TRF2 promotes dynamic and stepwise looping of POT1 bound telomeric overhang

Abstract Human telomeres are protected by shelterin components including POT1 and TRF2 but the molecular mechanism remains elusive. Here, we report an unexpected activity of POT1 in imparting conformational dynamics of the telomere overhang, even at a monomer level. Such POT1 induced overhang dynamics is greatly enhanced when TRF2 engages with the telomere duplex. Strikingly, TRF2 brings a POT1 bound overhang to the duplex and induces a stepwise movement up and down the axis of telomere duplex, thereby generating a highly dynamic loop formation. Interestingly, TRF2, but not TRF2ΔB, recruits POT1 bound overhang to the telomere duplex-single stranded junction by discrete steps of movement with a step size of TTAGGG. The same steps are observed regardless of the length of the POT1 bound overhang, suggesting a tightly regulated mechanism. Based on our results, we propose a plausible dynamic mechanism of t-loop formation coordinated by POT1 and TRF2.