Telomere end-binding proteins control the formation of G-quadruplex DNA structures in vivo

Telomere end-binding proteins (TEBPs) bind to the guanine-rich overhang (G-overhang) of telomeres. Although the DNA binding properties of TEBPs have been investigated in vitro, little is known about their functions in vivo. Here we use RNA interference to explore in vivo functions of two ciliate TEBPs, TEBP alpha and TEBP beta. Silencing the expression of genes encoding both TEBPs shows that they cooperate to control the formation of an antiparallel guanine quadruplex (G-quadruplex) DNA structure at telomeres in vivo. This function seems to depend on the role of TEBP alpha in attaching telomeres in the nucleus and in recruiting TEBP beta to these sites. In vitro DNA binding and footprinting studies confirm the in vivo observations and highlight the role of the C terminus of TEBP beta in G-quadruplex formation. We have also found that G-quadruplex formation in vivo is regulated by the cell cycle-dependent phosphorylation of TEBP beta.