Cell cycle-dependent modification of Pot1 and its effects on telomere function

Telomere functions are tightly controlled throughout the cell cycle to allow telomerase access while suppressing a bona fide DNA damage response (DDR) at linear chromosome ends. However, the mechanisms that link cell cycle progression with telomere functions are largely unknown. Here we show that a key S-phase kinase, DDK (Dbf4-dependent protein kinase), phosphorylates the telomere binding protein Pot1, and that this phosphorylation is crucial for DNA damage checkpoint inactivation, the suppression of homologous recombination (HR) at telomeres, and the prevention of telomere loss. DDK phosphorylates Pot1 in a very conserved region of its most amino-terminal-proximal OB fold, suggesting that this regulation of telomere function may be widely conserved. Mutation of Pot1 phosphorylation sites leads to telomerase independent telomere maintenance through constant HR, as well as a dependence of telomere maintenance proteins involved in checkpoint activation and HR. These results uncover a novel and important link between DDR suppression and telomere maintenance. The failure in Pot1 phosphorylation and DDR inactivation could potentially lead to uncontrolled cell proliferation without a requirement for telomerase by switching cells to HR dependent telomere homeostasis. In mammals this could result in ALT (Alternative Lengthening of Telomeres), a recombination dependent mode of telomere maintenance, uncontrolled cell proliferation and cancer.