The telomere-binding protein forms a heterodimer in ciliates consisting of an alpha and a beta subunit. This complex may function as a protective cap for the single-stranded telomeric overhang. Alpha subunit consists of 3 structural domains, all with the same beta-barrel OB fold.
This domain binds single stranded telomeric DNA and adopts an OB fold [ (PUBMED:11935027) ]. It includes the proteins POT1 and Cdc13 which have been shown to regulate telomere length, replication and capping [ (PUBMED:11230149) (PUBMED:18066078) (PUBMED:16943437) ]. POT1 is one component of the shelterin complex that protects telomere-ends from attack by DNA-repair mechanisms [ (PUBMED:1239117) (PUBMED:19228335) ].
Pot1 and cell cycle progression cooperate in telomere length regulation.
Nat Struct Mol Biol. 2008; 15: 79-84
Display abstract
Removal of the vertebrate telomere protein Pot1 results in a DNA damageresponse and cell cycle arrest. Here we show that loss of chicken Pot1causes Chk1 activation, and inhibition of Chk1 signaling prevents the cellcycle arrest. However, arrest still occurs after disruption of ATM, whichencodes another DNA damage response protein. These results indicate thatPot1 is required to prevent a telomere checkpoint mediated by another suchprotein, ATR, that is most likely triggered by the G-overhang. We alsoshow that removal of Pot1 causes exceptionally rapid telomere growth uponarrest in late S/G2 of the cell cycle. However, release of the arrestslows both telomere growth and G-overhang elongation. Thus, Pot1 seems toregulate telomere length and G-overhang processing both through directinteraction with the telomere and by preventing a late S/G2 delay in thecell cycle. Our results reveal that cell cycle progression is an importantcomponent of telomere length regulation.
Vertebrate POT1 restricts G-overhang length and prevents activation of atelomeric DNA damage checkpoint but is dispensable for overhangprotection.
Mol Cell Biol. 2006; 26: 6971-82
Display abstract
Although vertebrate POT1 is thought to play a role in both telomerecapping and length regulation, its function has proved difficult toanalyze. We therefore generated a conditional cell line that lackswild-type POT1 but expresses an estrogen receptor-POT1 fusion. The cellsgrow normally in tamoxifen, but drug removal causes loss of POT1 from thetelomere, rapid cell cycle arrest, and eventual cell death. The arrestedcells have a 4N DNA content, and addition of caffeine causes immediateentry into mitosis, suggesting a G(2) arrest due to an ATM- and/orATR-mediated checkpoint. gammaH2AX accumulates at telomeres, indicating atelomeric DNA damage response, the likely cause of the checkpoint.However, POT1 loss does not cause degradation of the G-strand overhang.Instead, the amount of G overhang increases two- to threefold. Some cellseventually escape the cell cycle arrest and enter mitosis. They rarelyexhibit telomere fusions but show severe chromosome segregation defectsdue to centrosome amplification. Our data indicate that vertebrate POT1 isrequired for telomere capping but that it functions quite differently fromTRF2. Instead of being required for G-overhang protection, POT1 isrequired to suppress a telomeric DNA damage response. Our results alsoindicate significant functional similarities between POT1 and Cdc13 frombudding yeast (Saccharomyces cerevisiae).
Conserved structure for single-stranded telomeric DNA recognition.
Science. 2002; 296: 145-7
Display abstract
The essential Cdc13 protein in the yeast Saccharomyces cerevisiae is asingle-stranded telomeric DNA binding protein required for chromosome endprotection and telomere replication. Here we report the solution structureof the Cdc13 DNA binding domain in complex with telomeric DNA. Thestructure reveals the use of a single OB (oligonucleotide/oligosaccharidebinding) fold augmented by an unusually large loop for DNA recognition.This OB fold is structurally similar to OB folds found in the ciliatedprotozoan telomere end-binding protein, although no sequence similarity isapparent between them. The common usage of an OB fold for telomeric DNAinteraction demonstrates conservation of end-protection mechanisms amongeukaryotes.
Cdc13 both positively and negatively regulates telomere replication.
Genes Dev. 2001; 15: 404-14
Display abstract
Cdc13 is a single-strand telomeric DNA-binding protein that positivelyregulates yeast telomere replication by recruiting telomerase tochromosome termini through a site on Cdc13 that is eliminated by thecdc13-2 mutation. Here we show that Cdc13 has a separate role in negativeregulation of telomere replication, based on analysis of a new mutation,cdc13-5. Loss of this second regulatory activity results in extensiveelongation of the G strand of the telomere by telomerase, accompanied by areduced ability to coordinate synthesis of the C strand. Both the cdc13-5mutation and DNA polymerase alpha mutations (which also exhibit elongatedtelomeres) are suppressed by increased expression of the Cdc13-interactingprotein Stn1, indicating that Stn1 coordinates action of the laggingstrand replication complex with the regulatory activity of CDC13. However,the association between Cdc13 and Stn1 is abolished by cdc13-2, the samemutation that eliminates the interaction between Cdc13 and telomerase. Wepropose that Cdc13 participates in two regulatory steps-first positive,then negative-as a result of successive binding of telomerase and thenegative regulator Stn1 to overlapping sites on Cdc13. Thus, Cdc13coordinates synthesis of both strands of the telomere by first recruitingtelomerase and subsequently limiting G-strand synthesis by telomerase inresponse to C-strand replication.
Crystal Structure of the N-terminal domain of Oxytricha nova telomere end binding protein alpha subunit both uncomplexed and complexed with telomeric ssDNA
