Helicase mediated vectorial folding of telomere G-quadruplex.

The G-rich single-stranded telomere overhang can self-fold into G-quadruplex (G4) structure both in vivo and in vitro. In somatic cells, telomeres shorten progressively due to the end-replication. In stem cells, however, telomeres are replenished by a special enzyme, telomerase which synthesizes single-stranded telomere overhang. The active extension by the telomerase releases G-rich overhang segmentally in 5' to 3' direction as the overhang folds into G4 structure after successive elongation. To replicate such vectorial G4 folding process, we employed a superhelicase, Rep-X to release the G-rich sequence gradually. Using single-molecule assay we demonstrated that the folded conformation achieved by the vectorial folding is inherently different from the post-folding where the entire overhang is allowed to fold at once. In addition, the vectorially folded overhangs are less stable and more accessible to a complementary C-rich strand and the telomere binding protein, POT1 compared to the post-folded state. The higher accessibility may have implications for the facile loading of shelterin proteins after DNA replication.