Although t-loops protect telomeres, they are at risk of cleavage by Holliday junction (HJ) resolvases if branch migration converts the three-way t-loop junction into four-way HJs. T-loop cleavage is repressed by the TRF2 basic domain, which binds three- and four-way junctions and protects HJs in vitro. By replacing the basic domain with bacterial-protein domains binding three- and four-way junctions, we demonstrated the in vivo relevance of branched-DNA binding. Branched-DNA binding also repressed PARP1, presumably by masking the PARP1 site in the t-loop junction. Although PARP1 recruits HJ resolvases and promotes t-loop cleavage, PARP1 activation alone did not result in t-loop cleavage, thus suggesting that the basic domain also prevents formation of HJs. Concordantly, removal of HJs by BLM helicase mitigated t-loop cleavage in response to loss of the basic domain. We propose that TRF2 masks and stabilizes the t-loop three-way junction, thereby protecting telomeres from detrimental deletions and PARP1 activation.

中文翻译：

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TRF2结合分支DNA以保护端粒完整性

尽管t环可保护端粒，但如果分支迁移将三向t环连接转变为四向hj，则它们有被霍利迪结（HJ）分解分裂的风险。T环裂解受TRF2基本结构域抑制，该结构域结合三向和四向连接并在体外保护HJ 。通过用结合三向和四向连接的细菌-蛋白质结构域取代基本结构域，我们证明了体内支链DNA结合的相关性。分支DNA的结合也抑制了PARP1，大概是通过掩盖t环连接处的PARP1位点。尽管PARP1募集HJ分解并促进t环切割，但单独的PARP1活化并不能导致t环切割，因此表明基本域也可以阻止HJ的形成。相应地，通过BLM解旋酶去除HJ减轻了响应于基本结构域丧失的t环裂解。我们建议TRF2掩盖并稳定t环三向结，从而保护端粒免受有害的缺失和PARP1激活。