Distinct Roles of TRF1 in the Regulation of Telomere Structure and Lengthening

Yoichi Shinkai,Tomohiko Iwano,Keiji Okamoto,Makoto Tachibana

doi:10.1074/jbc.m802395200

Abstract

The telomere is a functional chromatin structure that consists of G-rich repetitive sequences and various associated proteins. Telomeres protect chromosomal ends from degradation, provide escape from the DNA damage response, and regulate telomere lengthening by telomerase. Multiple proteins that localize at telomeres form a complex called shelterin/telosome. One component, TRF1, is a double-stranded telomeric DNA binding protein. Inactivation of TRF1 disrupts telomeric localization of other shelterin components and induces chromosomal instability. Here, we examined how the telomeric localization of shelterin components is crucial for TRF1-mediated telomere-associated functions. We found that many of the mTRF1 deficient phenotypes, including chromosomal instability, growth defects, and dysfunctional telomere damage response, were suppressed by the telomere localization of shelterin components in the absence of functional mTRF1. However, abnormal telomere signals and telomere elongation phenotypes were either not rescued or only partially rescued, respectively. These data suggest that TRF1 regulates telomere length and function by at least two mechanisms; in one TRF1 acts through the recruiting/tethering of other shelterin components to telomeres, and in the other TRF1 seems to play a more direct role.

Highlights

Large complex termed the “shelterin/telosome,” which is important for regulating telomeric structure and function [3,4,5]
Creation of a Chicken-Mouse TRF1 Chimera That Can Localize to Telomeres but Does Not Associate with mTIN2—We previously described the phenotype of a murine TRF1 conditional knock-out model in ES cells in which both alleles of the endogenous mTRF1 gene were inactivated but exogenous mTRF1 cDNA flanked by two loxP sequences and a transgene encoding a Cre-estrogen receptor fusion molecule, Mer-Cre-Mer, were expressed [11]
To further examine whether telomere localization of other shelterin components, mTPP1 and mPOT1, are affected in mTRF1⌬ ES cells, we introduced the GFP-mTPP1, GFP-mPOT1a, and GFPmPOT1b expression vectors

Summary

Introduction

Large complex termed the “shelterin/telosome,” which is important for regulating telomeric structure and function [3,4,5]. On the other hand TRF2 is a telomere capping molecule, and deletion of TRF2 immediately induces end-to-end fusion, cell senescence, or cell death via the activation of telomere dysfunctional DNA damage responses (14 –16). These proteins are associated with TIN2, which contributes to the stabilization of telomeric localization of them [17, 18]. Each shelterin component plays a crucial role in telomere length regulation and the maintenance of the functional telomere structure It is still not well understood how shelterin components functionally interact in terms of telomere dynamics and the regulation of chromosomal stability.

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