Cell-cycle-dependent Xenopus TRF1 recruitment to telomere chromatin regulated by Polo-like kinase.

Telomeres are regulated by a homeostatic mechanism that includes telomerase and telomeric repeat binding proteins, TRF1 and TRF2. Recently, it has been hypothesized that telomeres assume distinct configurations in a cell-cycle-dependent manner, although direct biochemical evidence is lacking. Here we demonstrated that Xenopus TRF1 (xTRF1) associates with telomere chromatin specifically in mitotic Xenopus egg extracts, and dissociates from it upon mitotic exit. Both the N-terminal TRF-homology (TRFH) domain and the linker region connecting the TRFH domain and the C-terminal Myb domain are required for this cell-cycle-dependent association of xTRF1 with chromatin. In contrast, Xenopus TRF2 (xTRF2) associates with chromatin throughout the cell cycle. We showed that Polo-like kinase (Plx1) phosphorylates xTRF1 in vitro. Moreover, the mitotic xTRF1-chromatin association was significantly impaired when Plx1 was immunodepleted from the extracts. Finally, high telomerase activities were detected in association with replicating interphase chromatin compared with mitotic chromatin. These results indicate that telomere chromatin is actively regulated by cell-cycle-dependent processes, and provide an insight for understanding how telomeres undergo DNA metabolisms during the cell cycle.