Comparison of Telomere Structure in Eukaryotes

Abstract

Telomeres are DNA-protein complexes that are located at the ends of eukaryotic chromosomes. The fusion of broken chromosome ends is prevented by the presence of telomeres, which act to inhibit this process. This specific function of telomeres serves to distinguish normal chromosome ends from double-stranded breaks in DNA. Telomeres contain a series of short, repeated sequences arranged in a tandem array. The number of repeats varies between different organisms, with a range of 20 to 1,000 repeats being typical. A G-rich strand is replicated by lagging strand synthesis, which creates a 3' overhang. In addition, a complementary C-rich strand is replicated by leading strand synthesis. The objective of this study is to undertake a comparative analysis of the structure of telomeres in Saccharomyces cerevisiae, Saccharomyces pombe and mammals. In Saccharomyces cerevisiae, the Rap1 protein binds to the double-stranded telomeric sequences, as well as to the Rif1 and Rif2 proteins, which regulate telomere length. Cdc13 and the Cdc13-interacting factors Ten1 and Stn1 bind to the single-stranded overhang. In Saccharomyces pombe telomeres, Taz1 binds to the double-stranded DNA (dsDNA), and Rap1 and Rif1 also bind to the ds region via Taz1. Pot1 interacts with Tpz1, forming a complex that binds to the 3' overhang. The protein Poz1 serves to connect the dsDNA binding complex, comprising Taz1 and Rap1, to the ssDNA binding complex, which includes Pot1 and Tpz1. Furthermore, Ccq1 interacts with Tpz1 and facilitates the recruitment of telomerase. The Stn1/Ten1 complex exhibits a binding affinity for a single-stranded telomere. In mammalian telomeres, the shelterin complex that binds double-stranded telomeric DNA is composed of six subunits. The double-stranded telomeric DNA is bound by TRF1 and TRF2. TPP1 and POT1 are capable of binding single-stranded DNA. TIN2 serves to connect the dsDNA binding complex TRF1/TRF2 to the ssDNA binding complex POT1/TPP1. Rap1 binds to the telomere by interacting with TRF1 and TRF2. Moreover, this study will address the regulation and comparison of the shelterin complex. Additionally, in mammals, the activation of DNA damage response pathways is necessary when double-strand DNA is broken. This, in turn, elucidates the specific repair pathways that are employed. We conclude by discussing the T-loop structure, as telomeres in several species have been shown to fold back into a structure called a T-loop, which is believed to mediate telomere protection.

Keywords: Shelterin; T-Loop Structure; Telomere.

Figure 1

The end replication problem. Lagging strand DNA synthesis does not complete the replication of the 5' end resulting in a 3' overhang. Leading strand DNA synthesis, however, complete the replication of the 5' end resulting in a blunt end.

Figure 2

Evolutionary Conservation of Telomeric repeat sequences. Telomeric repeat sequences and also telomere function are conserved throughout eukaryotes.

Figure 3

Structure of Saccharomyces cerevisiae telomeres. Rap1 binds the double-stranded telomeric sequences and also binds Rif1 and Rif2 which regulate telomere length. Cdc13 and the Cdc13-interacting factors Ten1 and Stn1 bind to the single strand overhang.

Figure 4

Summary of some of the proposed steps in TG tract length-dependent regulation of telomerase action. The Rap1-Rif complex creates a negative signal which blocks telomerase recruitment. Rif2 competes with Tel1 to bind to the Xrs2 component of MRX, hence it inhibits Tel1 binding. Cdc13 is phosphorylated by Cdk1, which creates a positive signal for telomerase recruitment by interacting with Est1 subunit. The DNA polymerase α QUOTE α /primase complex interacts with the CST complex, thereby inhibiting telomerase recruitment.

Figure 5

Structure of Saccharomyces Pombe Telomeres.Taz1 binds the dsDNA and also Rap1 and Rif1 bind to the ds region via Taz1. Pot1 interacts with Tpz1 and binds to the 3′ overhang. Poz1 connects the dsDNA binding complex Taz1/Rap1 to the ssDNA binding complex Pot1/Tpz1. Ccq1 also interacts with Tpz1, recruits telomerase. Stn1/Ten1 complex binds to single strand telomere.

Figure 6

TRF1 and TRF2 which bind directly the double-stranded telomeric DNA. TPP1 and POT1 bind single-stranded DNA.TIN2 connects the dsDNA binding complex TRF1/TRF2 to the ssDNA binding complex POT1/TPP1. Rap1 binds the telomere by interacting with TRF1 and TRF2.

Figure 7

The telomerase complex. Telomerase is composed of TERT that provides reverse transcriptase activity to the complex and uses TERC, the RNA component of telomerase, as a template.

Figure 8

Comparison of shelterin to telomeric proteins in other eukaryotes. The proteins with the same colour could have similarity in their functions, whereas, similarity could not be observed in the case of their sequence and their structure. Mammals: Shelterin complex of TRF1, TRF2, Rap1, TIN2, TPP1, and POT1. Fission yeast: Shelterin-like complex of Taz1, Rap1, Poz1, Tpz1, Ccq1, and Pot1; also Ten1 and Stn1 are present. Budding yeast: dsDNA-binding complex of Rap1, Rif1, and Rif2; ssDNA- binding complex of Cdc13, Stn1, and Ten1. Oxytricha nova: TEBP α/β.

Figure 9

The end-protection problem. When a mammalian chromosome breaks, two signalling pathways can be activated (the ATM and ATR kinase pathways) which can induce cell death. One of two different DNA repair pathways (HDR and NHEJ) repairs the broken chromosome. Activation of DNA repair or DNA damage signalling has a problem for the ends of linear chromosomes, thus, telomeres can solve this end-protection problem by shelterin.

Figure 10

Different solutions of the end protection problem. POT1 inhibits the ATR kinase signalling pathway and NHEJ especially after DNA replication, however, TRF2represses the ATM kinase signalling pathway and NHEJ at telomeres. Both POT1 and TRF2 inhibit HDR at telomeres. In budding yeast, Rap1 represses NHEJ and also Cdc13 inhibits Mec1 kinase.

Figure 11

Different components of shelterin are dedicated to different aspects of the end-protection problem. (A), T-loop structure blocks telomeres end from the MRN (DNA end sensr) that activates the ATM kinase pathway, and the Ku70/80 that activates NHEJ. In (B), POT1 can repress ATR kinase signalling by inhibiting the binding of RPA that activates the ATR kinase pathway.