others are normal. The effects of STN1-OBM do not align with the telomeric versus non-telomeric roles of CST but instead separate out different aspects of CST function at telomeres and genome-wide. In vitro binding studies indicate that STN1-OBM disrupts binding to only short DNA substrates. Since CST is likely to encounter DNA substrates of varied length and structure in vivo as it helps resolve different replication problems, this finding starts to explain why STN1-OBM affects only certain aspects of CST function. Our in vitro binding studies also shed light on how CST actually binds to DNA and they suggest a novel “dynamic binding model” that provides a mechanistic explanation for how CST helps resolve a diverse array of replication problems to preserve genome stability.

Aspergillus flavus SUMO Contributes to Fungal Virulence and Toxin Attributes

Small ubiquitin-like modifiers (SUMOs) can be reversibly attached to target proteins in a process known as SUMOylation, and this process influences several important eukaryotic cell events. However, little is known regarding SUMO or SUMOylation in Aspergillus flavus. Here, we identified a novel member of the SUMO family in A. flavus, AfSumO, and validated the existence of SUMOylation in this pathogenic filamentous fungus. We investigated the roles of AfsumO in A. flavus by determining the effects of AfsumO mutations on the growth phenotype, stress response, conidia and sclerotia production, aflatoxin biosynthesis, and pathogenicity to seeds, and we found that SUMOylation plays a role in fungal virulence and toxin attributes. Taken together, these results not only reveal potential mechanisms of fungal virulence and toxin attributes in A. flavus but also provide a novel approach for promising new control strategies of this fungal pathogen.

Dynamic Interactions of Arabidopsis TEN1: Stabilizing Telomeres in Response to Heat Stress

Telomeres are the essential nucleoprotein structures that provide a physical cap for the ends of linear chromosomes. The highly conserved CST (CTC1/STN1/TEN1) protein complex facilitates telomeric DNA replication and promotes telomere stability. Here we report three unexpected properties of Arabidopsis thaliana TEN1 that indicate it possesses functions distinct from other previously characterized telomere proteins. First, we show that telomeres in ten1 mutants are highly sensitive to thermal stress. Heat shock causes abrupt and dramatic loss of telomeric DNA in ten1 plants, likely via deletional recombination. Second, we show that AtTEN1 has the properties of a heat-shock induced molecular chaperone. At elevated temperature, AtTEN1 rapidly assembles into high molecular weight homo-oligomeric complexes that efficiently suppress heat-induced aggregation of model protein substrates in vitro. Finally, we report that AtTEN1 specifically protects CTC1 from heat-induced aggregation in vitro, and from heat-induced protein degradation and loss of telomere association in vivo. Collectively, these observations define Arabidopsis TEN1 as a highly dynamic protein that works in concert with CTC1 to preserve telomere integrity in response to environmental stress.

Multi-step coordination of telomerase recruitment in fission yeast through two coupled telomere-telomerase interfaces

Tightly controlled recruitment of telomerase, a low-abundance enzyme, to telomeres is essential for regulated telomere synthesis. Recent studies in human cells revealed that a patch of amino acids in the shelterin component TPP1, called the TEL-patch, is essential for recruiting telomerase to telomeres. However, how TEL-patch—telomerase interaction integrates into the overall orchestration of telomerase regulation at telomeres is unclear. In fission yeast, Tel1^ATM/Rad3^ATR-mediated phosphorylation of shelterin component Ccq1 during late S phase is involved in telomerase recruitment through promoting the binding of Ccq1 to a telomerase accessory protein Est1. Here, we identify the TEL-patch in Tpz1^TPP1, mutations of which lead to decreased telomeric association of telomerase, similar to the phosphorylation-defective Ccq1. Furthermore, we find that telomerase action at telomeres requires formation and resolution of an intermediate state, in which the cell cycle-dependent Ccq1-Est1 interaction is coupled to the TEL-patch—Trt1 interaction, to achieve temporally regulated telomerase elongation of telomeres.

DOI:http://dx.doi.org/10.7554/eLife.15470.001

The genetic blueprints for animals, plants and fungi are mostly contained within long strands of DNA and packaged into more compact thread-like structures called chromosomes. As such, most cells need to duplicate their chromosomes before they divide so that the two new cells each get a complete set of genetic instructions.

The machinery that copies DNA is unable to make it to the very ends of the chromosomes. Instead, an enzyme called telomerase adds new DNA to the chromosome ends to prevent them becoming too short. Problems with this process can cause serious issues, such as cell death or cancer, and so the activity of telomerase is carefully controlled. Other proteins guide telomerase to the ends of the chromosome only after the rest of the DNA has been copied. However, scientists do not know exactly how cells correctly time the arrival of telomerase.

A group of proteins called shelterin protects the chromosome ends, and studies with human cells have shown that telomerase attaches to a specific patch on one of shelterin proteins, called the TEL-patch, to begin its work. Now, Hu, Liu et al. have identified a similar TEL-patch in a shelterin protein from a type of yeast called fission yeast; this patch is also needed to attach telomerase to the chromosome ends. Further experiments with this yeast then showed that telomerase only arrives at the ends of the chromosomes after two parallel interaction interfaces have formed. Importantly, one of these interactions only takes place after most of the chromosomes are have been copied. As such, this “two-pronged interaction” mechanism ensures that the telomerase enzyme arrives at the end of the chromosomes at the right time.

Other similarities between human and fission yeast chromosome ends make it plausible that a comparable process controls the timing of telomerase attachment in human cells. However, more studies will be needed to confirm if this is the case.

DOI:http://dx.doi.org/10.7554/eLife.15470.002

Targeting of SUMO substrates to a Cdc48-Ufd1-Npl4 segregase and STUbL pathway in fission yeast

In eukaryotes, the conjugation of proteins to the small ubiquitin-like modifier (SUMO) regulates numerous cellular functions. A proportion of SUMO conjugates are targeted for degradation by SUMO-targeted ubiquitin ligases (STUbLs) and it has been proposed that the ubiquitin-selective chaperone Cdc48/p97-Ufd1-Npl4 facilitates this process. However, the extent to which the two pathways overlap, and how substrates are selected, remains unknown. Here we address these questions in fission yeast through proteome-wide analyses of SUMO modification sites. We identify over a thousand sumoylated lysines in a total of 468 proteins and quantify changes occurring in the SUMO modification status when the STUbL or Ufd1 pathways are compromised by mutations. The data suggest the coordinated processing of several classes of SUMO conjugates, many dynamically associated with centromeres or telomeres. They provide new insights into subnuclear organization and chromosome biology, and, altogether, constitute an extensive resource for the molecular characterization of SUMO function and dynamics.

High Confidence Fission Yeast SUMO Conjugates Identified by Tandem Denaturing Affinity Purification

Covalent attachment of the small ubiquitin-like modifier (SUMO) to key targets in the proteome critically regulates the evolutionarily conserved processes of cell cycle control, transcription, DNA replication and maintenance of genome stability. The proteome-wide identification of SUMO conjugates in budding yeast has been invaluable in helping to define roles of SUMO in these processes. Like budding yeast, fission yeast is an important and popular model organism; however, the fission yeast Schizosaccharomyces pombe community currently lacks proteome-wide knowledge of SUMO pathway targets. To begin to address this deficiency, we adapted and used a highly stringent Tandem Denaturing Affinity Purification (TDAP) method, coupled with mass spectrometry, to identify fission yeast SUMO conjugates. Comparison of our data with that compiled in budding yeast reveals conservation of SUMO target enrichment in nuclear and chromatin-associated processes. Moreover, the SUMO "cloud" phenomenon, whereby multiple components of a single protein complex are SUMOylated, is also conserved. Overall, SUMO TDAP provides both a key resource of high confidence SUMO-modified target proteins in fission yeast, and a robust method for future analyses of SUMO function.

Pli1(PIAS1) SUMO ligase protected by the nuclear pore-associated SUMO protease Ulp1SENP1/2

Covalent modification of the proteome by SUMO is critical for genetic stability and cell growth. Equally crucial to these processes is the removal of SUMO from its targets by the Ulp1 (HuSENP1/2) family of SUMO proteases. Ulp1 activity is normally spatially restricted, because it is localized to the nuclear periphery via interactions with the nuclear pore. Delocalization of Ulp1 causes DNA damage and cell cycle defects, phenotypes thought to be caused by inappropriate desumoylation of nucleoplasmic targets that are normally spatially protected from Ulp1. Here, we define a novel consequence of Ulp1 deregulation, with a major impact on SUMO pathway function. In fission yeast lacking Nup132 (Sc/HuNUP133), Ulp1 is delocalized and can no longer antagonize sumoylation of the PIAS family SUMO E3 ligase, Pli1. Consequently, SUMO chain-modified Pli1 is targeted for proteasomal degradation by the concerted action of a SUMO-targeted ubiquitin ligase (STUbL) and Cdc48-Ufd1-Npl4. Pli1 degradation causes the profound SUMO pathway defects and associated centromere dysfunction in cells lacking Nup132. Thus, perhaps counterintuitively, Ulp1-mediated desumoylation can promote SUMO modification by stabilizing a SUMO E3 ligase.

Quantitative Fitness Analysis Identifies exo1∆ and Other Suppressors or Enhancers of Telomere Defects in Schizosaccharomyces pombe

Synthetic genetic array (SGA) has been successfully used to identify genetic interactions in S. cerevisiae and S. pombe. In S. pombe, SGA methods use either cycloheximide (C) or heat shock (HS) to select double mutants before measuring colony size as a surrogate for fitness. Quantitative Fitness Analysis (QFA) is a different method for determining fitness of microbial strains. In QFA, liquid cultures are spotted onto solid agar and growth curves determined for each spot by photography and model fitting. Here, we compared the two S. pombe SGA methods and found that the HS method was more reproducible for us. We also developed a QFA procedure for S. pombe. We used QFA to identify genetic interactions affecting two temperature sensitive, telomere associated query mutations (taz1Δ and pot1-1). We identify exo1∆ and other gene deletions as suppressors or enhancers of S. pombe telomere defects. Our study identifies known and novel gene deletions affecting the fitness of strains with telomere defects. The interactions we identify may be relevant in human cells.

Sample Collection Method Bias Effects in Quantitative Phosphoproteomics

Current advances in selective enrichment, fractionation, and MS detection of phosphorylated peptides allowed identification and quantitation of tens of thousands phosphosites from minute amounts of biological material. One of the major challenges in the field is preserving the in vivo phosphorylation state of the proteins throughout the sample preparation workflow. This is typically achieved by using phosphatase inhibitors and denaturing conditions during cell lysis. Here we determine if the upstream cell collection techniques could introduce changes in protein phosphorylation. To evaluate the effect of sample collection protocols on the global phosphorylation status of the cell, we compared different sample workflows by metabolic labeling and quantitative mass spectrometry on Saccharomyces cerevisiae cell cultures. We identified highly similar phosphopeptides for cells harvested in ice cold isotonic phosphate buffer, cold ethanol, trichloroacetic acid, and liquid nitrogen. However, quantitative analyses revealed that the commonly used phosphate buffer unexpectedly activated signaling events. Such effects may introduce systematic bias in phosphoproteomics measurements and biochemical analysis.

RPA prevents G-rich structure formation at lagging-strand telomeres to allow maintenance of chromosome ends

Replication protein A (RPA) is a highly conserved heterotrimeric single-stranded DNA-binding protein involved in DNA replication, recombination, and repair. In fission yeast, the Rpa1-D223Y mutation provokes telomere shortening. Here, we show that this mutation impairs lagging-strand telomere replication and leads to the accumulation of secondary structures and recruitment of the homologous recombination factor Rad52. The presence of these secondary DNA structures correlates with reduced association of shelterin subunits Pot1 and Ccq1 at telomeres. Strikingly, heterologous expression of the budding yeast Pif1 known to efficiently unwind G-quadruplex rescues all the telomeric defects of the D223Y cells. Furthermore, in vitro data show that the identical D to Y mutation in human RPA specifically affects its ability to bind G-quadruplex. We propose that RPA prevents the formation of G-quadruplex structures at lagging-strand telomeres to promote shelterin association and facilitate telomerase action at telomeres.

SUMO-mediated regulation of DNA damage repair and responses

Sumoylation has important roles during DNA damage repair and responses. Recent broad-scope and substrate-based studies have shed light on the regulation and significance of sumoylation during these processes. An emerging paradigm is that sumoylation of many DNA metabolism proteins is controlled by DNA engagement. Such 'on-site modification' can explain low substrate modification levels and has important implications in sumoylation mechanisms and effects. New studies also suggest that sumoylation can regulate a process through an ensemble effect or via major substrates. Additionally, we describe new trends in the functional effects of sumoylation, such as bi-directional changes in biomolecule binding and multilevel coordination with other modifications. These emerging themes and models will stimulate our thinking and research in sumoylation and genome maintenance.

POT1a and components of CST engage telomerase and regulate its activity in Arabidopsis

Protection of Telomeres 1 (POT1) is a conserved nucleic acid binding protein implicated in both telomere replication and chromosome end protection. We previously showed that Arabidopsis thaliana POT1a associates with the TER1 telomerase RNP, and is required for telomere length maintenance in vivo. Here we further dissect the function of POT1a and explore its interplay with the CST (CTC1/STN1/TEN1) telomere complex. Analysis of pot1a null mutants revealed that POT1a is not required for telomerase recruitment to telomeres, but is required for telomerase to maintain telomere tracts. We show that POT1a stimulates the synthesis of long telomere repeat arrays by telomerase, likely by enhancing repeat addition processivity. We demonstrate that POT1a binds STN1 and CTC1 in vitro, and further STN1 and CTC1, like POT1a, associate with enzymatically active telomerase in vivo. Unexpectedly, the in vitro interaction of STN1 with TEN1 and POT1a was mutually exclusive, indicating that POT1a and TEN1 may compete for the same binding site on STN1 in vivo. Finally, unlike CTC1 and STN1, TEN1 was not associated with active telomerase in vivo, consistent with our previous data showing that TEN1 negatively regulates telomerase enzyme activity. Altogether, our data support a two-state model in which POT1a promotes an extendable telomere state via contacts with the telomerase RNP as well as STN1 and CTC1, while TEN1 opposes these functions.

Tpz1TPP1 SUMOylation reveals evolutionary conservation of SUMO-dependent Stn1 telomere association

Elongation of the telomeric overhang by telomerase is counteracted by synthesis of the complementary strand by the CST complex, CTC1(Cdc13)/Stn1/Ten1. Interaction of budding yeast Stn1 with overhang-binding Cdc13 is increased by Cdc13 SUMOylation. Human and fission yeast CST instead interact with overhang-binding TPP1/POT1. We show that the fission yeast TPP1 ortholog, Tpz1, is SUMOylated. Tpz1 SUMOylation restricts telomere elongation and promotes Stn1/Ten1 telomere association, and a SUMO-Tpz1 fusion protein has increased affinity for Stn1. Our data suggest that SUMO inhibits telomerase through stimulation of Stn1/Ten1 action by Tpz1, highlighting the evolutionary conservation of the regulation of CST function by SUMOylation.