Sumoylation of RecQ helicase controls the fate of dysfunctional telomeres

Sumoylation of the BLM ortholog, Sgs1, promotes telomere-telomere recombination in budding yeast

BLM and WRN are members of the RecQ family of DNA helicases, and in humans their loss is associated with syndromes characterized by genome instability and cancer predisposition. As the only RecQ DNA helicase in the yeast Saccharomyces cerevisiae, Sgs1 is known to safeguard genome integrity through its role in DNA recombination. Interestingly, WRN, BLM and Sgs1 are all known to be modified by the small ubiquitin-related modifier (SUMO), although the significance of this posttranslational modification remains elusive. Here, we demonstrate that Sgs1 is specifically sumoylated under the stress of DNA double strand breaks. The major SUMO attachment site in Sgs1 is lysine 621, which lies between the Top3 binding domain and the DNA helicase domain. Surprisingly, sumoylation of K621 was found to be uniquely required for Sgs1's role in telomere-telomere recombination. In contrast, sumoylation was dispensable for Sgs1's roles in DNA damage tolerance, supppression of direct repeat and rDNA recombination, and promotion of top3Delta slow growth. Our results demonstrate that although modification by SUMO is a conserved feature of RecQ family DNA helicases, the major sites of modification are located on different domains of the protein in different organisms. We suggest that sumoylation of different domains of RecQ DNA helicases from different organisms contributes to conserved roles in regulating telomeric recombination.

A non-canonical function of topoisomerase II in disentangling dysfunctional telomeres

The decatenation activity of topoisomerase II (Top2), which is widely conserved within the eukaryotic domain, is essential for chromosomal segregation in mitosis. It is less clear, however, whether Top2 performs the same function uniformly across the whole genome, and whether all its functions rely on decatenation. In the fission yeast, Schizosaccharomyces pombe, telomeres are bound by Taz1, which promotes smooth replication fork progression through the repetitive telomeric sequences. Hence, replication forks stall at taz1 Delta telomeres. This leads to telomeric entanglements at low temperatures ( Collapse

Key Words

• dna replication
• schizosaccharomyces pombe
• telomeres
• topoisomerase ii

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MESH Headings

• Alleles
• Catalysis
• Chromosomes/ultrastructure
• DNA/chemistry
• DNA Topoisomerases, Type II/chemistry
• Dimerization
• Escherichia coli/metabolism
• Introns
• Mutagenesis, Site-Directed
• Mutation
• Point Mutation
• Protein Structure, Tertiary
• Schizosaccharomyces/enzymology
• Schizosaccharomyces/genetics
• Telomere/ultrastructure
• Temperature

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Grants

• Cancer Research UK

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Affiliation(s)

Thomas Germe Telomere Biology Laboratory, Cancer Research, London, UK
Kyle Miller Telomere Biology Laboratory, Cancer Research, London, UK
Julia Promisel Cooper Telomere Biology Laboratory, Cancer Research, London, UK

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RecQ helicases: multifunctional genome caretakers

Around 1% of the open reading frames in the human genome encode predicted DNA and RNA helicases. One highly conserved group of DNA helicases is the RecQ family. Genetic defects in three of the five human RecQ helicases, BLM, WRN and RECQ4, give rise to defined syndromes associated with cancer predisposition, some features of premature ageing and chromosomal instability. In recent years, there has been a tremendous advance in our understanding of the cellular functions of individual RecQ helicases. In this Review, we discuss how these proteins might suppress genomic rearrangements, and therefore function as 'caretaker' tumour suppressors.

Taming the tiger by the tail: modulation of DNA damage responses by telomeres

Telomeres are by definition stable and inert chromosome ends, whereas internal chromosome breaks are potent stimulators of the DNA damage response (DDR). Telomeres do not, as might be expected, exclude DDR proteins from chromosome ends but instead engage with many DDR proteins. However, the most powerful DDRs, those that might induce chromosome fusion or cell-cycle arrest, are inhibited at telomeres. In budding yeast, many DDR proteins that accumulate most rapidly at double strand breaks (DSBs), have important functions in physiological telomere maintenance, whereas DDR proteins that arrive later tend to have less important functions. Considerable diversity in telomere structure has evolved in different organisms and, perhaps reflecting this diversity, different DDR proteins seem to have distinct roles in telomere physiology in different organisms. Drawing principally on studies in simple model organisms such as budding yeast, in which many fundamental aspects of the DDR and telomere biology have been established; current views on how telomeres harness aspects of DDR pathways to maintain telomere stability and permit cell-cycle division are discussed.