10779/crick.11638269.v1 Grzegorz Sarek Grzegorz Sarek Panagiotis Kotsantis Panagiotis Kotsantis Phil Ruis Phil Ruis David Van Ly David Van Ly Pol Margalef Pol Margalef Valerie Borel Valerie Borel Xiao-Feng Zheng Xiao-Feng Zheng Helen R Flynn Helen R Flynn Ambrosius P Snijders Ambrosius P Snijders Dipanjan Chowdhury Dipanjan Chowdhury Anthony J Cesare Anthony J Cesare Simon J Boulton Simon J Boulton CDK phosphorylation of TRF2 controls t-loop dynamics during the cell cycle. The Francis Crick Institute 2020 Boulton FC001048 PC-ack PRT General Science & Technology 2020-01-17 16:58:56 Journal contribution https://crick.figshare.com/articles/journal_contribution/CDK_phosphorylation_of_TRF2_controls_t-loop_dynamics_during_the_cell_cycle_/11638269 The protection of telomere ends by the shelterin complex prevents DNA damage signalling and promiscuous repair at chromosome ends. Evidence suggests that the 3' single-stranded telomere end can assemble into a lasso-like t-loop configuration1,2, which has been proposed to safeguard chromosome ends from being recognized as DNA double-strand breaks2. Mechanisms must also exist to transiently disassemble t-loops to allow accurate telomere replication and to permit telomerase access to the 3' end to solve the end-replication problem. However, the regulation and physiological importance of t-loops in the protection of telomere ends remains unknown. Here we identify a CDK phosphorylation site in the shelterin subunit at Ser365 of TRF2, whose dephosphorylation in S phase by the PP6R3 phosphatase provides a narrow window during which the RTEL1 helicase can transiently access and unwind t-loops to facilitate telomere replication. Re-phosphorylation of TRF2 at Ser365 outside of S phase is required to release RTEL1 from telomeres, which not only protects t-loops from promiscuous unwinding and inappropriate activation of ATM, but also counteracts replication conflicts at DNA secondary structures that arise within telomeres and across the genome. Hence, a phospho-switch in TRF2 coordinates the assembly and disassembly of t-loops during the cell cycle, which protects telomeres from replication stress and an unscheduled DNA damage response.