Subramanian Lakxmi, Moser Bettina A, Nakamura Toru M
Department of Biochemistry and Molecular Genetics, University of Illinois at Chicago, 900 S. Ashland Ave., MC669, Chicago, IL 60607, USA.
Mol Cell Biol. 2008 Mar;28(5):1443-55. doi: 10.1128/MCB.01614-07. Epub 2007 Dec 26.
Fission yeast cells survive loss of the telomerase catalytic subunit Trt1 (TERT) through recombination-based telomere maintenance or through chromosome circularization. Although trt1Delta survivors with linear chromosomes can be obtained, they often spontaneously circularize their chromosomes. Therefore, it was difficult to establish genetic requirements for telomerase-independent telomere maintenance. In contrast, when the telomere-binding protein Taz1 is also deleted, taz1Delta trt1Delta cells are able to stably maintain telomeres. Thus, taz1Delta trt1Delta cells can serve as a valuable tool in understanding the regulation of telomerase-independent telomere maintenance. In this study, we show that the checkpoint kinase Tel1 (ATM) and the DNA repair complex Rad32-Rad50-Nbs1 (MRN) are required for telomere maintenance in taz1Delta trt1Delta cells. Surprisingly, Rap1 is also essential for telomere maintenance in taz1Delta trt1Delta cells, even though recruitment of Rap1 to telomeres depends on Taz1. Expression of catalytically inactive Trt1 can efficiently inhibit recombination-based telomere maintenance, but the inhibition requires both Est1 and Ku70. While Est1 is essential for recruitment of Trt1 to telomeres, Ku70 is dispensable. Thus, we conclude that Taz1, TERT-Est1, and Ku70-Ku80 prevent telomere recombination, whereas MRN-Tel1 and Rap1 promote recombination-based telomere maintenance. Evolutionarily conserved proteins in higher eukaryotic cells might similarly contribute to telomere recombination.
裂殖酵母细胞可通过基于重组的端粒维持或染色体环化来在端粒酶催化亚基Trt1(TERT)缺失的情况下存活。尽管可以获得具有线性染色体的trt1Δ存活细胞,但它们的染色体常常会自发环化。因此,很难确定不依赖端粒酶的端粒维持的遗传需求。相比之下,当端粒结合蛋白Taz1也缺失时,taz1Δtrt1Δ细胞能够稳定维持端粒。因此,taz1Δtrt1Δ细胞可作为理解不依赖端粒酶的端粒维持调控的宝贵工具。在本研究中,我们表明检查点激酶Tel1(ATM)和DNA修复复合物Rad32-Rad50-Nbs1(MRN)是taz1Δtrt1Δ细胞中端粒维持所必需的。令人惊讶的是,Rap1对于taz1Δtrt1Δ细胞中的端粒维持也是必不可少的,尽管Rap1招募到端粒依赖于Taz1。催化失活的Trt1的表达可以有效抑制基于重组的端粒维持,但这种抑制需要Est1和Ku70两者。虽然Est1对于将Trt1招募到端粒是必不可少的,但Ku70是可有可无的。因此,我们得出结论,Taz1、TERT-Est1和Ku70-Ku80可防止端粒重组,而MRN-Tel1和Rap1则促进基于重组的端粒维持。高等真核细胞中进化上保守的蛋白质可能同样有助于端粒重组。